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High Pressure |
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An area of high pressure is usually associated with hot clear summer days, but it can bring fog, frost and even cloud.
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High pressure areas are generally larger and move more slowly than low pressure systems. The winds circulate around the centre in a clockwise, 'anticyclonic' movement. The winds are generally weaker than those around a low pressure, especially in the centre. As air falls it warms, preventing clouds from forming.
In a high pressure, the air is generally sinking slowly, or 'subsiding'. As air falls it warms, preventing clouds from forming. This is why highs are generally clear. However, sometimes the ground may be warm enough to cause some air to rise, and this can form a layer of cloud.
High pressure does not necessarily mean warm weather. A 'cold anticyclone' has cold air near the ground, and is common in Siberia and Canada, where the cold air is cooling and subsiding. Another cold anticyclone is an area of high pressure sandwiched between two areas of low pressure. This is quite common during the British winters, giving us fog and frost. 'Warm anticyclones' include the Azores high. The Azores have a warm climate, which normally indicates rising air, but at this latitude, the earth's air has a tendency to sink and so a high pressure is formed. This high pressure can extend all the way to the UK, giving us a period of hot sunny weather. a 'blocking high' ... forces other weather systems to go round it.
Sometimes a high pressure will 'get stuck', and can stay in the same position for over a week. This is called a 'blocking high' and forces other weather systems to go round it. These blocks often reoccur in the same place, including just west of the UK and over Scandinavia. This gives us some more unusual weather, either dragging air from further north or further south than usual.
A common phrase used by weather forecasters is a 'ridge of high pressure'. This often indicates settled weather. A ridge is an area of high pressure that does not have a closed circulation, it either extends from a high pressure or is sandwiched between a couple of lows. Although in the UK we have been taught to love high pressure, in winter it can bring frost and fog. In the Summer it can cause problems with smog, as the pollution gets 'caught' in the sinking air. However, there is a chance that a high pressure will bring the sun and in the UK we try to be optimistic! | |||||
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