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Air Pressure

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Skies which show how air pressure can create different types of cloud conditions.
Air pressure is an important factor which affects the resulting atmospheric climate conditions.

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Air pressure is the force of the air pressing down on the Earth's surface. As air is all around us, there is air pressure all around us too. Near sea level there is more air above you than there would be on the top of Mount Everest, therefore the air pressure is higher near the sea than it is on the top of a mountain.

A mass of low pressure is an area of air that is rising.
If air temperature falls, the air pressure will increase as cool air sinks towards the ground. Warm air near the earth's surface will tend to rise, so a mass of low pressure is an area of air that is rising.

As air rises it expands and cools. Cooler air cannot hold as much water as warmer air, so as the air rises the water will condense and form clouds. This is why an area of low pressure will often be accompanied by clouds and rain.

Conversely, an area of high pressure is a section of air that is sinking. As the air sinks it warms, so is able to hold more water, and therefore areas of high pressure are often accompanied by fair weather.

Points of the same pressure can be joined up to form lines called 'isobars'. These form rings around high and low pressure centres.

Winds blow in an attempt to combat the differences in air pressure. They try to flow directly from a high to a low pressure, but due to the rotation of the earth and friction caused as air moves over the earth's surface, wind flows around the high or low pressure centres, following the isobars. The larger the difference in pressure the stronger the winds will blow.

When isobars on a weather chart are close together, it will be a blustery day...
When isobars on a weather chart are close together, they indicates a sharp pressure gradient, and it will be a blustery day. When this is accompanied by an area of low pressure, it will also be wet. A wet and windy day is not exactly unusual for the British Isles nor indeed are areas of low pressure and closely packed isobars.

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Key Points

• Points of the same pressure can be joined up to form lines called 'isobars'.
• Low pressure areas are more likely to produce clouds.
• The closer the isobars on a weather chart, the stronger the winds.

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Winter High Pressure
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