Frequently, television meteorologists are heard referring to high pressure weather systems that bring us picture perfect skies and pleasant conditions while low pressure is typically blamed for the “not so perfect” weather scenarios. But in the name of perfect weather, let us investigate this high pressure phenomenon a bit further. High pressure is simply large scale weather system in which the wind flow is counter clock-wise, or anti-cyclonic, around the center.
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 center.
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 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.
‘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.
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 the southeast U.S 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 US 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 US we always try to be optimistic!
Here’s hoping that if your jon finds you outside, rest assured the next few days it will indeed be a ‘High Pressure Job’!
Keller Watts, Meteorologist / Storm Force 31
