Weather

Weather and climate

It is important that pupils understand what weather is and appreciate that there is a difference between weather and climate. Weather can be described as what is happening now – whether it is hot or cold, raining or dry, sunny or clouded over. It describes what is happening within the atmosphere. On the other hand, the word climate describes the average weather you might expect in any one place and is based on measurements taken over many years.

World climates

The world can be divided into many different climate zones found within three main bands of latitude around the Earth. These are Tropical, Polar and Temperate. Tropical climates are usually very hot, wet and humid. Polar climates are extremely cold, but often quite dry. Temperate climates lie between the Tropics and the Polar regions and tend to be less extreme than the other two types of climate.

Cloud formation

One activity all pupils can take part in that will give them a good idea as to what the weather is doing is cloud watching. Changes in cloud formation can provide valuable clues as to the weather. All clouds begin as water, and the sun’s heat causes the water to evaporate into the air as vapour. This moisture is essential for cloud formation – without it we would not have any clouds. Warm air is lighter than cool air and rises into the atmosphere. This process is called convection. As water evaporates into the air, it is carried upwards. The water vapour cools as it rises, turning back into water droplets and ice crystals, which form clouds. This process is called condensation. The water droplets form around tiny particles of dust in the atmosphere. There are, of course, different cloud types:

• Cumulus: large cauliflower-shaped masses of clouds. Small cumulus clouds are a sign of fair weather; larger cumulus clouds indicate showers or thunderstorms.
• Cirrus: sometimes resemble horses’ tails or appear as feathery wisps. These clouds are made up of tiny particles of ice.
• Stratus: grey, flat-looking cloud that sits unbroken across the sky.

Rain

In the United Kingdom, rain is common and we are all pretty much used to it. But, did you realise that there are actually four different types of rain:

• Convection
• Relief
• Frontal
• Monsoon

Convection rain

Convection rain requires warm ground (so lots of sun) and low pressure. It causes short showers interspersed with sunny periods and is the main cause of England’s April showers.

Relief rain

This type of rain is caused by air moving along the ground or over the sea, passing over hills or mountains. These force the air to rise in order to pass the obstruction. The rising air cools and, if moist, will form mist, clouds or rain on the side of the obstruction where the air is rising.

Frontal rain

Frontal rain is very common in temperate regions and is caused when two air masses meet. If a warm air mass meets a cold air mass, a Warm Front is the result. The two air masses will generally not mix. The warm air, being less dense, will gently slide over the cold air. As it rises, it will cool and condense into cloud and rain. A Cold Front occurs when a cold air mass meets a warm air mass. Again, the two air masses do not mix. The cold air, being more dense, forces its way under the warm air. The warm air is forced up quickly. This leads to large Cumulonimbus clouds giving heavy showers, often with thunder, lightning and hail.

Monsoon rain

Monsoon rain is caused by cyclic air movements changing with the time of year. During the dry season, air rises over the sea. It moves over the land and descends. Descending air does not produce clouds or rain. The air then moves out to sea, where it rises again. This cycle of air requires that the sea is warmer than the land. At a certain point in the year, the air cycle reverses. The land becomes warmer than the sea. This causes the air to rise over the land, pass out to the sea, descend, then come back over the land. The rising air is now over the land and, since it will be moist after passing over the sea, it will condense and rain will fall.

Hail and snow

Hail is formed inside large thunderclouds, when droplets of water are carried to great height where they cool and freeze. As the hailstones travel up and down, new layers of ice are added. Snow is essentially the same water droplets that form rain, just frozen into crystals. Snowflakes are formed when individual ice crystals join together. As many as 50 individual ice crystals may be contained in a single snowflake.

Thunder and lightning

Scientists believe that the violent movements of raindrops and ice particles inside thunderclouds create static electricity. As the electric charge in the cloud builds up, it is attracted by an opposite electric charge on the ground or elsewhere in the cloud. Eventually, a large spark occurs between the two areas and this creates lightning. Lightning heats the air up around it very quickly, causing it to expand. When this happens, air rushes into the ‘hole’ that the lightning has made in the air. This creates shock waves in the air that we hear as thunder. Light reaches your eyes instantly so you see the lightning as soon as it happens. However, sound is slower and takes roughly 5 seconds to travel one mile. This is why if you count from when you see a lightning flash to when you hear the thunder, you can roughly tell how far away the storm is.

Droughts and floods

A drought is a long period of dry weather. After 21 days with only 30 per cent normal rainfall, a ‘dry spell’ is reclassified as a drought. During a drought more and more moisture is taken from the air as water literally ‘dries up’. If the drought lasts a long time, and plants begin to die, things get even worse. As the plants die and soil dries, large areas of land can turn to dust. The opposite of a drought (sort of) is a flood. When water falls onto or covers the land in the form of rain, or there are high tides or rising levels in rivers, some is absorbed into the land and some evaporates into the air. Any water that is not absorbed or evaporated is left on the land. This water is called surface run-off and, when there is a lot of surface runoff and nowhere for it to go, floods can be caused.

Did you know?

• A tornado’s spinning speed can reach up to 300 miles per hour.
• The biggest snowflake on record measured 8 inches in width.
• The biggest hailstone recorded weighed 2 1/4 pounds.
• A bolt of lightning can strike the Earth with a force of 100 million volts.
• Poplar trees flip up their leaves when air pressure is low and rain is imminent.
• Some scientists believe moisture (impending rain) makes your nose more sensitive – so some people can smell rain coming!
• Why are storm clouds dark? Because they have a high ice crystal content, light has trouble passing through them, making the clouds appear dark.

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