Category Environment

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.

The most important factor that influences the climate of a place is its distance from the equator – the imaginary line around the centre of the Earth. Places further away from the equator are usually cooler than places that are nearer. This is because the Sun’s light is spread over a larger area towards the Earth’s poles and it has to travel through more of the atmosphere to get there.

Climate and the sea

Places far from the sea, or hidden from the sea by great mountain ranges, often have very little rainfall – the air reaching them has already lost most of its moisture as rain on its journey over the land.

Places near to the sea do not usually have great temperature changes. The sea heats up much more slowly than the land and cools more slowly. Therefore, it keeps the land warm in winter and cool in summer.

Deserts often have little rainfall because they are sheltered by mountain ranges.

The climate is affected by how high up you are and where in the world you are. As you go higher, the climate becomes cooler and eventually too cold for trees to grow. Even higher, the ground is covered by snow all year round. The ‘snow line’ and ‘tree line’ become lower as you move away from the equator.

The snow line and the tree line on the side of the mountain.

As the Earth spins on its axis, it orbits about the Sun. This means that at different times of year, different parts of the world directly face the Sun in the middle of the day. In January, places about 2,000 kilometres south of the equator have the hottest weather. In July, the hottest weather is about 2,000 kilometres north of the equator. This means that the warmest season in the northern parts of the world is during July and in the southern part during January.

Many climates further away from the equator have four seasons. The weather in winter is often too cold for most plants to grow, and there is a good deal of frost. In summer and winter the weather is often stable for longer periods of time. The weather in spring and autumn often changes from day to day, with high winds and sudden showers. The main season of growth is spring.

As the hottest regions change, the directions of winds and positions of fronts change around the world. These winds and fronts affect rainfall, and so some regions have distinct rainy and dry seasons. In India, for example, there is a very rainy summer season, called the wet ‘monsoon’, but little or no rain from December to April.

The wet monsoon often causes flooding.