What is high pressure?

The Earth’s atmosphere puts pressure on the surface. This pressure is measured in hectoPascals (hPa), also called millibars. Standard pressure at sea level is defined as 1013hPa.

Lines that have an equal sea-level pressure join each other on a weather chart and are called isobars.  Weather charts showing isobars identify areas of high and low pressure.

A high-pressure area is a region where the atmospheric pressure at the surface of the Earth is greater than its surrounding environment. Areas with above-average pressure are called anticyclones. Anticyclones occur when air is sinking. As a result, there are usually very few clouds in the air. In the UK, anticyclones bring cold clear days in winter and hot and sunny days in summer. Winds within high-pressure areas flow outward from the higher-pressure areas near their centres towards the lower-pressure areas further from their centres. These light winds blow in a clockwise direction in the northern hemisphere.

On a weather chart, an area of low pressure is indicated by an H in the centre of a series of isobars.

What is the weather like during a winter anticyclone?

In winter, the skies are cloudless, so heat is allowed to escape. Therefore temperatures are usually very cold. The ground cools rapidly at night so frost often forms. Fog can also develop as the cold air makes water vapour condense into tiny droplets. Fog can last long into the day as there is insufficient heat from the sun to evaporate the water droplets away.

What is the weather like during a summer anticyclone?

Summer anticyclones bring very different weather. As the air descends, it is heated, causing water in the air to evaporate. Therefore there are few clouds in the atmosphere. The skies are clear allowing the sun’s rays to reach the surface of the earth. This causes temperatures to rise. Heat waves can occur if anticyclones remain over Britain for a number of weeks.