School Science Lessons
Weather experiments
Updated: 2008-07-23
Please send comments to: J.Elfick@uq.edu.au
History
Table of contents
37.1.0 Make weather instruments and a weather station
37.8.0 Measure relative humidity
37.9.0 Winds and weather
37.15.0 How moisture gets into the air
37.25.0 How moisture comes out of the air
37.31.0 Weather projects
37.37.0 Clouds and weather
37.38.0 Observe and describe warm and cold fronts
37.39.0 Weather studies
37.1.0 Make weather instruments and a weather station
37.1 Make a wind vane
37.2 Make a wind speed indicator
37.3 Make a deflection anemometer
37.4 Make a pressure tube anemometer
37.5 Make a rain gauge, precipitation gauge
37.6 Make a hair hygrometer
37.7 Make a housing box (Stevenson screen) for weather instruments
5.22 Rain gauge (Primary)
37.8.0 Measure relative humidity
37.8.1 Wet and dry bulb hygrometer (wet and dry bulb thermometers)
37.8.2 Sling psychrometer
37.8.3 Dew point hygrometer
37.8.4 Relative humidity table, depression of the wet bulb
6.17 Measure relative humidity (Primary)
37.9.0 Winds and weather
37.9 Air expands when heated
37.10 Air has mass
37.11 Air exerts pressure
37.12 Cold air is heavier than warm air, inverted paper bag balance
37.13 Make a convection box
37.14 Trace convection currents
4.24 Convection currents in a test-tube
4.25 Convection currents in a container
4.26 Convection current from an ink bottle
4.27 Convection currents in air, convection box
4.28 Trace convection currents from a lighted candle
4.29 Convection disc, heat snake, convection wheel
4.30 Convection currents and ventilation
4.31 Temperature of water at maximum density, 4oC
5.23 Wind speed and direction (Primary)
5.26 Air pressure (Primary)
6.18 Measure air pressure (Primary)
4.223 Atmospheric pressure
37.15.0 How moisture gets into the air
37.15 Atmospheric moisture
37.16 Weigh water "lost" by evaporation
37.17 Moisture evaporates from soil
37.18 Moisture comes from plants
37.19 Moisture comes from other plants
37.20 Moisture from breathing
37.21 Surface area affects evaporation
37.22 Temperature affects the rate of evaporation
37.23 Moving air affects the rate of evaporation
37.24 Moisture in the air affects the rate of evaporation
37.25.0 How moisture comes out of the air
37.25 Moisture condenses on cool surfaces
37.26 Study the water cycle
37.27 Make a rain cycle
37.29 Make cloud in a bottle
37.30 Study snowflakes
4.41 Ice experiments (Primary)
37.31.0 Weather projects
37.31 Keep a weather record
37.31.1 Make a weather picture
37.32 Miniature weather fronts
37.33 Measure the upper winds with a simple astrolabe, simple sextant
37.34 Avoid solar ultraviolet radiation
37.34.1 Measure solar ultraviolet radiation
37.35 Measure the dust in the air
37.36 Thunderstorm experiment
27.1.80 Rainbows
37.37.0 Clouds and weather
37.37.1 The ten main types of cloud
37.37.2 Descriptions of the main types of clouds
37.37.3 Contrails
37.37.4 Cloud seeding
5.24 Describe clouds (Primary)
37.337.0 Observe and describe warm and cold fronts
37.38.1 Warm front
37.38.2 Cold front
37.38.3 Tornadoes
37.38.4 Hurricanes
37.39.0 Weather studies
37.39.1 Layers of the atmosphere
37.39.2 Inversion layers
37.40 Coriolis force (Coriolis effect) and weather rotations
37.41 Plug hole experiments
37.42 Weather maps, Buys Ballots law, geostrophic wind and gradient wind
37.43 Model greenhouse to simulate the greenhouse effect
37.44 Navigation data used by a ship at sea
37.1 Make a wind vane
See diagram 37.1: Wind vane
Make an arm from a piece of wood about 25 cm in length and 1 cm2 cross-section. Saw 6 cm deep slots in the centre at each end of the arm. From a piece of wood 10 cm wide and thin enough to just fit into the slots in the arm, cut the head of an arrow and the tail. The head of the arrow should have a smaller area than the tail. Push the head and tail of the wind vane into the slots in the arm and fasten with glue. Balance the wind vane on the blade of a knife and mark the point of balance on the arm. Close the open end of a medicine dropper by rotating it in a flame. At the point of balance of the arm drill a 0.75 cm deep hole wider than the medicine dropper tube. Put the closed end of the medicine dropper tube up in the hole and fasten it with glue. Make a 1 metre supporting pole for the wind vane. Hammer a nail into the end of the pole and sharpen it to a point with a file. Place the medicine dropper in the arm over the nail. Put the wind vane on top of a building or in a place where it is exposed to the wind. Fix four arms to the pole with the symbols N, E, S, W, at the ends of the arms.
37.2 Make a wind speed indicator
See diagram 37.2: Wind speed indicator
Wind speed (at 10 m height): 1 m / s = 3.6 km / h = 2.237 mph = 1.944 knots
1 knot = 1 nautical mile per hour = 0.5144 m / s = 1.852 km / h = 1.125 mph
m / s
 Knots
 Beaufort
scale
 Wind
0.0-0.4
 0.0-0.9
 0
 Calm
0.4-1.8
 0.9-3.5
 1
 Light
1.8-3.6
 3.5-7.0
 2
 Light
3.6-5.8
 7-11
 3
 Light
5.8-8.5
 11-17
 4
 Moderate
8.5-11
 17-22
 5
 Fresh
11-14
 22-28
 6
 Strong
14-17
 28-34
 7
 Strong
17-21
 34-41
 8
 Gale
21-25
 41-48
 9
 Gale
25-29
 48-56
 10
 Strong gale
29-34
 56-65
 11
 Strong gale
>34
 >65
 2
 Hurricane
Use two pieces of light wood 50 cm long and 1 cm2 in cross section. Cut a notch 1 cm wide and 0.5 cm deep in the exact centre of each piece of wood and fit them to form cross arms. Close the open end of a medicine dropper by rotating it in a flame. Drill a 0.75 cm deep hole wider than the medicine dropper tube at the exact centre of the cross arms. Put the closed end of the medicine dropper tube in the hole and secure it with glue. Fasten four small plastic cups to the ends of the cross arms. The cups must all face in the same direction. Make a 1 metre supporting pole for the wind speed indicator. Hammer a nail into the end of the pole and sharpen it to a point with a file. Place the medicine dropper in the cross arms over the nail. The wind speed indicator will spin in the wind. To estimate the speed of the wind in kilometres per hour, count the number of turns made in 30 seconds and divide by 3. Calibrate the wind speed indicator by asking a driver to drive a car on a calm day at five kilometres per hour while holding the speed indicator out of the front window. Count the number of turns in 30 seconds for this speed. Repeat at 10, 15, 20, 25, 30 km per hour. Mount your wind speed indicator in a place exposed to the wind from any direction.
37.3 Make a deflection anemometer
See diagram 37.3: Anemometer
Use a 25 x 2 x 1 cm piece of wood for an arm. Make a saw cut at one end of the arm, insert the base of a plastic protractor then secure with glue. Before the glue sets, drill a hole 0.5 cm diameter through the end of the arm and the protractor. Bend a 12 cm length of coat hanger wire into an U-shape and pass it though the hole just drilled. Cut a 10 x 8 cm piece of plywood and fasten each end of the wire to it. The vane is slotted so that it will swing up and each side of the protractor as the wind blows against the vane. Balance the arm (with protractor, wire and vane attached) on the blade of a knife and mark the point of balance on the arm. Attach screws to the lighter end of the arm if needed. Close the open end of a medicine dropper by rotating it in a flame. At the point of balance of the arm drill a 0.75 cm deep hole wider than the medicine dropper tube. Put the closed end of the medicine dropper tube up in the hole and fasten it with glue. Make a 1 metre supporting pole for the deflection anemometer. Hammer a nail into the end of the pole and sharpen it to a point with a file. Place the medicine dropper in the arm over the nail. The anemometer should pivot freely and face into the wind. Calibrate this device by asking a driver to drive a car on a calm day at five kilometres per hour while holding the anemometer out of the front window. Make a mark on the protractor where the vane hangs. Repeat for speeds of 10, 15, 20, 25, 30 km per hour.
37.4 Make a pressure tube anemometer
See diagram 37.4: Pressure tube anemometer
Fit an U-tube, with a funnel attached, to a vertical flat board. Attach a triangular vane to keep the mouth of the funnel facing directly into the wind. Attach a ruler to the board to measures the height of water in the arm of the U-tube. Attach a pole to the vertical board. The bottom end of the pole must turn freely in a hole in a wooden base. Lubricate the points of contact between the end of the pole and the base. The wind speed is proportional to the difference in height of water in the arms of the U-tube. Keep the same amount of water in the U-tube to ensure accuracy of readings. If you put ink in the water, a "dirty" ring inside the glass tube will show the highest wind gust velocity between the times of instrument reading. If the winds in your location are light, the device will be more useful if the left hand tube is sloping at 30o, instead of being vertical. A similar device in an aeroplane, the Pitot-static tube, measures the dynamic air pressure to be displayed on the airspeed indicator.
37.5 Make a rain gauge, precipitation gauge
See diagram 37.5: Rain gauge
Use a large metal can, e.g. 203 mm diameter and 200 mm capacity, or 127 mm diameter and 650 mm capacity. Use a measuring cylinder about 3 cm in diameter and at least 25 cm high, which will stand inside the can. Select a funnel whose diameter is equal to that of the metal can. Set the rain gauge in an open spot where it will not be upset easily. The whole apparatus should be fixed so that the funnel is 300 mm above ground level, horizontal and at a distance of at least four times the height of the nearest object. Find the rainfall in centimetres by graduating the measuring cylinder in terms of its radius and the radius of the collecting funnel by use of the formula: Height in measuring cylinder for each centimetre of rainfall = (radius of funnel / radius of bottle)2. Measure the contents of the measuring cylinder by placing it on a flat horizontal surface or hold the measuring cylinder from the top between thumb and forefinger so that it hangs vertically. Bring the eye level with the surface of the liquid to read the position of the surface on the scale. If using a dipstick, keep it vertically against the side of the container. Melt frozen precipitation before measuring the contents of the measuring cylinder.
37.6 Make a hair hygrometer
See diagram 37.6: Hair hygrometer
The length of human hair is related to air humidity. When air is damp, the hair is longer. When air is dry, the hair is shorter. Dip 30 cm long human hairs in dilute sodium hydroxide solution to dissolve grease. Attach one hair to the upper end of a stand and stretch it with a 50 g weight. Pass the hair twice around a spool. Fix the spool to an axle that is free to rotate in bearings made from a piece of tin. Fix a pointer to the axle and make a circular scale. Changes in atmospheric humidity will affect the length of the hair and the position of the pointer. To calibrate the scale, place the hair hygrometer above warm water in a bucket and cover with a wet towel. When the pointer has moved as far as it can, mark this point 100 on your scale because the air in the bucket will be 100% saturated. Mark other points on the scale by noting readings on a wet and dry bulb hygrometer or according to the relative humidity reported by radio or TV station.
37.7 Make a housing box (Stevenson Screen) for weather instruments
See diagram 37.7: Stevenson screen
Weather instruments that you must expose to the weather include the wind vane, the wind speed indicator and the rain gauge. Protect the metal parts of these instruments with either grease or aluminium paint. Weather instruments that you must shield from rain, wind, and direct sunlight, include the barometer, thermometer, and the hygrometer. Keep them in an open wooden box 1 metre above the ground. Place them in the box so that one closed side forms a roof and another a floor for your house. Fit the open side and the two ends with louvres to provide free access of air and to protect the instruments.
Measure relative humidity
37.8.1 Wet and dry bulb hygrometer
See diagram 37.8.1: Wet and dry bulb thermometers
Use two thermometers that have the same reading under similar conditions. Sew together a strip of muslin to make a snug fitting "sock" over the bulb of one thermometer. You can purchase wicks ready for immediate attachment. Mount a small narrow mouthed bottle to the board so that the top of the bottle is at the same level or slightly lower than the top of the bulb. Keep the bottle filled with water. Before taking a reading, fan the air across the wet bulb for a minute or two. Read the wet bulb and dry bulb thermometers, calculate the difference and read the relative humidity from the relative humidity table.
37.8.2 Sling psychrometer
If a sling psychrometer is not available, the instrument shown in 37.37.1 may be converted into one by boring a hole in the top of the board, adding a strong rope and removing the reservoir of water. When it is spun around in the air, maximum evaporation occurs and more accurate readings are possible. Mount the thermometers securely before swinging the instruments. Instruct students on how to swing the device safely to avoid striking their bodies or a desk. Read the wet bulb and dry bulb thermometers, calculate the difference and read the relative humidity from the relative humidity table below.
37.8.3 Dew point hygrometer
1. The dew point temperature is the temperature at which the moisture in the air begins to condense. Use a shiny drink-can containing some water. The drink-can must not have fingerprints on it. Put the drink-can on a page of printing so that the printing is clearly reflected from the can. Do not breathe on the drink-can. Slowly add ice to the water while stirring with a thermometer. Keep scraping the surface of the drink-can with a piece of newspaper or cotton wool. Note the temperature when dew forms and the print are no longer clearly visible.
2. The dew point hygrometer consists of a brightly polished metal cup and an accurate thermometer suspended in the water partially filling the cup. Hold the thermometer in the cup by inserting it in a pencil clip attached inside the cup. Put a cube of ice in the water and stir. Continue stirring until the first evidence of dew appears outside the cup. Read the temperature of the chilled water, the dew point temperature and the atmospheric temperature. Use the following Relative Humidity Table to find the relative humidity.
3. Measure the dew point temperature with a shiny can containing some water, a thermometer and some ice. The dew point temperature is an important weather observation. It is the temperature at which the moisture in the air begins to condense. The dew point temperature changes from day to day. Be sure that the outside of the can is clean and free from fingerprints. Stand the can on a page of printing so that the printing is clearly reflected from the can. Now add ice, a little at a time, to the water and carefully stir with the thermometer. Keep a close watch of the temperature and read the thermometer at the temperature where dew begins to form on the outside of the can, that is, when the print is no longer clearly visible. This will be near the dew point temperature.
4. Temperature that make unsaturated steam becomes saturated steam is called dew point. Dew point is also an expression of air humidity. Predicting the dew point has direct significance for agriculture, if the dew point is below 0oC, the water steam in the air can condense into frost which is harmful to crops. To observe the dew point of air at temperature indoor, use a beaker that is clean and dry outside. Fill a beaker to 2 / 3 with water with temperature slightly higher than the indoor temperature. Insert a thermometer into the beaker and gradually add some pieces of ice while stirring slowly. To avoid your breath affecting the apparatus, put a piece of plastic between beaker and your month or nose. Concentrate your thought on observing the readings of water temperature. If after all pieces of ice melt still no dew appears, add more ice again and stir gently. As the temperature decreases until "thin frost" appears first, record the temperature t at the moment. In order to take advantages of confirming the observation, you can rub the surface of beaker with a cotton stick. Record the dew point of air in the room.
37.8.4 Relative humidity table
% Dry bulb = temperature of dry bulb oC
Wet bulb = depression of wet bulb -oC
Example: If dry bulb reads 30oC, and wet bulb reads 28oC, depression = (30oC - 28oC) = -2oC, so relative humidity = 86%
Depression of the wet bulb, oC
Dry Bulb -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -12 -14 -16 -18 -20
50oC 94 89 84 79 74 70 65 61 57 53 46 40 33 28 22
45oC 94 88 83 78 73 68 63 59 55 51 42 35 28 22 16
40oC 93 88 82 77 71 65 61 56 52 47 38 31 23 16 10
35oC 93 87 80 75 68 62 57 52 47 42 33 24 16 8 -
30oC 92 86 78 72 65 59 53 47 41 36 26 16 8 -
 -
25oC 91 84 76 69 61 54 47 41 35 29 17 6 -
 -
 -
20oC 90 81 73 64 56 47 40 32 26 18 5 -
 -
 -
 -
15oC 89 79 68 59 49 39 30 21 12 4 -
 -
 -
 -
 -
10oC 87 75 62 51 38 27 17 5 -
 -
 -
 -
 -
 -
 -
Winds and weather
37.9 Air expands when heated
1. Fit a flask with a one-hole stopper with a 30 cm length of glass tubing through it. Support the bottle with the end of the glass tubing dipping into water. Heat the flask and observe the bubbles of air through the water. When most of the air has been removed from the flask, cool the flask under tap water. Water rises up the glass tubing as the air in the flask cools and contracts.
2. Fit a toy balloon over the neck of a small flask and put the flask in a pan of warm water. The balloon expands because the air in the flask becomes heated and expands, increasing the air pressure inside the balloon.
37.10 Air has mass
Inflate a balloon with a bicycle pump. Put the balloon on a platform balance and note its mass. Remove the balloon but leave the weights on the other platform. Deflate the balloon and replace it on the balance pan. Disregarding the effect of buoyancy, the balloon has less mass.
37.11 Air exerts pressure
See diagram 37.11: Glass tube drinking straws
Players of golf and baseball may complain about "heavy air" during damp weather but the weight of a volume of air is actually less during bad weather caused by low pressure.
1. Prepare a flask with a two-holes stopper fitted with a bent glass tube and a glass tube bent at right angles. Add water to the flask and adjust the tubes so that the end of the bent glass tube is under water and the end of the glass tube bent at right angles is above water. Close the end of the glass tube bent at right angles with a finger. Sucking liquid up through the other tube is difficult. Remove the finger and sucking is easy.
2. Blow through the glass tube bent at right angles. Increased atmospheric pressure on the surface of the water causes water to rise up the bent tube.
37.12 Cold air is heavier than warm air, inverted paper bag balance
See diagram 37.12: Balanced flasks
1. Open two same size paper bags. Attach identical pieces of string to the bottom of each bag with an identical pieces of adhesive tape. Make a loop in the other end of each piece of string. Put the loops over each end of a balanced rod. Adjust the positions of the loops until the rod is horizontal. Heat the air below one paper bag. The end of the rod supporting that paper bag rises. Leave the balance to stand without heating a bag. The rod becomes horizontal again. Heat the air below the other bag. The other end of the rod rises. This experiment shows that a volume of warm air weighs slightly less than a volume of cool air. However, the experiment does not give any information about the weight of a volume of air. The flame under the paper bag heats the air in it and it expands, following Charles' law. Some heated air spills out of the paper bag leaving less air and less dense air in the paper bag. The air in the heated paper bag weighs less than the air it displaces so by Archimedes' principle there is an upthrust greater than its weight that causes the paper bag to rise. When you remove the flame, the warm air in the paper bag cools and contracts drawing in air at atmospheric pressure. The weight of a paper bag full of air and the bag crunched together, with all the air squeezed out, is the same. Air in a hot air balloon is heated, it expands and becomes lighter and the balloon is pushed up because the air left in the balloon is less dense than the surrounding atmospheric air.
37.13 Make a convection box
See diagram 37.13: Convection box
Use an open box and cut a pane of glass so that it just covers the opening of the box to make a window. Cut two holes in the roof of the box. Place two lamp chimneys or plastic tubes over the holes. Place a short piece of candle on the floor of the box under one chimney. Light the candle. This represents a land area that the sun has heated. Close the window. Trace the air current in each chimney with a smoking piece of piece of paper. Observe the movement of smoke inside the box. Move the candle so that it is under the other chimney and repeat the experiment. The smoke moves because of convection currents.
37.14 Trace convection currents
See diagram 37.14: Convection currents
1. Shield a burning candle to protect it from stray air currents. Trace the air currents about it with smouldering paper. Open a door a little way between a warm and a cool room. With a piece of smouldering paper explore the air currents about the opening at various levels above the floor.
2. Explore the air currents in a room heated with a radiator or a stove.
3. Explore the air currents in a room ventilated with windows open at the top and the bottom.
37. Use an attached wire to lower a lighted candle into a milk bottle. Note what happens to the candle. Ventilate the bottle with fresh air. Again place the lighted candle in the bottle but this time separate the warm and cold air currents with a piece of cardboard cut in the shape of the letter "T ". Use smoke from burning newspaper to see the air currents each side of the piece of cardboard.
5. Cut out the top of a drink-can to make a metal disc. Punch a depression in the centre. Cut along radial lines almost to the centre to make blades. Twist all the blades in the same direction to make a wheel. Mount the wheel on a pointed wire and hold it over a lighted candle to make the wheel turn.
6. Make a more sensitive wheel from the metal foil top of a milk bottle. Place the top on a piece of absorbent paper with the flat side down. Press the point of a ball pen into the middle to make a dent. Cut "petals " in the turned-up edge to form the vanes of a turbine. Pivot it on a pointed wire or a needle stuck into a cork.
How moisture gets into the air
37.15 Atmospheric moisture
You cannot see atmospheric moisture but you can prove its presence. Put some water in a kettle over a fire. Alternatively, use a flask with a one-hole stopper fitted with a right angle bend of glass tubing, put water in the flask and heat the flask. When the water is boiling, observe the cloud formed. This is not steam, but condensed water. Observe the space right next to the spout or opening of the tubing. You cannot see anything. Hold a candle or a Bunsen burner in the cloud of condensed steam. The candle flame goes out.
37.16 Weigh water "lost" by evaporation
See diagram 37.16: Wet towel
Wet a bath towel with water and wring it out. Hang it on a coat hanger. Hang the coat hanger on one end of a long stick balanced on a triangular file lying on a corner of a table. An hour later, hang weights on the hanger until the balance is restored and note the weight of water evaporated.
37.17 Moisture evaporates from soil
Weigh a flowerpot plastic container filled with wet soil. Put it in a shady place then weigh it again every 2 hours until the weight no longer changes.
37.18 Moisture comes from plants
Fix a transparent plastic bag over the leaves of a plant or a seedling in a flowerpot. Tie around the mouth of the bag with string so that no moisture can enter or leave the plastic bag. Later, observe the water drops inside the plastic bag.
37.19 Moisture comes from other plants
Plant some bean or pea seedlings in a flower pot and let them grow until they are 10 or 15 cm in height. Cover the top of the pot with plastic. Attach it closely around the stems of the plants so that no soil is left uncovered. Invert a clean, dry glass jar over the plants and observe after an hour. Note the moisture appearing in the jar.
37.20 Moisture from breathing
Show that moisture coming from breathing by blowing on a cool mirror or into a cool glass or bottle.
37.21 Surface area affects evaporation
Half fill a large flat dish with water, e.g. a baking pan. Put the same volume of water in a tall can with a smaller diameter than the dish. Place the two containers side by side. The next day, measure the volume of water in each container. The difference in the volume of water evaporated is related to the surface areas of the water in the two containers.
37.22 Temperature affects the rate of evaporation
Put water drops of equal size on a hot and a cool surface. Note the difference in change of the size of the drops of water.
37.23 Moving air affects the rate of evaporation
With a moist sponge or cloth, wet two areas of equal size some distance apart on a cool blackboard surface. Fan one area with a piece of cardboard and leave the other to evaporate without fanning.
37.24 Moisture in the air affects the rate of evaporation
Half fill two identical dishes with water. Suspend a wet cloth over one dish. Note the different rates of evaporation in the two dishes.
How moisture comes out of the air
37.25 Moisture condenses on cool surfaces
Put salt and ice in a shiny metal drink-can. Add alternate layers of ice then salt so that the volume of salt is twice the volume of ice. Stir the salt and ice to mix them evenly. Observe the outer surface of the drink-can. Water drops appear then freeze. Later a thin white frost forms. The moisture comes from the water vapour in the air.
37.26 Study the water cycle
See diagram 37.26: Condensation on a drinking glass
Heat some water until it is near the boiling point. Place it in a drinking glass and rotate the glass to moisten the sides right to the top. Put some very cold water in a round flask and place the flask on the glass at an angle as shown in the figure. Water will evaporate from the hot water, condense on the cool surface of the flask and fall back in droplets into the glass. Here you have evaporation, condensation and precipitation. You have seen the water cycle as it is in nature.
37.27 Make a rain cycle
See diagram 37.27: Miniature rain cycle
1. Put pieces of cracked ice in a metal tray. Boil water in a flask with a one-hole stopper with a right angle piece of glass tubing inserted through it. Direct steam to beneath the metal tray. The moisture condenses on the tray and drips back as "rain". The cool tray represents the upper layers of air above the Earth, cooled by expansion.
2. Use dry ice (frozen carbon dioxide gas) to show rain circulation at a small way in a classroom. Place a box at which a small seeding grows on a tabletop. Hang a large metallic dish 35-40 cm above the box. On the dish place some small dry ice or ice taken off a refrigerator just now. Near the box place a flash with a bend tube (or a teapot), its end close to the space between the box and the dish. Use an alcohol burner to heat the flash with hot water to produce enough steam into the space between he box and the dish. Observe that steam condenses into water at the bottom of the metallic dish and the water falls into the box the small seeding grows. Here the flash is regarded as water source on the Earth surface and the metallic dish as cold air at high altitude. Water on the Earth surface vaporizes into steam to enter air and it condenses into rain due to cold at high altitude to fall down on the ground.
37.29 Make cloud in a bottle
See diagram 37.29: Cloud in a bottle
1. Add a small amount of water to a plastic drink bottle. Light a match, blow it out and drop the smoking match stick into the bottle. Immediately screw on the lid and start squeezing the bottle. The cloud in the bottle from the burning match stick smoke disappears when you squeeze the bottle. Inside the bottle water vapour condenses on the smoke particles that act as nucleation sites. When you squeeze the bottle the temperature of the air inside it increases and the condensed water vapour on the smoke particles evaporates.
2. Use a big bottle with a one-hole stopper fixed with a 10 cm length of glass tubing. Put 3 cm of warm water in the bottle and drop chalk dust into the air inside the bottle. Connect the end of the glass tubing to a bicycle pump with rubber tubing. Hold the stopper tight in the bottle and pump air in. When the air becomes compressed inside the bottle, close the rubber tubing with a strong clamp and remove the bicycle pump. Keep a tight hold on the stopper and open the clamp to let air blow out through the glass tubing. A cloud forms in the bottle. When the air comes out of the bottle, the air left in the bottle expands and cools, reducing the temperature in the bottle below the dew point. The moisture then condenses and forms a cloud. Similarly, when warm air rises above the Earth the air pressure is reduced. The air expands and cools, and clouds form when the cooling goes below the dew point.
3. Make a better cloud by using smoke instead of chalk dust or by dropping a lighted match into the bottle. The smoke or lighted match provides small particles of the carbon that float in the air and act as condensation cores as steam sticks to them and condenses to water.
4. Pour several mL water in a bottle, cover the bottle tightly with a rubber stopper on which a hole is punched. Insert a short glass tube into the stopper, then connect the pump used to pump the bike with the glass tube on the stopper by a piece of rubber tube and short glass tube. The mouth in the pump should be removed first. Pump air to the bottle and let the students observe the phenomenon in the bottle, especially at the moment that the stopper jumps up suddenly. If it is not easy to observe clearly, you may throw a lighted piece of match in the bottle, then a little fog appears in the bottle. Repeat the step, much fog will appear.
5. When the stopper jumps up suddenly, the volume of the air in the bottle increase suddenly leading to reduce the pressure in the bottle, and the temperature decreases with it. This results the evaporation of water and cool condensation later. The reason that the burning match can make the process of condensation faster is due to the burning match provided the small particles of the carbon. It is these small solid particles floating in the air that acted as a condensation core to cause the water steam sticks to them and condenses. The experiment shows why it is easy to produce fog and rain in the areas contaminated. For example, in those cities of heavy industry there are more fog days than the non-industrial cities.
37.30 Study snowflakes
See diagram 37.30: A snowflake
Collect snowflakes on a piece of dark wool cloth and examine them with a magnifying glass. Note the many different six-sided shapes.
Weather projects
37.31 Keep a weather record
See diagram 37.31: Wind direction scale
Record the date, hour, temperature, sky and wind in a table. Take readings at the same time each day, e.g. 9.00 am and 3.00 p.m. Make entries in a notebook under the headings:
Date Time Temperature Sky Wind Rain
Rain drops are > 0.5 mm in diameter. Supercooled rain, called sleet, freezes on objects .Drizzle drops are < 0.5 mm in diameter. A falling raindrop seen from the side is indented in the middle from below, not tear-shaped.
Draw graphs of temperature / time, rainfall / time, change in appearance of the sky over a period of time and changes in wind intensity. Record the velocity of the wind as follows:
Light wind moves smoke, but not wind vanes
Moderate wind raises dust and just moves twigs
Strong wind large branches move
High wind blows dust, papers. moves whole trees.
Gale breaks off twigs from trees.
Show the direction of the wind by an arrow in the wind column of your records or construct a paper star and draw a line each day along the arm that most nearly coincides with the direction of the wind.
37.31.1 Make a weather picture
Immerse a piece of white absorbent paper in a solution containing two parts cobalt chloride to one part common salt. While wet, the paper will remain pink, but when dried in the sun or near a Bunsen burner it turns blue. This is the basis of the weather pictures. A home made picture can be made in the following way. Obtain a picture in which there is some sky or water and make an inset of the prepared absorbent paper to replace the sea or the sky. The picture should then be mounted on a card and hung near a window where it will quickly respond to changes in the hygrometric state of the atmosphere.
37.32 Miniature weather fronts
See diagram 37.32: Weather fronts in a fish tank
1. Fit a plastic or glass partition in a plastic box or fish tank. Put warm water in one compartment and cool water in the other compartment. Add red ink to the warm water and blue ink and salt to the cool water. Remove the partition. The blue cool water sinks to the bottom and the red warm air water rises to the top, without much mixing. Similarly a warm air mass will remain over a cool air mass.
2. Reinsert the partition. Stir together the warm and cool water on one side of the partition to form an intermediate mass. Remove the partition. The intermediate mass forces its way between the warm and cold layers to form three distinct layers, similar to an occluded weather front.
37.33 Measure the upper winds with a simple astrolabe, simple sextant
See diagram 37.33: Wind direction astrolabe
1. Attach a protractor to the side of a wooden ruler. Hang a weight from the centre of the protractor to serve as a plumb bob. Attach a drinking straw to the ruler to aid in sighting. When the plumb line indicates 90o on the protractor, the transit is horizontal. When the plumb line indicates 80o, the transit is inclined 10o. Release a balloon filled with lighter than air gas. Keep the balloon in sight through the drinking straw and read the angle of the plumb bob every 30 seconds.
2. Tack a protractor to the side of a wooden stick, keeping the straight edge of the protractor parallel to the top of the stick. Hang the weight on a piece of thread from the centre of the protractor to serve as a plumb bob. A soda straw taped to the top of the stick will improve the sighting. When the plumb bob indicates 90o on the protractor, the transit is horizontal. When the plumb bob indicates 80o, the transit is inclined 10o. An indicated angle on the protractor must be subtracted from 90o to find the inclination of the transit. Stand at a measured distance, 3 to 5 metres, from the wall of the classroom. From that point find the angle that your line-of-sight to the top of the wall makes with the horizontal. Do this by finding how many degrees above the horizontal the soda straw had to be elevated in order to sight the top of the wall. On graph paper, measure off horizontally the number of units equivalent to the distance from the wall. At the end of this horizontal distance, copy the angle of elevation indicated by the transit. The scale drawing of the distance from the wall, and the angle of elevation, will indicate the height of the ceiling above your eye level. For example, if standing 7 metres from the wall and the top of the wall is 30o above the horizontal, a protractor reading of 60o, the ceiling is nearly 3.5 metres above eye level. To find the height of the room, add the eye height to the 3.5 metres.
Tie a long thread to a gas filled balloon so it can be pulled down if released in the high ceiling room. Pull the balloon to the floor, release it and measure the time it takes to strike the ceiling. Divide the ceiling height by the time of rise of the balloon to find its rate of ascent. Take the balloon out to measure the upper winds. Assign the following tasks:
2.1 Keep the balloon in sight through the soda straw
2.2 Read the angle of the plumb bob every 30 seconds
2.3 Keep time by calling off each 30 seconds to the angle reader
2.4 Record the elapsed time and the angle of sight at the end of each time.
2.5 When the data for a few minutes sighting has been recorded, plot the position of the balloon at the end of each 30 second interval. When the position of the balloon's at the end of each time interval has been plotted, its horizontal movement can be measured using the same scale for both vertical and horizontal distance.
37.34 Avoid solar ultraviolet radiation
The risk of developing non-melanoma skin cancer is related to the cumulative ultraviolet radiation (UV) exposure. The risk of melanoma skin cancer increases with the number of sunburns, specially during childhood. The UV waveband consists of UVA (320 to 400 nm), UVB (280 to 320 nm) and UVC (200 to 280 nm) wavelengths. No UVC reaches the Earth's surface because of absorption by oxygen and ozone in the atmosphere. The total solar UV radiation at the Earth's surface consists of a direct component and a diffuse component. The direct component comes directly from the sun. The diffuse component is the radiation scattered by the atmosphere, clouds and the surroundings. The scattering is more significant at the shorter UV wavelengths. The reflected UV radiation is the UV reflected from any surface, e.g. the ground surface. The shorter wavelengths are the most damaging and produce the greatest erythema, redness of the skin, due to dilation of the capillaries (sunburn), i.e. the UVB wavelengths. The total and diffuse UV radiation varies with the cloud cover. On the overcast days the diffuse irradiance forms a high relative proportion of the total UV irradiance. The relative proportions of the diffuse and total UV irradiance also vary with the seasons because of the change in solar zenith angle resulting in a different atmospheric optical path length. So the amount of scattering and absorption in the atmosphere varies. A UV meter can be designed to have a response that approximates the response of human skin to UV radiation and measures the erythemal UV irradiance or the UV irradiance weighted with the response of human skin. Protect yourself from the UV of the sun with long sleeves and a shady hat. Avoid direct exposure between 10 a.m. to 3 p.m. Use sunblocks with a sun protection factor (SPF) of 15 or more.
37.34.1 Measure solar ultraviolet radiation
See also 3.50: Ozone | See also 7.16: Albedo
After Alflio Parisi and Michael Kimlin, Australian Science Teachers' Journal, 44 (3)
The risk of developing non-melanoma skin cancer is related to the cumulative ultraviolet radiation (UV) exposure. The risk of melanoma increases with the number of sunburns, specially during childhood. The UV waveband consists of UVA (320 to 400 nm), UVB (280 to 320 nm) and UVC (200 to 280 nm) wavelengths. (Note: nm = nanometre = 10 Angstrom units = 10-9 m.) No UVC reaches the Earth's surface because of absorption by oxygen and ozone in the atmosphere. The total solar UV radiation at the Earth's surface consists of a direct component and a diffuse component. The direct component comes in a direct path from the sun. The diffuse component is the radiation scattered by the atmosphere, clouds and the surroundings. The scattering is more significant at the shorter UV wavelengths. The reflected UV radiation is the UV reflected from any surface, e.g. the ground surface. The shorter wavelengths are the most damaging and produce the greatest erythema, redness of the skin due to dilation of the capillaries (sunburn), i.e. the UVB wavelengths. Take two sets of readings per day, at 10.30 am and 1.00 pm over a period of two weeks. Measure the total solar UV irradiance (UVTotal) with the meter pointing upwards. Measure the diffuse UV reflectance with the detector facing two ground surfaces, e.g. dead grass and asphalt. Cover the detector so that it is in shadow. Calculate the percentage reflectance (albedo) of the surface (R%), e.g. 3.3% for dead grass and 3.8% for asphalt: R% = (UVReflected / UVTotal) X 100. Albedo is the reflecting power of a non-luminous body. Plot the total and diffuse reflectance for two readings each day on a bar graph. Note the cloud cover at each measuring time. The total and diffuse UV radiation varies with the cloud cover. On the overcast days the diffuse irradiance forms a high relative proportion of the total UV irradiance. The relative proportions of the diffuse and total UV irradiance also vary with the seasons due to the change in solar zenith angle resulting in a different atmospheric optical path length. So there is a change in the amount of scattering and absorption in the atmosphere. A UV meter can be designed to have a response that approximates the response of human skin to UV radiation measures the erythemal UV irradiance or the UV irradiance weighted with the response of human skin.
37.35 Measure the dust in the air
See diagram 37.35: Dust collector
1. Use three wide 5 litre wide-mouthed glass jars or plastic containers. Partly fill them with demineralized water, fix wire screens and leave them for 30 days in different places. Heat the water until it all evaporates without burning the dust. Weigh the dust on an accurate balance. Calculate the weight of dust fall per square metre per unit time.
2. To measure the amount of dust that falls in your neighbourhood, you will need at least three wide mouthed glass jars, 5 litre size. You will also need about 10 litres of deionized water. Ordinary tap water may contain tiny particles that would affect your measurements of dust fall. You will also need a 2 litre pan or other container that can be heated without breaking. Last but not least, you will need to use a balance that weighs things to the nearest centigram or milligram. Make sure that the jars are clean, then rinse them out with some of the deionized water. Pour 1.5 litres of deionized water into each jar. Mark the water level with fingernail polish, a file mark, or anything else that rain will not wash away. Cover the top with a wire screen to keep insects out. Put each of the jars in a different location outdoors. They should be about 1.5 metres above the ground, and not under trees or eaves of buildings. Leave the jars in their places for 30 days. Visit them every few days and add deionized water to the original water level. If the jar dries out, the wind may blow away the dust. Rain may fall into the jar, but this causes no trouble unless the jar overflows: if this happens, the experiment will have to be repeated. After 30 days, bring the jars indoors. To find out how much dust is in each jar, first weigh the 2 litre pan on the balance and write down the result, then pour the water from the jar into the pan. Use more deionized water to rinse out the jar, making sure all the dust particles have been removed. Then, heat the water until it all evaporates. Do not overheat the pan or the dust will be burnt. Let the pan cool and then weigh it on the balance. This gives the weight of the dust and the pan. The weight of the dust alone can be found by subtracting the weight of the pan, which you have already noted. If the balance used only weighs in centigrams, multiply the weight of the dust by 10 to change it to milligrams. Your figures tell you only how many milligrams of dust fell through the mouth of the jar in a month. If you want to know how many metric tons per square kilometre this is equal to, first find the area of the mouth of the jar in square centimetres. Simple division will give the fall in milligrams per square centimetre. If this answer is multiplied by 10 you will have the number of metric tons per square kilometre. If the fall is required in tons per square mile the answer can be found quickly by multiplying the number of milligrams per square centimetre by 25.5. Do you get the same sort of figure for each of the jars you set out? If the numbers vary a lot, take an average of them to get a more accurate idea of the dust fall in your area. Can you think of reasons why one jar would catch more dust than another? Repeat your investigations in another month, or next year, to see if the amount of dust in the air varies.
37.36 Thunderstorm experiment
See also 31.7.4: Lightning conductor, lightning stroke
Prepare a sketch map of the local area within a radius of 15 kilometres. Draw concentric circles on the map showing places that are 1, 2, 3 15 kilometres away from the observer.
When a thunderstorm occurs, locate lightning strokes by:
1. the direction by visual observation,
2. the distance by dividing the interval between flash and sound of the thunder in seconds by three, to give the approximate distance in kilometres.
When the storm has passed, examine this record of the thunderstorm.
37.37.0 Clouds and weather
1. Clouds are visible evidence of moisture in the air, the more clouds, the more moisture. The moisture may be in the form of liquid water droplets or ice crystals, or both. The type of cloud is an indicator of the stability of the atmosphere in which it forms. Layered clouds, stratiform indicate generally stable conditions which change rather slowly. Clouds with vertical development, cumuli form indicate a degree of instability in the atmosphere which produces rapid changes in the clouds. Because of these indications, weather forecasters find it most helpful to have accurate descriptions of the clouds at each weather observing site. Since clouds are continuously in the process of growth or decay they appear in an infinite variety of forms. However, it is possible to define a limited number of characteristic forms generally observed all over the world into which clouds can be broadly grouped. In addition to the stratiform and cumuliform types, clouds may be grouped according to the average heights of their bases from the ground as low, middle and high, but this is not a precise classification, since the heights of cloud bases will vary with terrain, available average moisture and weather patterns.
The three main types of cloud are cirrus - hair-like, cumulus - puff balls, stratus - layered. The rain cloud is called nimbus.
The three altitude bands are high cloud, middle cloud and low cloud.
2. The high clouds are 1. cirrus, 2. cirrocumulus and 3. cirrostratus. The bases are generally above 5, 500 metres. They are always made up of ice crystals and vary greatly in density. A distinguishing feature of cirriform clouds is the halo they produce around the sun or moon as a result of refraction of the sunlight or moonlight shining through the ice crystals. Lower clouds, altostratus containing water droplets show the solar or lunar corona phenomenon rather than the halo. Another major category of clouds includes "those showing great vertical development ". This category includes all the low cumulus type, except the fair weather cumulus and stratocumulus. The cumulonimbus or "thunderhead " is a special category cloud because it may extend through all levels from the very lowest to the very highest and during its life cycle may actually produce nearly all the other cloud types. The following notes will help you to identify cloud types.
3. The middle clouds are 4. altocumulus, 5. altostratus and 6. nimbostratus. The bases range from 2, 200 to 7, 700 metres. They are made up of water droplets or ice crystals or both usually both, and they show considerable variation in density. An aircraft pilot flying in a dense water droplet cloud may be able to see only a few metres, while in an ice crystal cloud he may be able to see as far as a kilometre.
4. The low clouds 7. stratocumulus, 37. stratus, and 9. cumulus. Low clouds includes fog. Also, 10. Cumulonimbus may continues though the three altitude bands. These are found when masses of air move over the Earth surfaces which are warmer or colder than the air. Uneven heating transferred from the Earth's surface to a cooler air layer of ten causes the formation of cumulus clouds which continue to develop vertically until they become cumulonimbus or thunderheads. The average height of low cloud bases ranges from the Earth's surface up to 2, 200 metres. Low clouds are usually made up entirely of water droplets and are normally quite dense.
5. Clouds are classified by using the following words: cirrus (hair curl), cumulus (heap), nimbus (rain), stratus (layer) alto (mid level). Also, high cloud names begin with "cirr". In the 1896 International Cloud Atlas the highest cloud, cumulonimbus, was called "cloud 9" which nowadays means "very happy".
See diagram 37.37: Altocumulus and cumulus clouds
6. Stand where you can see the whole sky and estimate the amount of the sky covered by cloud. The amount is estimated in the number of eighths of the sky covered. If no blue sky is visible, the amount of cloud is 37. If the sky has no clouds, the amount of cloud is 0. Report intermediate values between 0 and 37. 3. Estimate cloud height. The thinner the cloud the higher it will be within the height range. Estimate the direction of movement of cloud, i.e. the direction from which the clouds are coming to the nearest sixteen points of the compass.
37.37.1 The ten main types of cloud
Low level cloud (surface to 2500 m)
1. Stratus 150 to 600 m, base forms below 500 m
2. Stratocumulus 600 to 1500 m
3. Cumulus 600 to 1500 m
4. Cumulonimbus 600 to 1500 m (Cumulonimbus may extend through low, middle and high levels.)
Middle level cloud (2500 to 6000 m)
5. Nimbostratus 150 to 2500 m (Nimbostratus may extend through more than one level.)
6. Altostratus 2500 to 6000 m
7. Altocumulus 2500 to 6000 m
High level cloud (above 6000 m)
37. Cirrus 6000 to 12 000 m
9. Cirrocumulus 6000 to 12 000 m
10. Cirrostratus 6000 to 12 000 m
37.37.2 Descriptions of the main types of clouds
1. Stratus, the lowest cloud, is usually a grey featureless layer with a uniform base. It is associated with drizzle and occasional snow. It makes people feel sad. The outline of the sun or moon is usually visible through it but it may mask the sun and moon. It is often the result of fog or mist lifting from the surface of the Earth. It occurs when the air is stable. Stratus also forms when moist air is lifted up a frontal surface or up sloping terrain, or by advection such as when warm moist air moves over a colder surface. So it is typically found around coasts or mountains.
1.1 Fog is stratus cloud with a base on the Earth's surface. In mountainous country it is possible to have a single layer of stratus cloud reported as a cloud layer at a valley station and as fog at a mountain observatory. Fog occurs as either water or ice. Visibility in fog is less than 1000 metres. Visibility in mist is 1000 to 2000 metres. Advection fog occurs when air from over a warm sea surface moves over a cold land surface. Radiation fog occurs over land when stationary air at ground level cools quickly on a clear night with no clouds. The fog may accumulate in the denser air in valleys, valley fog, or be moved up slopes by the wind, upslope fog. Fog also forms in upslopes and valleys. Freezing fog causes rime, frozen water droplets on solid objects.
2. Stratocumulus clouds have grey to white patches or sheets of cloud with the dark parts in rounded masses or clumps. It may be formed when a temperature inversion stops cumulus clouds from rising so they move sideways and join together. It is associated with very light rain, drizzle or snow. This is low level cloud composed mainly of water droplets or ice particles with bases below 2,000 metres.
3. Cumulus (Latin: heap) clouds are dense lumpy or stacked clouds with sharp outlines. The sunlit parts may be brilliant white but the base is dark and nearly horizontal. Fair weather cumulus is the common "puff ball" clouds of spring and summer skies. They form usually at a uniform height above the ground, grow in size during the hottest part of the day and dissipate towards sundown. They are the most common of cloud types and give an appearance of boiling, so that their shapes are continuously changing. In tropical regions they commonly form rising towers with upper part often resembling a cauliflower. Brief showers of rain or snow may be associated with this large cumulus that may have vertical development up to 12,000 metres. They may form rows or "cloud streets" parallel to the wind direction. Cumulus clouds are formed individually when thermals rising from ground level carry up water vapour that condense at higher levels to form the minute water droplets that constitute cumulus cloud. Similarly, pyrocumulus cloud can form very quickly above forest fires in humid tropical areas.
4. Cumulonimbus is heavy and dense cloud with considerable vertical extent as a mountain or huge tower. The base of the cloud appears dark and stormy looking. Low ragged clouds are frequently observed below the base and generally other varieties of low cloud are joined or are close. The upper portion consists of ice crystals and is usually fibrous or striated due to often appearing as an anvil, incus, or a vast plume as it reaches the tropopause. It is associated with lightning and thunder and heavy showers of rain, snow or hail often in succeeding groups of short thunderstorms called cells. Huge hurricane systems may be associated with "super cells". This cloud can have vertical development up to 12,000 metres.
5. Nimbostratus is a dark grey cloud layer covering the whole sky and thick enough throughout to hide the sun. The base appears diffuse due to continually falling moderate to heavy rain or snow. It is the thickest of the layer clouds.
6. Altostratus is grey to blue sheet of fibrous or uniform appearance totally or partly covering the sky, having parts thin enough to reveal the sun as if through ground glass. Precipitation as rain or snow can occur. The sun or moon shining through altostratus may show the corona effect that distinguishes altostratus from cirrostratus. Under altostratus cloud you usually cannot see your shadow on the ground. Altostratus cloud may thicken and darken downwards to form nimbostratus.
7. Altocumulus is a layer or patch of flattened globular masses, white or grey in colour. The elements are arranged in groups, or in lines, or in waves, that may be joined to form a continuous layer or in broken patches. Coloured rings around the sun are a characteristic of this cloud. It includes orographic clouds, stationary clouds formed when moist air is forced over a mountain, e.g. the lens-shaped lenticularis cloud, banner clouds seen behind mountains and cloud caps on mountains. Altocumulus castellanus is like small towering cumulus clouds with scalloped tops and my indicate imminent thunderstorm activity.
37. Cirrus clouds, the highest and fastest clouds, are separate clouds as white delicate filaments or white or mostly white patches m narrow bands. They are composed entirely of falling ice crystals in wavy fallstreaks that later evaporate so no precipitation reaches the ground. They may appear as downwards hooks called mare's tails. Cirrus clouds may be carried along in high altitude jet streams.
9. Cirrocumulus is thin, white patches or layers of regularly spaced cloudlets. It is composed of very small elements as grains or ripples, joined or separate called mackerel sky.
10. Cirrostratus is a transparent white veil of fibrous or smooth appearance, totally or partly covering the sky. Cirrostratus are ice crystal clouds at high levels in a sheet or layer that may vary in density from so thin that you have to look very carefully to see them to clouds dense enough to hide the sun. It produces the halo phenomenon, a luminous white ring around the sun or moon with a faint red fringe on the inside. Usually this cloud form signals the approach of a storm frontal system. Under cirrrostratus cloud you can see your shadow on the ground. A bright patch of cloud called a sundog or parhelia is caused by diffraction through ice crystals shaped like hexagonal plates.
37.37.3 Contrails
Contrails are artificial clouds formed when moist gases from the exhaust of an airplane cool rapidly to form ice crystals. They may soon disappear as the cryutals evaporate or persist across the sky in the warm humid conditions in advance of a warm front. Many contrails may spread to form a layer of cirrostratus cloud.
37.37.4 Cloud seeding
Cloud seeding is attempts to add artificial nuclei into cloud to increase precipitation, reduce hailstorms and dissipate fogs near airports. The "seeds" may be ice crystals, salt particles and silver iodide crystals. However, it is difficult to prove that cloud seeding works because no control is possible, i.e. another unseeded cloud for comparison with exactly the same characteristics as the seeded cloud. Also, it is hard to judge the correct concentration of seeding. A "under seeded" cloud is a waste of resources because no rain falls. An "over seeded" cloud may result in less precipitation because too many resulting nuclei may form to make many small droplets that do not precipitate. However, the use of dry ice (frozen carbon dioxide) dispersion around airports to make cold fog precipitate as ice crystals has been reported as successful.
37.38.0 Observe and describe warm and cold fronts
37.38.1 Warm front
Warm fronts are preceded by a slowly falling barometer. Cirrus clouds will be observed and precipitation can usually be expected in 24 to 36 hours. The cloud pattern gradually thickens as it progresses from cirrus to cirrostratus, then altocumulus or altostratus, and finally to nimbostratus or cumulonimbus. Precipitation often begins from dense altostratus clouds before being obscured by the lower stratus or cumulus types of clouds. As the front passes the wind changes direction, the barometer rises a little, precipitation ends, the sky begins to clear, and the temperature begins to rise noticeably. In summer, afternoon thundershowers may develop behind a warm front.
37.38.2 Cold front
When a cold front approaches the barometer falls rapidly. Cold fronts move faster than warm fronts, having an average speed of 32 to 40 km per hour, although they sometimes move at less than 16 km per hour and occasionally at more than 56 km per hour. The procession of cloud types will be proportionately faster than those associated with a warm front. The transition from cirrus to cirrostratus and then to altostratus or altocumulus of ten takes place within a period of a few hours. Precipitation may start from 12 to 30 hours after the cirrus is first seen. In the summer, cumulus clouds will build into cumulonimbus and produce thundershowers. In winter, nimbostratus or stratocumulus will bring rain or snow. When the front passes the wind will shift abruptly, the barometer will rise steadily, and the temperature will fall. If the front is moving rapidly, clearing will begin quickly, but if the front is comparatively slow moving, cloudiness and some precipitation may last for several hours.
- Altitude Altitude Altitude
- Polar climate Temperate climate Tropical climate
Upper level 3 - 8 km 5 - 13 km 6 - 18 km
Middle level 2 - 4 km 2 - 7 km 2 - 8 km
Lower level Ground level to 2 km Ground level to 2 km Ground level to 2 km
37.38.3 Tornadoes
Tornadoes are created by the same atmospheric conditions that cause hail and thunderstorms, a collision of warm moist air and cold dry air masses. If different wind speeds occur at different altitudes, a horizontal spinning column of air, a vortex, may form. If the vortex collides with a violent updraft it may be knocked into a vertical position which on reaching the ground causes a tornado. The tornado is usually cone-shaped because the higher pressure near the ground squeezes the bottom of the vortex. At very low pressure, water vapour in the air condenses to produce a funnel-shaped cloud. Tornadoes cannot be predicted, but the air conditions which breed them are not completely understood. Covering an area from 70 to 330 metres in width, a tornado usually travels with an average speed of 32 to 63 km per hour, though the wind velocity may be 300 km per hour. In the Northern hemisphere, tornadoes most frequently occur between 1 April and 15 July, and generally in the late afternoon. A tornado is possible whenever the air is humid, with temperature above 26oC, and a cold air mass arrives. Mammatocumulus clouds, like smooth globular udders, may extend down from different cloud types and are often seen under cumulonimbus clouds before and after a tornado. The first sign of a developing tornado or waterspout in a tuba, a column of cloud in the centre of a vortex extending down from Cumulonimbus cloud. It may touch the ground.
37.38.4 Hurricanes
The tropical hurricane is the most devastating of storms. Though occurring all over the world, but under different names, all hurricanes originate in the equatorial regions. North of the equator their usual travel direction is north to north-west to north-east. However, south of the equator hurricanes travel in the opposite direction. Hurricane cloud formations are similar to a warm front with the usual sequence of changes as follows: 1. cirrus 2. cirrostratus about 1, 600 km in advance of the hurricane 3. altostratus 4. nimbostratus rain clouds or cumulonimbus. A halo is often seen about the sun or moon. Although a hurricane travels only 12 to 24 km per hour, it is accompanied by winds that may reach 240 km an hour. In its life of about ten days a hurricane covers an area of 800 to 3, 200 square kilometres. When the barometer begins to rise and the winds change direction, the worst of the hurricane is over.
37.39.0 Weather studies
Make weather instruments and a weather station
Study of weather is a topic that is close to the life of every student. Even at the lowest levels of primary instruction, you may make observations of the weather from day to day. At the intermediate levels you may construct a simple weather station. At the level of general science and later, you may study the causes of weather phenomena. At all stages of the work it is an advantage to represent readings and observations in graphical form.
Weather systems usually come from the west, hence the saying: "A red sky in the morning gives a shepherd warning. A red sky at night gives a shepherd delight." Similarly in the Bible, Matthew 16:3 (King James version): "And in the morning, It will be foul weather today: for the sky is red and lowering. O ye hypocrites, ye can discern the face of the sky; but can ye not discern the signs of the times?"
37.39.1 Layers of the atmosphere
See diagram 37.39.1: Atmosphere divided into vertical divisions
The troposphere
The upper limit, the tropopause, varies from a height of 28 km in the tropics and 7 km in polar regions. In this layer most of the components of weather occur including winds, water vapour, clouds, rainfall and lightning. Temperature decreases with height, lapse rate, by about 6.5oC per km.
The stratosphere
The upper limit, the stratopause, is about 50 km, when the temperature ceases to rise.
The absorption of ultraviolet radiation from the sun in the stratosphere causes a rise in temperature resulting in it being a stable layer.
The mesosphere
In the lower part, the isothermal layer, the temperature hardly changes, then temperature decreases with height, down to -95oC, at about 80 km above the earth., the mesopause.
The thermosphere
This layer of rising temperature above the mesopause may extend for 400 to 500 km depending on the activity of the sun. Ultraviolet rays and X-rays from the sun break molecules into atoms and ions. 37.39.5 The exosphere
This level contains a very low density of neutral atoms and molecules, and electrically charged particles. This level merges with the interplanetary region.
The ionosphere (the upper mesosphere and the thermosphere)
This region of concentration of ions, the ionosphere, acts as a reflector of radio waves, sky waves,. Reflection properties change with height, from 50 to 150 km level, and change diurnally and with different solar activity. Television waves are have shorter wavelengths than longer wavelength radio waves and are not reflected by the ionosphere. The wavelengths used by satellites must be shorter than radio waves so that they can penetrate the ionosphere.
37.39.2 Inversion layers
3.50 Ozone, O3
Most of the atmosphere, and nearly all of the water vapour, is contained in the troposphere, a layer up to 20 km deep in the tropics and 8 km deep at the poles. Weather occurs in the troposphere. Between the troposphere and the stratosphere is the tropopause where the temperature no longer decreases with altitude but starts to increase. The temperature, density and pressure of the atmosphere decreases with height. The drop in temperature, the lapse rate, is about 6.5oC per km increase of height. During the day, radiation from the sun heats the ground much faster than it heats the air. The ground then heats the air in direct contact with it and this warm air rises. As the volumes of ground-heated air rise, they expand to match the lower density of the air around them, like a hot air balloon. Expansion of an air volume causes it to cool at about 10oC per km height. However, this rising volume of air may remain warmer than the air surrounding it. So it can continue to rise, causing unstable turbulent conditions when the warmer and cooler air mix.
On dry cloudless nights, the ground cools faster than the air due to radiation of heat out to space. The ground cools the air in contact with it so the temperature of the atmosphere increases with height to produce a temperature inversion that traps pollutants in this lower layer, e.g. smoke from fires and exhaust gases from motor cars. The boundary where the switch of temperature change occurs can be clearly seen from above the inversion layer, like a hill. In the morning, the sun heats the ground. The ground then heats the air in direct contact with it and this warm air rises. Unstable conditions begin, the inversion layer is broken, and the usual cycle starts again.
Use a transparent square tank with an immersion heater at the bottom. Fill the tank half full of cold water. Carefully pour hot water on top to form a separate layer. Shine a strong torch through the water onto a screen. Note the sharp boundary because of the difference in refractive index of the water at different temperatures if no mixing between the two layers occurs. Turn on the heater at the bottom of the tank. Note the warm water rising through the cold layer as a turbulent swirling shadow on the screen. When the rising water reaches the hot water layer, it rises no further and is trapped. In the atmosphere, the inversion layer prevents mixing between the troposphere and higher atmosphere. Water does not move through and no clouds occur beyond the stratosphere, otherwise the Earth would eventually lose all its water. Material that does get into the stratosphere layer stays there for years and can take part in chemical reactions, e.g. reactions with ozone and oxygen atoms. Radiation at around 265 nm is most dangerous to living things, including plants. Ozone prevents radiation below 290 nm from reaching the ground. Ozone also stops great deal of radiation in the 290 nm to 320 nm range. This radiation causes skin cancer. Concentrations of ozone in the stratosphere fluctuate with natural changes in rates of production and destruction. In any one year, the maximum concentrations in the spring can be half as high again as the minimum in the autumn. While the rates of ozone production appear to be out of our control, the compounds added to the atmosphere will affect the destruction. The oxides of nitrogen, both natural and from car pollution, account for perhaps two thirds of the destruction.
37.40 Coriolis force (Coriolis effect) and weather rotations
1. The circumference of the Earth at the equator is larger than near the north pole or south pole and the Earth rotates once every 24 hours, so the surface of the Earth at the equator must move faster at the equator than near the north pole or south pole. A super missile fired from the north pole and aimed at the south pole would to be deflected to the right in the Northern hemisphere and deflected to the left in the Southern hemisphere. The Coriolis force does not only operate on objects travelling in a north south direction. The size of the Coriolis force is independent of the direction in which something is moving. The missile goes similarly off track after being fired in an easterly or westerly direction. Some people regard "Coriolis force" as a fictional force used to account for movement of air and water over the spinning Earth and "Coriolis acceleration" refers to the apparent tendency of a moving body to swing to one side when its motion is defined by rotating axes. Other people do not use the term "Coriolis effect" because it is too vague. They say that in a rotating co-ordinate system there is a Coriolis force that causes a mass to be accelerated. The Coriolis force does no work but that does not disqualify from being called a force.
2. The rotation of the Earth does influence the direction of rotation of large weather systems and large vortices in the oceans. These long-lived phenomena allow the very weak Coriolis force to produce a significant effect, given enough time. The Coriolis force causes the air to rotate around a low pressure centre in a cyclonic direction, i.e. air or water rotates in the same direction as the Earth. The air flowing around a cyclone (hurricane, typhoon) spins anticlockwise in the Northern hemisphere, and clockwise in the Southern hemisphere. If the Earth did not rotate, the air would flow directly in towards the low pressure centre.
3. The Coriolis force, operating on its own causes a moving object to experience a force to the right of its path in the Northern hemisphere and to the left of its path in the Southern hemisphere. In a geophysical flow in the atmosphere or oceans, there is always another force operating, e.g. the pressure gradient force that cause material to start to move. The direction of any rotation depends upon the net force where the Coriolis force and other forces are present. Around a high atmospheric pressure area, the pressure gradient force points radially outward. Around a low atmospheric pressure area, the pressure gradient force points radially inward.
4. If a body of air moves horizontally at constant speed where friction with uneven terrain is negligible, the two horizontal forces on it are the pressure gradient force and the Coriolis force. If the magnitudes of these forces are equal then the Coriolis force does not cause a deflection to left or right. The Coriolis force may be larger or smaller than the pressure gradient force, depending on the wind speed. If the pressure gradient force is greater than the Coriolis force, the flow will be curved around a low pressure area.
In the Northern hemisphere the flow of the gradient wind is anticlockwise around the low pressure areas because the Coriolis force acts to the right.
In the Southern hemisphere the flow of the gradient wind is clockwise around the low pressure areas because the Coriolis force acts to the left.
Study the direction of rotation winds around Highs and Lows from weather charts in the newspaper or on television.
It is unlikely that the construction of wind farms affects the rotation of the earth. The relative forces are not comparable. Some people have suggested that half the wind farms could face east and the other half face west to counteract any effect on the rotation of the earth!
37.41 Plug hole experiments
Some people think that when water goes down a bath plug hole its direction of spin is determined by the Coriolis effect. These people say that if you leave the water still in the bath for a long time then pull out the plug it will spin anticlockwise in USA and spin clockwise in Australia. Other people say that the Coriolis effect is too small to affect the small amount of water in a bath tub and they cannot produce the effect by experiment in a bath. The Coriolis force is very small compared with common rotations, e.g. water down a plug hole, because the rotation of the Earth is only one rotation per day. The direction of rotation of water down a plug hole depends on the way it was filled or by vortices due to washing action. If you want to obtain the rotation down a plug hole that is always cyclonic, use a 1 m smooth pan with a very small hole in the centre and a stopper that can be removed from below. Leave the water undisturbed for weeks before removing the stopper so that the water takes hours to drains through the hole. As a fluid parcel moves towards a wall it will be deflected and turn. It is this rotary motion that is accentuated when the water converges towards the drain. Similarly, some people report that if you put a flat round dish full of still water in a refrigerator, the water heaps up as it freezes to form a roughly north-south ridge because of the Coriolis force.
37.42 Weather maps, Buys Ballots law, geostrophic wind and gradient wind
See diagram 37.149.1: Geostrophic wind flow | See diagram 37.149.2: Gradient wind flow | See diagram 37.149.3: Veering and backing
1. Study laminar and turbulent flow from weather maps. Daily weather maps show large scale fluid dynamics. The usual weather map is a mean sea level synoptic chart. Synoptic means overall view. Lines on a weather map joining places of equal atmospheric pressure are called isobars. Buys Ballot was a Dutch meteorologist who described the relationship between wind direction and pressure as shown by isobars. The Buys Ballots law states that if an observer stands with the back to the wind, in the Northern hemisphere the lower pressure is to the observer's left and in the Southern hemisphere the lower pressure is to the observer's right. If an observer facing south feels a southerly on the face then feels a wind on the right side of the face, the wind has veered from a southerly wind to an easterly wind. If the wind then moves back to a southerly wind, the wind has backed. So the direction of a veering wind moves clockwise, to the right, and the direction of a backing wind moves anticlockwise, to the left.
2. An air mass moving horizontally at constant speed with no friction with the surroundings has two forces on it 1. the pressure gradient force from high pressure to low pressure and 2. the Coriolis force. If 1. and 2. are exactly equal and opposite, the air mass continues moving as a geostrophic flow horizontally in a great circle around the world, i.e. in a straight line on a synoptic weather chart. For any given latitude, at a certain wind speed, called the geostrophic speed 1. = (b). As no Coriolis force exists at the equator, air masses there move in the direction of the pressure gradient force from high pressure to low pressure. Similarly, no geostrophic flow occurs between 15o north and 15o south because the Coriolis force is too weak.
3. If 1. is not equal to (b), the air mass moves to the left or right tangential to the isobar as a gradient wind, i.e. along the curved isobars on a synoptic weather chart. If the pressure gradient force is greater than the Coriolis force, the air mass moves in a curve around a low pressure area, anticlockwise in the Northern hemisphere and clockwise in the Southern hemisphere. This curved motion is called cyclonic flow and is in the direction of the Earth's rotation. Remember that an observer above the north pole would observe the Earth spinning anticlockwise and an observer below the south pole would observe the Earth spinning clockwise.
37. If the pressure gradient force is less than the Coriolis force, then the movements of the air mass is the opposite to the movements as in 3.
5. Measure surface wind at a standard level of 10 m above the Earth's surface where forces of friction with the rough surface of the Earth decrease the geostrophic wind speed and cause the wind to move across the isobars at an angle of about 30o over land and 10o over sea. At about 1 km above the ground the friction force is zero.
6. Horizontal convergence refers to a gain of air mass above a place causing increased atmospheric pressure. Horizontal divergence refers to a loss of air mass above a place causing decreased atmospheric pressure. Low pressure at X causes air to move towards X due to the pressure gradient force, followed by slow upward movement of air. If the upward moving air contains moisture, cloud will form at X1. Low pressure areas are associated with a low, low pressure centre, depression or cyclone and wet weather. A trough is an elongated area of low pressure. High pressure at Y causes air to move away from Y, followed by a slow downward movement of air. So clouds do not form at Y1. High pressure areas are associated with a high or anticyclone and fine weather with light winds. A ridge is an elongated area of high pressure. Horizontal winds, called advection winds, are always much greater than vertical winds, called convection winds.
37.43 Model greenhouse to simulate the greenhouse effect
1. Use a thermometer to read the ambient temperature in the shade. Leave a closed bottle in direct sunlight for some time. Put the bottle in the shade, open it and read the inside temperature with the thermometer.
2. Hold a sheet of glass between your hand and the sun. You can feel the increase in temperature due to the radiant heat passing through the glass. Hold the sheet of glass between a fire and your hand. You cannot feel any temperature change due to the radiant heat from the fire passing through the glass.
3. Line a household bowl with aluminium foil. Put a piece of food, e.g. cheese, on the end of a tooth pick and fix it in the centre of the bowl. Cover the bowl with clear food wrap and leave the bowl in the sun. The bowl acts as a greenhouse and the cheese melts.
37. Use two small identical packets or cardboard boxes. Cut identical square holes in the upper surface of each box. Punch a hole in the side of each box and insert a thermometer through the hole. Find a piece of window glass to cover the square hoe of one box or make a glass roof with microscope slides. The other box has no cover over the square hole in the roof. The initial readings of the two thermometers should be the same. Take the boxes out of the room and put them in the direct sunlight for 20 minutes. Read the thermometers and record the temperature. The box with the glass covering the square hole is a model greenhouse. It absorbs radiant energy though the glass roof. The temperature in the model greenhouse box is greater than the temperature in the other box.
The sun emits light and short wavelength infrared radiation that can pass through gases in the atmosphere and the glass roof of a greenhouse to heat the Earth and the contents of the greenhouse that in turn emit longer wavelength infrared radiation as their temperature rises. Some longer wavelength radiation emitted by the Earth is absorbed by gases in the atmosphere, e.g. carbon dioxide, methane and water vapour, which in turn emit some of this radiation to n be absorbed by the Earth. However, the longer wavelength radiation cannot pass through glass so the contents of the greenhouse get hotter than if outside the greenhouse. If you place a sheet of glass between a red-hot fire and your hand, you cannot feel any heat from this longer length radiation. So the "greenhouse effect" is a natural process accentuated in the last two hundred years by industrial and agricultural development causing increases in the concentration of "greenhouse gases" in the atmosphere. This increase has probably caused "global warming" the steady increase in average temperature now being experienced. Some people say that "greenhouse effect" is a misnomer because the main function of a greenhouse is to stop loss of heat by convection yet allow plants to receive the radiation necessary for photosynthesis. If that is true then the above experiment is not a good simulation of the greenhouse effect.
A plant greenhouse traps insolation. The glass roof and sides transmits most radiation wavelengths except the infra-red and ultraviolet wavelengths. The radiation that passes through the glass is absorbed by the plants which then get warmer and radiate infra-red radiation that cannot pass through the glass. So a greenhouse is a "hot house".
37.44 Navigation data used by a ship at sea
Position: 10.23 UTC (Co-ordinated Universal Time (UTC) replaced Greenwich Mean Time (GMT) as the World standard for time in 1986. It is based on atomic measurements rather than the earth's rotation. Greenwich Mean Time (GMT) is still the standard time zone for the Prime Meridian (Zero Longitude).
20o57.05' S
039o52.82' W
Course: 32o
Speed: 18.8 Kts (knots)
Relative wind: 55 Km \ h
Depth of sea: 47 metres (154 feet)
N | | NE | | E
Ships time: 07.29
Water temperature: 25oC
Air temperature: 29oC
Conditions: Cloudy sky
Air pressure hPa
Beaufort Wind Scale 3 (Beaufort number 0 --> 12) (See Interesting websites "Beaufort scale")
Wind direction: 8 km / h from south
Barometer: 1015 mb, 761 mm Hg, 30.00 inch
Tendency: Slowly increasing
History of experiments in this document
This version of selections from the New UNESCO source book for science teaching, Third impression 1979, ISBN 92-3-101058-1, was edited by Dr J. Elfick, School of Education, University of Queensland, Brisbane, Australia, assisted by Mr R. Smith, Central Queensland University, Australia, working under UNESCO contract 8347201, 2001-12-15.