School Science Lessons
2017-02-16 SP
Please send comments to: J.Elfick@uq.edu.au

37. Weather
Table of contents
Websites: Meteorology, weather, climate
Weather stations, climate, "Scientrific" (commercial website)
21.0 Weather, (Primary)
37.1 Weather science, climate change, global warming, Great Barrier Reef, coal seam gas, fracking
37.1.0 Atmosphere, (Weather)
Climate change
37.5.0 Clouds
37.3.0 Humidity
37.7.0 Moisture enters the air, evaporation
37.8.0 Moisture leaves the air, precipitation
37.9.0 Weather science
37.10.0 Weather science, Instruments in a weather station
37.11.0 Winds, convection

37.1.0 Atmosphere
Air, atmosphere, (Chemistry)
37.9 Air expands when heated
37.5.4 Dust in the air
37.32 Simulated weather fronts
37.5.8 Smoke
Experiments
37.34 Solar ultraviolet radiation and skin cancer
37.5.9 Thunderstorm

37.5.0 Clouds
37.37.0 Clouds and weather
37.29 Cloud in a bottle
37.37.1 Cloud classification, ten main types of clouds
37.37.4 Cloud seeding, rain making
37.37.3 Contrails (condensation trails)

37.3.0 Humidity
37.48 Dew point
37.8.3 Dew point hygrometer
37.8.4 Relative humidity table, depression of the wet bulb
12.3.1 SVP, Saturation Vapour Pressure
12.3.2 Saturation vapour pressure over water
37.8.2 Sling psychrometer
37.8.1 Wet and dry bulb thermometer (hygrometer, psychrometer)

37.7.0 Moisture enters the air, evaporation
Experiments
37.15 Atmospheric moisture
37.20 Moisture from breathing
37.19 Moisture from plants
37.17 Moisture evaporates from soil
37.24 Moisture in the air affects the rate of evaporation
37.23 Moving air affects the rate of evaporation
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 Moisture leaves the air, precipitation
37.5.0 Precipitation, rain gauge, rain, drizzle, snow, hail
37.5.2 Precipitation, Classification of precipitation
37.5.5 Frost
37.5.6 Haze
37.5.7 Mist
37.37.5 Raindrops, stair rods illusion
Experiments
37.28 Dew point temperature
37.25 Moisture condenses on cool surfaces
37.27 Rain cycle
37.30 Snowflakes
37.26 Water cycle

37.9.0 Weather and climate, instruments in a weather station
37.1.0 Weather and climate, instruments in a weather station
53.1 Coconut weather indicator, Hawaii, (JPG)
37.6.0 Hair hygrometer
37.7.0 Housing box (Stevenson screen)  for weather instruments
37.1.0 Weather and climate, instruments in a weather station
37.1.1 Wind speed, wind vanes
37.2.0 Wind speed indicator, Rotation anemometer
37.3.0 Wind speed indicator, Deflection anemometer
37.4.0 Wind speed indicator, Pressure tube anemometer
37.5.0 Rain gauge, precipitation gauge, rain, drizzle, snow, hail

37.10.0 Weather science
37.39.0 Weather science
37.31.1 Weather pictures
37.31.2 Weather periods
21.0  Weather, (Primary)
37.31 Weather records
3.2.5.2 Weather indicator, Cobalt (II) chloride solution, invisible writing ink
37.46 Weather sayings

37.11.0 Winds, convection
Wind, "Scientrific", (commercial website)
"Eco Biker Wind Power Kit", clip to bike to power LED, (commercial)
"Mini Wind Turbine", electric motor as generator, LED, (commercial)
37.33 Upper winds, wind direction astrolabe
37.1.2 Wind speed measurements, m s, knots, Beaufort scale
37.1.3 Windmills

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, Mammatocumulus *
4.0 Cumulonimbus *
5.0 Nimbostratus
6.0 Altostratus
7.0 Altocumulus *
8.0 Cirrus
9.0 Cirrocumulus *
10.0 Cirrostratus

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 US,
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
At 10 m standard measurement height, wind speed: 1 m / s = 3.6 km / h = 2.237 mph = 1.944 knots
Anabatic winds move up slopes because the air mass over the ground becomes heated and cooler air moves in from over the ocean.

1. Gales are winds with speed of 34 to 47 knots (USA).
2. Katabatic winds move down slopes because of the weight of the cold air mass.
3. Pressure gradients cause wind to move from areas of high pressure towards areas of low pressure, the direction modified by the
Coriolis effect.
4. Prevailing wind is the most likely wind direction at a place.
5. 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.
6. 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.
7. Wind rose is a representation wind speed and direction as the spokes of a wheel.
8. 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.
9. 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.
10. Windage is the marksman's estimated lateral deflection by the wind when aiming at a target.

Experiment
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).
Table 37.1.2 Wind speed
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

37.1.3 Windmills
Windmill sails, or blades, convert wind energy into rotational energy.
Most windmills are windpumps to pump up water or wind turbines to generate electricity.
Formerly, windmills had four sails at right angles crossing at the centre. 
Modern wind turbines have three arms like aircraft propellers at 120o, which makes them more stable than four arms windmills in
strong winds. 
Power generated by wind turbines is the rate of change of the kinetic energy of the air when passing the arms, the loss in air speed
caused by the the arms.
The efficiency of a wind turbine is about 40%, down to 20% after transmission of power.

37.2.0 Wind speed indicator, rotation anemometer
Anemometers, wind turbines, "Scientrific", (commercial website)
See diagram 37.108 Wind speed indicator, rotation anemometer
Beaufort Wind Scale (Beaufort number 0 --> 12)
Experiments
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 the 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
Experiment
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
Experiment
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 ink is added to the water, a "dirty" ring inside the glass tube will show the highest wind gust velocity between the times of instrument
reading.
If the local winds 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 Rain gauge, precipitation gauge, rain, drizzle, snow, hail
See diagram 37.111: Rain gauge
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.

Experiment
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.

37.5.2 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.
3.1 Slight precipitation can be:
3.1.1 Rain, with individual drops easily identified, puddles form slowly, small streams may flow in gutters,
3.1.2 Drizzle felt on the face but not visible, it produces little runoff from road surfaces or roofs and visibility reduced < 1000 m,
3.1.3 Snow, as small sparse snow flakes, and visibility reduced < 1000 m,
3.1.4 Hail, as sparse hail stones of small size often mixed with rain.
3.2 Moderate precipitation can be:
3.2.1 Rain, with rapidly forming puddles, down pipes flow freely, and some spray visible over hard surfaces,
3.2.2 Drizzle, with window and road surfaces streaming with moisture, and visibility between 400 m to 1000 m,
3.2.3 Snow, as many large flakes and generally visibility between 400 m to 1000 m,
3.2.4 Hail with particles numerous enough to whiten the ground.
3.3 Heavy precipitation may be:
3.3.1 Rain, falling in sheets, misty spray over hard surfaces, and perhaps roaring noise on the roof,
3.3.2 Drizzle with visibility < 400 m,
3.3.3 Snow, with flakes of all sizes and visibility < 400 m
3.3.4 Hail, with some hail stones < 6 mm diameter.

37.5.3 Dew
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 in the air
See diagram 37.141: Measure dust in the air
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.
Experiments
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, use three wide mouth 5 litre containers and 10 litres of deionized
water.
Tap water may contain particles.
Use a balance that weighs to the nearest milligram.
Rinse the containers with deionized water.
Pour 1.5 litres of deionized water into each container and mark the water level with a grease pencil.
Cover the top with a wire screen to keep insects out.
Put each of the containers in a different location, 1.5 metres above the ground, and leave them for 30 days.
Every few days add deionized water to the original water level.
Rain may fall into the jar, but the water level must not overflow the containers.
After 30 days, weigh a 2 litre pan and add the contents of the containers.
Use more deionized water to rinse out the containers.
Gently heat the pan until the remaining contents are dry.
Let the pan cool and then weigh it to obtain how many milligrams of dust fell through the mouth of the containers in a month.
Convert the result to tonnes per km2, knowing the area of the mouths of the containers.

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.
A black frost is less severe than a frost with snow or rime and is called "black" because it blackens plants.
Black ice is thin, hard ice which is dangerous on road surfaces, especially when transparent and invisible to drivers when the tyres of a
motor vehicle may lose grip on the road.
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.
Bush fires 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 eucalyptus.

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.
So 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
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
1. A thunderstorm is a storm with thunder and lightning from an electrically charged cumulonumbus cloud and may form heavy rain and
hail.
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.
A thunderclap is a loud sound of thunder.
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.
A thunderbolt is a flash of lightning with almost simultaneous clap of thunder.

2. Thunder and lightning
Thunder is heard after the lightning flash is seen although they occur virtually together.
The extremely hot lightning bolt causes an explosive expansion of air around it.
The rumble of the thunder lasts longer than the lightning flash because thunder is caused by the rapid expansion of air that is heated to
very high temperatures along the entire length of the lightning bolt, which may often be a kilometres.
The sound waves from this explosion need different amounts of time to reach the ears, because some parts of the lightning bolt are
farther away from than others.
After hearing the bang of thunder, which is delayed and weakened with increasing distance, a weak rumbling sound is heard, which is
the reflected sound waves.
Thunder is the noise of the sound wave caused by the sudden expansion of air massively heated by lightning.
The reverberations or rolling of thunder is caused by reflection of the sound from different layers of air of different temperatures.
Similarly, a contestant in a race may start at the sight of the puff of smoke from the starter's gun and not wait for the sound to arrive.
3.0 The speed of sound in air at 0oC is about 331 m / s.
3.1 The speed of sound in air at 20oC is about 334 m / s. The speed of light is 299,792,458 m / s.
When the speed of sound is already known, an approaching storm's distance may be estimated by counting the seconds between the
lightning and the thunder, and multiplying this by the speed.
The speed of light from lightning is almost instantaneous, so if the sound of thunder, after seeing lightning from the same thunderstorm,
occurs 3 seconds later, the distance from the thunderstorm = 3 × 334 = 1002 m  1 kilometre.
3.2 The speed of sound is just over 1,000 feet / second (334 × 3.048).
So in 5 seconds sound travels just over 1 mile.
The distance from a lightning strike in miles is the time taken for the sounds of thunder to arrive divided by 5.

Experiments
1. During a thunderstorm calculate the distance to a lightning strike.
2. 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:
2.1 the direction by visual observation,
2.2 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.

37.6.0 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.
Experiment
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 the 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
a 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 must be exposed 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 must be shielded 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 the 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 thermometer (hygrometer, psychrometer)
Thermometers, wet and dry bulb, (hygrometer, psychrometer), relative humidity, dew point, "Scientrific", (commercial website)
See diagram 37.114: Wet and dry bulb hygrometer
The wet and dry bulb thermometer, psychrometer, was invented by John Leslie, 1810, and later improved by L. Gay-Lussac.
Experiment
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.
Wicks ready for immediate attachment can be purchased.
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:
37.8.4 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:
37.8.4 Relative humidity table

37.8.3 Dew point hygrometer
The dew point temperature is an important weather observation.
It is the temperature at which the moisture in the air begins to condense or the temperature when unsaturated steam becomes saturated
steam.
So dew point is also an expression of air humidity.
The dew point temperature changes from day to day.
Predicting the dew point has direct significance for agriculture, because if the dew point is below 0oC, the water or steam in the air can
condense into frost that is harmful to crops.
Dew point measures how much humidity is in the air.
Relative humidity measures much humidity is in the air compared with how much can be in the air at that room temperature.
Experiment
Use a brightly-polished metal cup or shiny drink-can.
Fill it 2/3 with water at temperature slightly higher than the indoor temperature.
Suspend a thermometer in the water by a pencil clip attached inside the cup.
The cup must not have any fingerprints on it.
Place the cup on a page of printing so that the printing is clearly reflected from it.
Do not breathe on the cup.
Slowly add ice to the water while stirring with a thermometer.
Keep scraping the surface of the cup with a piece of newspaper or cotton wool.
Note the temperature of the water when dew forms as a "thin frost", and the print is no longer clearly visible, the dew point temperature.
Also note the room temperature.
Absolute humidity = dew point temperature oC + 273 / room temperature oC + 273

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 of the wet bulb = (30oC - 28oC) = -2oC, so relative humidity = 86%
Table 37.8.4 Relative humidity table
.................................................Depression of the wet bulb, oC
Dry bulb temperature -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -12 -14 -16 -18
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.

37.14 Trace convection currents
See diagram 37.120: Convection currents
Experiments
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
Atmospheric moisture cannot be seen but its presence can be demonstrated.
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 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.

37.19 Moisture from plants
1. Place a plastic bag over a leaf of some house plant or garden plant and close the end about the stem with a rubber band.
After about one hour, moisture appears on the inside surface of the plastic bag.
2. Half fill a black plastic flower pot with potting mix.
Plant bean or pea seedlings in the pot and let them grow until they are 10 cm in height, but not higher that the edge of the flower pot.
Cover the top of the pot with plastic sheet and pin it closely around the pot with a rubber band.
Moisture appears on the inside surface of the plastic sheet.
3. Invert a clean, dry glass jar over a plant in the garden.
Moisture appears on the inside surface of the glass 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 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.
The wind difference causes the difference in rate of evaporation.

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 just taken off a refrigerator.
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 the 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.

37.28 Dew point temperature
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.

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
| See diagram 8.31: Wind direction star
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 following headings:
Table 37.31
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:
Table 37.31a
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 and 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.
37.31.2 Weather periods
Alexander Buchan, (1829-1907, Scotland),  used the average of many weather observations to determine six cold and two warm
periods, the latter being 12-15 July and 12-15 August.
Since his work, climatologists have determined different patterns of weather periods in different places.

37.32 Simulated weather fronts
See diagram 37.138: Simulated weather fronts | See diagram 8.32: 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 Upper winds, wind direction astrolabe
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: UV, (Ultraviolet), "Scientrific", (commercial website) 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.

Experiment
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 sun blocks with a sun protection factor (SPF) of 15 or more.

37.36 Thunderstorm experiment
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 (a) the direction by visual observation (b) 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 Clouds and weather
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".
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.

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 that change rather slowly.
Clouds with vertical development, cumuliform, indicate a degree of instability in the atmosphere that 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.

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.
See diagram 8.37: Altocumulus and cumulus clouds

4. The low clouds 7. stratocumulus, 8. 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 that 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.
The ten main types of cloud and their usual heights:
1. Low level cloud (surface to 2 500 m) Stratus 150 to 600 m, Stratocumulus 600 to 1 500 m, Cumulus 600 to 1 500 m,
Cumulonimbus 600 to 1 500 m (Cumulonimbus may extend through low, middle and high levels.)
2. Middle level cloud (2 500 to 6 000 m) Nimbostratus 150 to 2 500 m (Nimbostratus may extend through more than one level.),
Altostratus 2 500 to 6 000 m, Altocumulus 2 500 to 6 000 m
3. High level cloud (above 6 000 m) Cirrus 6 000 to 12 000 m, Cirrocumulus 6 000 to 12 000 m, Cirrostratus 6 000 to 12 000 m.
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.

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
1. 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.
2. In an experimental programme since 2008 -2012, approaching weather fronts have been monitored with balloons in the Australian
Alps and clouds selected for cloud seeding.
It is claimed that the seeding caused a 14% increase of snowfall that benefited the skiing industry and eventually benefited irrigation
areas.
However, the economic benefit of the project is still being evaluated in 2012.

37.37.5 Raindrops, stair rods illusion
See 6.17: Splash sticks, (soils)
Raindrops are > 0.5 mm in diameter.
Supercooled rain, 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 not 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 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.