Air is light, but because there is so much of it above us, it exerts pressure on us. Air pressure is measured by a barometer the units used to measure this are millibars –the greater the reading the higher the pressure. High pressure systems are also known as anti-cyclones. Air falls in an anti-cyclone so not clouds are formed. In summer, high pressure usually results in clear skies, gentle breezes and fine weather. In winter, high pressure leads to clear skies and colder conditions. Anticyclones are larger than low pressure systems which are also known as depressions, they last longer than low pressure systems. Anti-cyclones can be seen In summer, the clear settled conditions associated with anticyclones allow the Sun’s light to warm the ground. This can bring long sunny days and warm temperatures. The weather is normally dry, although occasionally, very hot temperatures can trigger localised thunderstorms. Sometimes, an anticyclone doesn’t move and remains stationary over the UK or nearby for a longer period of time. How do anti-cyclones produce fog and frost? In winter, the clear, settled conditions and light winds associated with anticyclones can lead to frost and fog. The clear skies allow heat to be lost from the surface of the Earth to space by radiation, especially overnight, allowing ground temperatures to fall steadily, leading to air or ground frosts. Sometimes in late winter or spring, the air near the ground cools so much that low cloud, or fog, can form. Because of the light winds associated with anticyclones; this can linger well into the following morning and be slow to clear. An area of low pressure is called a depression – air rises so clouds and rainfall are formed. Depressions bring unsettled weather than rain. Winds are strong in depressions. They usually form over the ocean and are carried across Britain by westerly winds.