Category Geography

An understanding of the weather is often vital to daily life – framers, for example, require information about the weather to try and protect their crops. Extremes of weather can ruin entire crops.

Sometimes even our safety depends on knowing about weather conditions. With advance warning of a major storm, ships or aeroplanes can be routed away from danger. Flood warnings can be issued and people moved to a safe area.

Farmers need to irrigate, or water, their crops if there is no rain.

Heavy rain can cause rivers to burst their banks and flood large areas of land.

When we say “The Sun is shining”, we are talking about the weather. However, when we say “It’s always sunny here”, we are talking about the climate. The climate describes the way weather behaves over many years. The climate depends on many factors, including how far from the equator a place is. Weather is caused by changes in the ‘atmosphere’ – the layer of air surrounding the Earth.

Generally, weather and climate are caused by parts of the world becoming warmer than others when the Sun’s rays heat the Earth’s surface. These differences in temperature make the air move and these air movements cause the different types of weather we know.

The changing weather – hot, sunny conditions in summer heavy snow in winter. Some parts of the world get hurricanes – very strong winds.

Winds carry warm and cold air around the world. Knowing which way the wind is blowing is a useful guide to the type of weather we can expect. Around the world, there are many differences in temperature. Air in contact with hot land or sea is warmed. As warm air is lighter than cooler air, it rises. Cooler air then moves in to take its place. This movement of air is what we call wind.

Winds can be extremely powerful; they can knock down trees, and can push along boats fitted with sails. The map shows the routes of the main winds across the globe. These routes were once followed by sailing ships carrying their cargoes around the world.

Sea breezes

As the Sun shines, air over the land is heated more than air over the sea, and this hot air rises. Cooler air from over the sea moves in to take its place.

The hot air cools as it rises, but it does not fall straight back down. Instead, it spreads out over the sea and falls down there. This circular air current causes an onshore breeze.

The map shows the major winds of the world.

Yachts with the wind in their sails

Clouds are formed from water vapour. The air collects this vapour as it passes over damp places, like the sea. We call the process of a liquid turning to gas ‘evaporation’. Normally, the vapour in the air is invisible, but if the air is cooled then clouds of tiny water droplets are formed. This process we call ‘condensation’.

The amount of water vapour that the air can carry depends on how hot the air is – warmer air can carry more moisture. When warm, moist air rises, either by moving over hills and mountains, or by meeting cooler air, it is cooled. As the air cools, it can carry less vapour. The excess moisture forms clouds of tiny droplets.

Storm clouds gathering

The fluffy, white clouds you see on fine, summer days are called ‘cumulus’.

Your breath

The way clouds form is like the way a misty cloud forms when you breathe out on a cold day. The air that you breathe out contains a lot of moisture. On a cold day, this air is cooled as it meets the cold air outside. As it cools, it can hold less water vapour, and the extra water forms tiny droplets, like a cloud. When a whale breathes out, the water vapour in its warm breath condenses into a misty spray.

The tiny water droplets inside a cloud may bump into each other and join together to form larger droplets. If the air inside a cloud is rising, these droplets are lifted up again and join with others to form yet larger droplets. When the droplets are very large, about the size of raindrops, the rising air can no longer lift the drops back up and so they fall as rain.

The water cycle

The evaporation of water caused by sunshine makes the air moist. Moist air travelling inland may have to rise over hills and this cools it. As the rising air is cooled, clouds form and rain may fall. The rain falling on the land runs into streams, which flow into rivers. The river water eventually returns to the sea.

Continue reading “RAIN AND SNOW”

We usually think of air as being weightless, but in fact air is quite heavy. The air in a large classroom has the same weight as a small car! The air of the Earth’s atmosphere reaches upwards for several hundred kilometres. The effect of this is that the air at ground level presses on everything it surrounds.

The exact air pressure changes from day to day. Studying air pressure, and the way it is changing, helps to tell us how the weather will change in the next few hours and days. Usually, high pressure brings good weather whereas low pressure brings bad weather. A device called a ‘barometer’ measures air pressure and is used to predict the weather.

On this map you can see areas of high pressure (H) and low pressure (L).

The effects of air pressure

You can see an effect of air pressure with a washing-up liquid bottle. If you remove most of the air from inside the bottle, by sucking it, the bottle collapses. This is because the air around it pushes inwards. Normally the air inside balances this force. Simple barometers measure changes in pressure in a similar way.

Large swirls of cloud like this one often indicate areas of low air pressure.