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.