School Science Lessons
37. Weather experiments, atmospheric pressure, clouds, precipitation,
relative humidity, warm fronts and cold fronts, wind
2012-05-05c SPw
Please send comments to: J.Elfick@uq.edu.au
37. Weather experiments
See: Interesting websites, Part 12, Meteorology, weather, climate
Table of contents
12.3.0 Atmospheric pressure, air pressure
37.37.0 Clouds and weather
2.0.3 Greek alphabet
37.25.0 Moisture comes out of the air
37.15.0 Moisture comes into the air
37.5.0 Precipitation
37.8.0 Relative humidity
37.38.0 Warm fronts and cold fronts
29.0 Weather experiments (Primary)
37.1.0 Weather and climate
37.31.0 Weather projects
37.39.0 Weather science
26.5.02 Wind shear and noise level
37.9.0 Winds and weather
37.9.1 Wind speed
History
37.37.0 Clouds and weather
37.37.0 Clouds and weather
37.37.1 Cloud classification, ten main types of clouds
Cloud types marked * are cumuliform clouds from the lifting and cooling of moist air
by convection.
1.0 Stratus
2.0 Stratocumulus *
3.0 Cumulus *
4.0 Cumulonimbus *
5.0 Nimbostratus
6.0 Altostratus
7.0 Altocumulus *
8.0 Cirrus
9.0 Cirrocumulus *
10.0 Cirrostratus
37.37.4 Cloud seeding, rain making
37.37.3 Contrails (condensation trails)
5.24 Describe clouds (Primary)
37.25.0 Moisture comes out of the air
37.29 Cloud in a bottle
4.41 Ice experiments (Primary)
37.25 Moisture condenses on cool surfaces
37.27 Rain cycle
37.37.5 Raindrops
37.30 Snowflakes
37.26 Water cycle
37.15.0 Moisture comes into the air
37.15 Atmospheric moisture
37.20 Moisture from breathing
37.18 Moisture from plants
37.17 Moisture from soil
37.24 Moisture in the air affects the rate of evaporation
37.23 Rate of evaporation and moisture
37.21 Surface area affects the rate of evaporation
37.22 Temperature affects the rate of evaporation
37.16 Water "lost" by evaporation
37.8.0 Relative humidity
37.5.3 Dew, dew point
24.4.0 Dew point and humidity
8.8.3 Dew point hygrometer
6.17 Relative humidity (Primary)
37.8.4 Relative humidity table, depression of the wet bulb
37.8.2 Sling psychrometer
37.8.1 Wet and dry bulb thermometer (hygrometer, psychrometer)
37.38.0 Warm fronts and cold fronts
37.38.0 Warm fronts and cold fronts
37.38.2 Cold front
37.38.4 Hurricane, tropical cyclone, typhoon
37.38.3 Tornadoes
37.38.1 Warm front
37.1.0 Weather and climate
29.0 Weather lessons (Primary)
Instruments in a weather station
13.7.0 Barometers
37.6.0 Hair hygrometer
37.7.0 Housing box (Stevenson screen) for weather instruments
37.5.0 Precipitation
20.0.6 Standard atmosphere
37.5.0 Precipitation
37.5.0 Precipitation, rain gauge, precipitation gauge
37.5.1 Precipitation, rain, drizzle, snow, hail
37.5.2 Classification of precipitation
37.5.3 Dew, dew point
24.4.0 Dew point and humidity
8.8.3 Dew point hygrometer
37.5.4 Dust storm
37.5.7.1 Fog and rime
37.5.5 Frost
37.5.6 Haze
31.7.4.0 Lightning, sparks
37.5.7 Mist
5.22 Rain gauge (Primary)
31.7.4.1 Saint Elmo's fire
37.5.8 Smoke
26.5.8 Thunder and lightning
37.5.9 Thunderstorm
37.5.10 Tornado or waterspout
37.31.0 Weather projects
37.35 Dust in the air
27.1.80 Rainbows
37.34 Solar ultraviolet radiation and skin cancer
37.36 Thunderstorms
31.7.4.0 Lightning, sparks
37.33 Upper winds
37.32 Weather fronts
37.31.1 Weather pictures
37.31 Weather records
37.39.0 Weather science
37.39.0 Weather science
4.132 Colours of sunlight, rainbow
4.144 Colours of the blue sky and the sunset
4.145 Colours of the sea
3.32.1 Composition of the atmosphere and greenhouse gases
37.43 Greenhouse effect in a model greenhouse, global warning
37.43.1 Global warming and climate change
37.39.2 Inversion layers
23.6.5 Lava lamps, cumulus cloud, convection cells, Hadley cells
37.39.1 Layers of the atmosphere
37.43 Model greenhouse to simulate the greenhouse effect
37.44 Navigation data used by a ship at sea
37.41 Plug hole experiments, [Coriolis effect, (Coriolis force)]
27.1.80 Rainbows
27.1.70 Scattering, Rayleigh scattering, Mie scattering, blue sky and red sun
37.45 Ship's compass
20.0.10 Standard atmosphere
21.3.03 Swell, ocean swell, "State of sea" and "State of swell"
37.40 Trade winds and weather rotations, [Coriolis force, (Coriolis effect)]
37.40.1 Trade winds, easterlies and westerlies
37.42 Weather maps, Buys Ballots law, geostrophic wind and gradient wind
37.46 Weather sayings
37.9.1 Wind speed
Order online: Wind Turbine
37.1.2 Wind speed measurements, m s, knots, Beaufort scale
5.23 Wind speed and direction (Primary)
37.3.0 Wind speed indicator, Deflection anemometer
37.4.0 Wind speed indicator, Pressure tube anemometer
37.2.0 Wind speed indicator, Rotation anemometer
37.1.1 Wind speed, wind vanes
37.9.0 Winds and weather
37.11 Air exerts pressure in all directions
37.9 Air expands when heated
37.10 Air has mass
5.26 Air pressure in all directions (Primary)
6.18 Atmospheric pressure (Primary)
12.1.04 Atmospheric pressure water spray
37.13 Convection box
37.14 Trace convection currents
4.28 Trace convection currents
23.7.8 Water is a poor conductor of heat, boil water in a balloon
37.1.0 Weather and climate, instruments in a weather station
Weather is a description of the condition of the atmosphere at a certain place and
time. It is a description of the main conditions necessary for human comfort and
survival, i.e. temperature, humidity, wind, precipitation and sunshine. Weather data
from many places can be recorded on synoptic charts. Weather recording
instruments include anemometer, barograph (recording atmospheric pressure),
barometer, hygrometer ( a pair of wet and dry bulb thermometers to find relative
humidity), maximum and minimum thermometer, pluvigraph (recording rainfall) rain
gauge, sunshine recorder, thermograph (recording air temperature), thermometer,
and wind vane.
Climate is a summary of average weather data over at least one year. The Köppen
Climate Classification System (Vladimir Köppen. 1900) classified climates as follows:
1. Moist tropical climates, high temperatures and large amounts of rain all the year or
rain from the monsoon
2. Dry climates, little rain and huge daily temperature range. semiarid (steppe) or arid
(desert)
3. Humid middle latitude climates, warm dry summers and cool wet winters.
4. Continental climates, interiors of large land masses, low precipitation, big difference
in seasonal temperatures
5. Cold climates, permanent ice and tundra, four months of the year above freezing
temperatures.
A classification of world climates into three major climate groups show the important
effects of air masses as follows:
1.0 Low latitude climates controlled by equatorial air masses
1.1 Tropical moist climate, rainforest, heavy rainfall in all months, annual precipitation
> 250 cm. (100 in.), high, high temperatures stay the same, high humidity, Amazon
Basin, Congo Basin, East Indies, Papua New Guinea.
1.2 Wet and dry tropical climate, savannah, very hot wet seasons and very dry cooler
seasons, annual precipitation 0.25 cm, 15o to 25o N and S, India, Indochina, West
Africa, South America, north coast of Australia.
1.3 Dry tropical climate, desert, 18o to 28o N and S, light winds, intense dry heat,
low annual precipitation, south-western United States, northern Mexico, north Africa,
central Australia.
2.0 Mid latitude climates affected by tropical air masses are moving towards the poles
and the polar air masses are moving towards the equator
2.1 Dry mid latitude climate, steppe, semiarid grasslands and tall grass prairie, warm
to hot summers but cold winters if mountainous, annual precipitation <10 cm to 50 cm,
latitudes 35o to 55o, N. Western north America, Europe Asian interior (Gobi Desert,
north China).
2.2 Mediterranean climate, cool wet winter and hot dry summer, Eucalyptus forests,
forest fires, annual precipitation 42 cm, latitudes 30o to 50o N and S, southern
California, coastal areas of Mediterranean Sea, coastal Western Australia, Cape
Town region of South Africa.
2.3 Dry mid latitude climate, grasslands, dry, interior of North America, Europe and
Asia, summers are warm too hot, winters cold, annual precipitation 81 cm, latitude
Range 30o to 55o N and S, western North America, Europe and Asia interior.
2.4 Moist continental climate, deciduous forest, polar and tropical air masses so large
seasonal changes and daily temperature changes, annual precipitation 81 cm, latitude
Range: 30o to 55o N and S, eastern United States, southern Canada, north China,
Korea, Japan, central and eastern Europe.
3.0 High latitude climates controlled by polar and arctic air masses, exist only in the
Northern hemisphere
3.1 Boreal forest climate, taiga, long, very cold winters and short cool summers.
Annual precipitation 31 cm small but increases during summer months, latitude 50o to
70o, N and S, Alaska, Canada, northern Europe (Siberia).
3.2 Tundra climate, arctic coastal areas. long winter season and short mild season,
annual precipitation 20 cm, latitudes 60o to 75o, North America (Hudson Bay region),
coastal Greenland, coastal northern Siberia.
3.3 Highland climate, alpine, cool to cold in mountains and high plateaux, seasons and
wet and dry periods as the region, annual precipitation 23 cm, North America (Rocky
Mountains), South America (Andes), Europe (Alps), Africa (Mt. Kilimanjaro),
Himalayas.
37.1.1 Wind speed, wind vanes
1 knot = 1 nautical mile per hour = 0.5144 m / s = 1.852 km / h = 1.125 mph
Anabatic winds move up slopes because the air mass over the ground becomes
heated and cooler air moves in from over the ocean.
At 10 m standard measurement height, wind speed: 1 m / s = 3.6 km / h =
2.237 mph = 1.944 knots
Gales are winds with speed of 34 to 47 knots (USA).
Katabatic winds move down slopes because of the weight of the cold air mass.
Pressure gradients cause wind to move from areas of high pressure towards areas of
low pressure, the direction modified by the Coriolis effect.
Prevailing wind is the most likely wind direction at a place.
Squall is a sudden increase in the wind speed by 16 knots or more, to at least 22
knots, and lasting for at least one minute.
Wind chill refers to the fact that we lose heat faster from the skin by conduction and
convection if the wind is blowing. So frostbite can occur if air is not moving at -70oC,
but at -35oC in a wind blowing at 3 knots.
Wind rose is a representation wind speed and direction as the spokes of a wheel.
Wind shear is a sudden change in wind speed or direction vertically or horizontally,
e.g. updrafts and down drafts in thunderstorms and clear air turbulence that is a
hazard to aircraft.
Wind vanes are metal flags, often decorated with an arrow or a crowing cock, that
turn to show the wind direction, i.e. where the wind is coming from. So a
south-westerly wind is blowing from the south-west towards the north-east and the
wind vane arrow is pointing from south-west to north east.
Windage is the marksman's estimated lateral deflection by the wind when aiming at a
target.
See diagram 37.107: Wind vanes
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.
37.1.2 Wind speed measurements, m s, knots, Beaufort scale
The Beaufort scale was invented in 1806 by admiral Sir Francis Beaufort (1774-1857).
m / s
 Knots
 Beaufort scale
 Wind
0.0 to 0.4
 0.0 to 0.9
 0
 Calm
0.4 to 1.8
 0.9 to 3.5
 1
 Light wind
1.8 to 3.6
 3.5 to 7.0
 2
 Light wind
3.6 to 5.8
 7 to 11
 3
 Light wind
5.8 to 8.5
 11 to 17
 4
 Moderate wind
8.5 to 11
 17 to 22
 5
 Fresh wind
11 to 14
 22 to 28
 6
 Strong wind
14 to 17
 28 to 34
 7
 Strong wind
17 to 21
 34 to 41
 8
 Gale
21 to 25
 41 to 48
 9
 Gale
25 to 29
 48 to 56
 10
 Strong gale
29 to 34
 56 to 65
 11
 Strong gale
> 34
 > 65
 12
 Hurricane
37.2.0 Wind speed indicator, Rotation anemometer
See diagram 37.108 Wind speed indicator, rotation anemometer
1. 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 km 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 5 km 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.
2. Make 3 paper cones about the size of a cork. Bore a hole through the cork big
enough to allow a long nail to pass through easily. Use pins to attach the cones to the
cork. Cut out a square piece of cardboard about three times the width of the cork.
Cut a hole in the centre of the cardboard the same size as through the cork. Place a
washer over the hole in the cardboard. Pass a long nail through the cork, washer and
cardboard into the top of a pole. The cardboard cones must be allowed to move
freely when the wind blows.
37.3.0 Wind speed indicator, Deflection anemometer
See diagram 37.109: Deflection anemometer
Use a 25 × 2 × 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 × 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 5 km per hour while holding the anemometer
out of the front window. Make a mark on the protractor where the vane hangs.
Repeat the observation for speeds of 10, 15, 20, 25, 30 km per hour.
37.4.0 Wind speed indicator, Pressure tube anemometer
See diagram 37.110: 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.
37.5.0 Precipitation, rain gauge, precipitation gauge
See diagram 37.111: 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.
37.5.1 Precipitation, rain, drizzle, snow, hail
The term precipitation refers to the following:
1. Rain as liquid water drops, > 0.5 mm diameter, that have a uniform direction of
fall, usually falls from stratus clouds, and falls steadily unlike rain showers. Rain is
measured in a rain gauge or automatic pluvigraph, usually every 6 hours. The rain
gauge catches all forms of precipitation.
2. Drizzle as very small sparse or thick liquid droplets, < 0.5 mm in diameter, that may
fall in various directions or float rather than fall from thin stratiform clouds. Owing to
the very small size of the droplets, their impact on a water surface is imperceptible.
Drizzle occurs during high humidity and may become mist or fog. Some people feel
depressed during periods of continuous drizzle.
3. Snow as ice crystals, often interlocked to form flakes, and near the poles, grains
and pellets in strong wind blizzards that restrict visibility. Wind piles up snow in
snowdrifts. The lower boundary of higher places permanently covered with snow is
the snow line.
4. Hail as small balls or pieces of hard ice. Soft hail refers to crisp and easily
compressible grains of ice, which may rebound or burst on striking a hard surface.
Hail is formed in the updrafts and downdrafts of cumulonimbus clouds. If carried
successively in updrafts hail may grow in size as layers of ice are added. Hail guns are
a useless attempt to shatter hailstones. Hail may be predicted with radar and a
green-yellow appearance of storm clouds.
37.5.2 Classification of precipitation
1. Showers of rain, snow or hail are associated with cumulus and cumulonimbus
clouds only. Showers usually commence and cease abruptly, may change intensity
rapidly and are of 15 to 30 minutes duration.
2. Intermittent precipitation refers to rain, drizzle and snow associated with stratiform
clouds, i.e. altostratus, nimbostratus, stratocumulus, and stratus. It occurs with breaks
of varying duration, i.e. it began within the hour or more before the time of the
observation but breaks occurred.
3. Continuous precipitation is the same as intermittent precipitation only longer.
Slight precipitation can be:
1. Rain, with individual drops easily identified, puddles form slowly, small streams may
flow in gutters,
2. Drizzle that can be felt on the face but is not visible, it produces little runoff from
road surfaces or roofs and visibility reduced < 1000 m,
3. Snow, as small sparse snow flakes, and visibility reduced < 1000 m,
4. Hail, as sparse hail stones of small size often mixed with rain,
Moderate precipitation can be:
1. Rain, with rapidly forming puddles, down pipes flow freely, and some spray visible
over hard surfaces,
2. Drizzle, with window and road surfaces streaming with moisture, and visibility
between 400 m to 1000 m,
3. Snow, as many large flakes and generally visibility between 400 m to 1000 m,
4. Hail with particles numerous enough to whiten the ground.
Heavy precipitation may be:
1. Rain, falling in sheets, misty spray over hard surfaces, and perhaps roaring noise on
the roof,
2. Drizzle with visibility < 400 m,
3. Snow, with flakes of all sizes and visibility < 400 m
4. Hail, with some hail stones < 6 mm diameter.
37.5.3 Dew
See 9.196: Guttation
Dew is a deposit of water drops on objects at ground level from condensation of
water vapour from the air near the ground cooled to below dew point, the saturation
point, i.e. the temperature at which the air, at constant pressure. becomes saturated
and condensation occurs.
37.5.4 Dust storm
Dust storm (fine particles) or sand storm (coarse particles) is caused by turbulent
winds raising large quantities of dust or sand into the air and reducing visibility to
< 1000 M. However, a severe dust storm or Sand storm may reduce visibility below
200 M. A dust devils is a whirling, narrow column of dust or sand, usually less than
30 m high.
37.5.5 Frost
Frost may be 1. hoar frost deposits of soft white ice crystals when the temperature of
the surface is below freezing point, produced by deposition of water vapour from the
surrounding clear air, 2. white dew from frozen dew drops. A black frost is a common
term for destruction of plant tissue is cold dry conditions when frost cannot be seen on
the ground. In Australia agricultural land may be listed as being "frost free". Frostbite
occurs when tissue in exposed parts, e.g. cheeks, nose, ears, fingers and toes are
frozen by intense cold and become pale and numb, lose blood circulation and die
because ice forms in the tissue.
37.5.6 Haze
Haze is a state of atmospheric obscurity caused by suspension in the air of extremely
small dry particles invisible to the naked eye. Haze is like a uniform veil over the
landscape that subdues its colours. Viewed against a dark background mountain it has
a blue tinge but against the sun or clouds it has a orange-yellow tinge. Dust haze is
caused by suspension in the air of dust or small sand particles, raised from the ground
prior by a dust storm or sand storm. Bushfires may cause a smoke haze. In the Blue
Mountains near Sydney, Australia, a bluish haze in the valleys with almost vertical
walls, is caused by mie scattering of the terpenoids evaporated from the leaves of
eucalypts.
37.5.7 Mist
Mist is usually a suspension in the air of microscopic water droplets reducing the
horizontal visibility to 1000 m to 10 km and giving the air a grey tinge.
37.5.7.1 Fog and rime
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 sideways 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. Fog is a suspension of very small water droplets in
the air reducing the horizontal visibility to < 1000 m. Shallow fog lies on the surface of
the ground or sea that does not obstruct visibility at a height of 2 m over land or 10 m
over sea. Freezing fog causes rime, frozen water droplets on solid objects.
During World War II fog prevented British bombing raids and contributed to many
accidents on air fields. A successful but very expensive method of clearing fog called
FIDO (Fog Investigation and Dispersal Operation) was developed that used burning
petroleum, but has never been used in peace time because of the cost.
37.5.8 Smoke is caused by fine ash particles suspended in the atmosphere. The
disc of the sun at sunrise and sunset appears very red and during daytime it has a
reddish tinge. Smoke at a distance, such as from bush fires, usually has a light greyish
or bluish colour and is usually evenly distributed in the upper air.
37.5.9 Thunderstorm is a combination of thunder and lightning with or without
precipitation. Thunder is the sound caused by the atmospheric disturbance created by
a lightning flash and may be audible up to about 16 km from the source. Thunder is
always associated with cumulonimbus clouds. Lightning is a brilliant momentary
discharge between two electrified clouds or between such a cloud and the ground or
within a cloud. If the path of the discharge is visible to the observer it is seen as a
forked streak but if the actual discharge is hidden from the observer it is seen as a
diffuse glow.
37.5.10 Tornado or waterspout has a characteristic funnel cloud is caused by
violent, vertical funnel-shaped vortex hanging from a cumulonimbus cloud base. It may
reach the surface. Violent winds near the axis may do great damage along a narrow
track. In Australia a small local tornado is called a dust devil or willy-willy.
37.6.0 Hair hygrometer
See diagram 37.112: 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.
Women with blond curly hair complain of "bad hair days". This phrase allegedly
originates from the 1992 film "Buffy the Vampire Slayer", and means unmanageable
hair or even "everything is going wrong". It probably refers to days of changeable
humidity. Similarly violin players have to pay careful attention to tuning their instrument
when the humidity changes.
37.7.0 Housing box (Stevenson Screen) for weather instruments
See diagram 37.113: 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 or barograph, thermometer, and
the hygrometer. Keep them in a white-painted 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.
37.8.1 Wet and dry bulb hygrometer, (wet and dry bulb thermometer,
psychrometer)
See diagram 37.114: Wet and dry bulb hygrometer
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 a wet and dry bulb hygrometer 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
Percentage 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 .
 .
 .
 .
 .
 .
 .
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 in all directions
See diagram 37.117: 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.
37.13 Convection box
See diagram 37.119: 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 37.120: Convection currents
See diagram 4.28: Air currents from a lighted candle
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.
4. 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.
37.15 Atmospheric moisture
See diagram 37.121: Demonstrate 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 Water "lost" by evaporation
See diagram 37.122: Water "lost" by evaporation, 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 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 from plants
1. 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
2. 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 the rate of 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 Rate of evaporation and moving air
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.
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 Water cycle
See diagram 37.132: Water cycle, (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 Rain cycle
See diagram 37.133: Simulated 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 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 Cloud in a bottle
See diagram 37.135: 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. Pour a little water into a jar wider than your fist. Screw on the lid tightly and leave
for half an hour. Put a stick of white blackboard chalk in a plastic bag and use a
hammer to crush it to a fine powder. Cut the neck off a party balloon. Open the jar
and put in the chalk powder and immediately stretch the balloon over the opening of
the jar until the rubber is horizontal over the opening. Fix a rubber band around the
neck of the jar to keep the balloon stretched tight. Push the balloon down with your
fist and hold it down for one minute. Raise your fist and take off the balloon to see a
cloud formed in the jar. Your fist compressed the air in the jar to warm it and absorb
more water vapour. When you remove the balloon cover, the air cools and some of
the water vapour condenses on the chalk dust to form cloud.
3. 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.
4. 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.
5. 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. 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 in 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 Snowflakes
See diagram 37.136: 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.
37.31 Weather records
See diagram 37.137: Wind direction record
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
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 Weather pictures
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 Weather fronts
See diagram 37.138: Simulated weather fronts
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 Upper winds
See diagram 37.139: 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 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 Solar ultraviolet radiation and skin cancer
See 36.16: Albedo | See 13.1.31: Ozone
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. So Non-SI unit "angstrom" =
0.1 nanometres.)
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) × 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 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.
37.35 Dust in the air
See diagram 37.141: Measure dust in the air
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. To measure how many metric tons per km2, find the area of the mouth of the
jar in cm2. 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 km2. 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.
37.36 Thunderstorms
Prepare a sketch map of the local area within a radius of 15 km. Draw concentric
circles on the map showing places that are 1, 2, 3 15 km 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 km.
When the storm has passed, examine this record of the thunderstorm.
37.37.0 Clouds and weather
Order online: Pressure Pumper kit, clouds, refrigeration, air conditioning
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.
Clouds form when water vapour in saturated cooling air condenses to form water and,
sometimes later, ice. The cooling can be caused by uplift into cooler layers of the
atmosphere or by advection, i.e. moving sideways to contact cooler land, water or
air. Uplift can be caused by convection over heated land, orographic lift over
mountains and lift by sliding over warm fonts or cold fronts. Most clouds do produce
any precipitation that reaches the ground. Usually only cumulonimbus clouds produce
showers and only stratus and nimbostratus clouds produce rainfall.
The three main types of cloud are as follows:
1. cirrus (hair-like),
2. cumulus (puff balls), 3. stratus (layered).
The rain cloud is called nimbus.
The three altitude bands are as follows:
1. high cloud,
2. middle cloud,
3. 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 "be very happy" if you feel you are on cloud 9.
See diagram 37.143: 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 8. If the sky has no clouds,
the amount of cloud is 0. Report intermediate values between 0 and 8.
7. 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 Cloud classification, Ten main types of cloud
Luke Howard (1772-1864) invented the classification of clouds, including the terms
cumulus, stratus, cirrus, cirrostratus and stratocumulus.
Low level cloud (surface to 2500 m)
1.0 Stratus cloud, 150 to 600 m, base forms below 500 m, 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,
sideways, such as when warm moist air moves over a colder surface. So it is typically
found around coasts or mountains.
See also: 37.5.7.1 Fog and rime
2.0 Stratocumulus cloud, 600 to 1500 m, 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, but not continuous
rain. This is low level cloud composed mainly of water droplets or ice particles with
bases below 2 000 metres.
3.0 Cumulus cloud, 600 to 1500 m, 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.
4.0 Cumulonimbus cloud, 600 to 1500 m, 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.
Cumulonimbus clouds may extend through low, middle and high levels so they are the
most spectacular clouds.
Middle level cloud (2500 to 6000 m)
5.0 Nimbostratus cloud, 150 to 2500 m, 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. Nimbostratus may extend through more than one level.
6.0 Altostratus cloud, 2500 to 6000 m, 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. Altostratus cloud is composed mainly of water droplets, not ice crystals.
7.0 Altocumulus cloud, 2500 to 6000 m, 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 cloud, seen behind mountains and cloud caps on
mountains. Altocumulus castellanus is like small towering cumulus clouds with scalloped
tops and may indicate imminent thunderstorm activity.
High level cloud (above 6000 m) (8.0, 9.0 and 10. are called cirriform clouds)
8.0 Cirrus clouds, 6000 to 12 000 m, 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.
9.0 Cirrocumulus cloud , 6000 to 12 000 m, 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 when it looks like fish scales smaller than the
little fingernail at arm's length.
10.0 Cirrostratus cloud, 6000 to 12 000 m, 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 (condensation trails)
Contrails are artificial clouds formed when moist gases from the exhaust of an
aeroplane cool rapidly to form ice crystals. They may soon disappear as the crystals
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, rain making
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. Cloud seeding is always an expensive process and may not be
justified commercially.
37.37.5 Raindrops
Raindrops are > 0.5 mm in diameter. Supercooled rain, called sleet, freezes on
objects. Thin clouds with weak updrafts cause drizzle drops, < 0.5 mm in diameter.
Raindrops from a sloping frontal surface are persistent in size but no big. The largest
drops are formed in deep cloud layers with turrets containing strong updrafts. These
raindrops may contain ice crystals. Very heavy rainfall may be seen as the optical
illusion of "stair rods" that bounce after hitting the ground. A falling raindrop seen from
the side is indented in the middle from below, not tear-shaped as is often shown in
illustrations. This error is caused by the observation that just before a drop of water
leaves a faucet or kitchen tap it hangs down in a teardrop shape. However, when it
becomes too heavy to remain attached to the tap it falls and assumes a near spherical
shape like a muffin with a concave lower surface. Raindrop size is limited by the action
of the slipstream when the raindrops achieve a high velocity of about 10 m / second.
However, larger drops may reach the ground in a downburst.
37.38.0 Warm fronts and cold fronts
An air mass is a huge mass of air with its own property of temperature and
humidity gained from the original land or water below it, e.g. tropical, equatorial and
arctic air masses. The air masses keep moving with cold air masses moving towards
the equator and warm air masses moving towards the poles. Where air masses with
different properties meet is called a front. A faster moving colder air mass pushing
under a warmer air mass produces a cold front. The warmer air is pushed up and its
water vapour condenses to quickly form clouds and rain. A warmer air mass passing
over a colder air mass produces a warm front with light longer period rain.
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 to 8 km 5 to 13 km 6 to 18 km
Middle level 2 to 4 km 2 to 7 km 2 to 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,
although 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 Hurricane, tropical cyclone, typhoon
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 km2. When the barometer begins to rise and the winds change
direction, the worst of the hurricane is over. The "eye of the storm" is an are of still air
in the centre of a tropical cyclone where air is descending instead of rising rapidly and
the sky may be clear. The winds each side of the "eye" blow in opposite directions.
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