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Anticyclone and Other Pressure Systems
Over the E sides of the oceans the movement of anticyclones, which are also known for synoptic purposes as highs, is generally slow and erratic and the anticyclone may remain stationary for several days giving settled weather.
The pressure gradient is usually slight, the winds are light and the weather is often fine or partly cloudy, but in winter overcast skies are common, producing gloomy conditions.
Precipitation is, however, rare except on the outskirts of an anticyclone.
Over the W parts of the oceans anticyclones are more likely to move quickly and consequently the weather is more changeable.
Movement is generally towards the E.
LOCAL MODIFICATION OF THE WEATHER NEAR THE COAST.
The following notes indicate how weather near the coast is likely to be modified by the shape of the land:
If the coast is formed by steep cliffs, or if the ground rises rapidly inland, onshore winds are usually deflected to blow nearly parallel to the coast, and with increased force. Near headlands, or islands with steep cliffs, there may be large and sudden changes in the direction and speed of the wind.
In a strait, especially if it is narrow and the sides steep, the wind will tend to blow along the strait in the direction most nearly corresponding to the general wind direction in the area, even though these two directions may differ considerably. Where the strait narrows the wind will increase.
Similarly, in a fjord or other narrow steep-sided inlet there is a tendency for the wind to blow along the inlet.
When a strong wind blows directly towards a very steep coast, there is usually a narrow belt of contrary, gusty winds close to the coast.
Where there is high ground near the coast, offshore winds are liable to be squally, especially when the air is appreciably colder than the sea, and also when the wind over the open sea is force 5 or more.
Land and sea breezes occur at certain latitudes.
Radiation fog sometimes forms near the shore.
Anticyclone is the name given to a system of high pressure. The wind circulation around this system will be clockwise in the Northern hemisphere and anticlockwise in the Southern hemisphere. Anticyclones are associated vital small pressure gradients and consequently light Winds.
As they are areas of subsidence rainfall is unlikely.
They may be classified as either cold or warm and each of these may be either permanent (long lasting) or temporary.
When the high pressure is brought about by the air over an area being denser than that nearby a cold anticyclone is formed, cold air having a greater density than warm air. On the other hand a high pressure can be formed by larger than normal amounts of warm air over an area, in which case a warm anticyclone is formed.
The cold anticyclone gives brilliant frosty weather in the centre but near the outer regions dull foggy weather is frequently experienced, as there is usually a temperature inversion close to the surface.
The subsiding air warming adiabatically forms the
inversion. The best known of the cold
anticyclones is that over
Temporary cold anticyclones occur between the middle latitude depressions and bring a welcome though brief spell of very bright weather. Occasionally the ridge with which the temporary anticyclone is associated will intensify to build up an anticyclone composed entirely of cold air. If this occurs over the land in winter it may merge with the continental high and be kept as a cold anticyclone by continuous cooling of the land.
If it occurs over land in summer or over the sea at any time it is hardly likely to persist as a cold anticyclone it may collapse or become a warm anticyclone.
Warm anticyclones occur when the air in the troposphere is warmer than the surrounding air and an excessive depth of this warm air causes the high pressure.
Permanent warm anticyclones are those, which are found over the oceans and are generally referred to as the subtropical highs. They are composed of warm dry air and the visibility is excellent.
Temporary warm anticyclones may occur when there is an extension from the sub-tropical high or when a temporary cold anticyclone warms adiabatically. Over the land the air is very dry and by day fine hot weather will be experienced during the summer. By night there may be sufficient radiation cooling to give fog, this being this being particularly likely during the autumn.