Category Weather

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.

Weather forecasting is partly done by looking at the movements of ‘fronts’ – regions where warm air meets cooler air. The warm air at a front rises over the cooler air. This cools the warm air and so rain often falls near fronts.

Where warm and cool air meets, the warm air may become partly surrounded by cooler air. As warm air causes lower pressure, this creates a low pressure area called a ‘depression’. When a depression moves over us, we can expect unsettled, rainy or stormy weather.

A high pressure region called an ‘anticyclone’ form where cool air is surrounded by warmer air. An anticyclone moves slowly and can mean a long period of dry or sunny weather.

 

 

 

Bad weather can make driving very dangerous.

 

 

 

 

 

 

Movement of fronts

The region where cold air pushes against a mass of warm air is called a ‘cold front’.

The cold air burrows under the warm air (1) causing it to rise, and so clouds and rain are formed in the rising air.

The cold front catches up with a warm front, where warm air moves into a region of colder air (2). Here, the warm air rises over the cold air, causing more clouds and rain.

Eventually the cold air on the left catches up with the cold air on the right, and the warm air is lifted above ground level (3).

Finally, the warm air disappears, and we just see a region of cool air moving over a region of colder air (4).

 

Sometimes the weather can be extremely violent. One of the most severe types of weather is a hurricane, which may happen near tropical oceans.

Another violent form of weather is a thunderstorm. Thunderstorms happen in extremely moist air, where the grey-black thundercloud stretches up several thousand metres. Inside a thundercloud there are fast air currents which cause ‘static electricity’, electric charge, to build up inside the cloud. Lightning and thunder occur when this electric charge leaps from cloud to cloud or to the ground. The fast air currents inside thunderclouds can hold up large raindrops and so produce very heavy rain.

 

 

 

Thunderstorms can cause bolts of lightning to jump from a cloud to the ground.

 

 

 

 

 

Inside a hurricane

In the centre of a hurricane, called the ‘eye’, there is very little wind. Around the eye, there are very strong winds spiralling round and upwards. Further out there are swirling regions of cloud, reaching perhaps 50 km across. These clouds produce torrential rain.