What makes the weather?

Press, 19 July 1983, Page 18

What makes the weather?

Have you ever wondered why it rains or where cold winds come from, and why there seems to be no shortage of either at this time of year?

The sun has a big effect on the weather, even in winter. Less than half of the heat from the sun that enters the Earth’s atmosphere reaches the Earth — the rest is reflected back into space or soaked up by the clouds. Wind Rays from the sun have a shorter journey through the atmosphere to the equator than to the north or south pole, so more heat reaches the equator.

The hot air near the equator rises, just as the air warmed by a heater rises towards the ceiling in a room. Cooler air, which has moved towards the equator from the north or south pole, moves in to take its place. The hot air moves outwards, and' when it is cool it falls again. This constant movement of air is what we call wind, and that is why we have warm winds and cool winds. Rain There is water all around us — not only in the sea, lakes and rivers, but also in fhp nir

When water on the surface of the sea or a river is warmed by the sun, it turns into a vapour which is carried upwards through the air. Not all the water vapour is carried up into the sky. The water vapour which is still near the ground at night may turn back into water when it touches something cold. This is what makes the dew on the grass and the condensation that forms on the outside of windows. If the night is cold enough, this water will freeze and there will be a frost. The water vapour that does rise into the atmosphere is cooled as it rises

and forms tiny droplets of water. Because these droplets are so small, they are light enough to stay there where they gather in large masses to form clouds. As the droplets of water move around inside the clouds, they join together to form bigger droplets. When these are big and heavy enough, they fall out of the cloud and down through the sky. That is what makes rain.

Snow If it is very cold, the droplets in the clouds may freeze and form snowflakes. Most snowflakes melt again before they reach the ground. The closer they get to sea level, the more likely they are to melt. That is why it snows so often on the mountains, and why the temperature must be very low if it is to snow on lower ground.

Whatever the weather is like, it does not please everyone. Rain during the middle of summer may please the farmer who wants his crops to grow, but

it may spoil the day for someone who wants to go to the beach. If an unexpected change in the weather spoils your plans, spare a thought for the people who forecast the weather — the meteorologists. They are often blamed when rain or a storm is not forecast. Meteorologists use instruments, space satellites and weather balloons to help them forecast the weather, but changes can occur very quickly. They may be as surprised as you are when it rains on the day you left your raincoat at home.