An area of high pressure is created where the air is cold. The cold air sinks, pushing down and creating high pressure. This causes the air molecules to be squashed together, creating heat. As the air warms up, it tends to bring warm and pleasant weather.
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Since surface air pressure is a measure of the weight of the atmosphere above any location, a high pressure area represents a region where there is somewhat more atmosphere overlying it. High pressure areas are usually caused by air masses being cooled, either from below (for instance, the subtropical high pressure zones that form over relatively cool ocean waters to the west of Califormia, Africa, and South America), or from above as infrared cooling of winter air masses over land exceeds the warming of those airmasses by sunlight. As the airmass cools, it shrinks, allowing air from the surroundings to fill in above it, thus increasing thte total mass of atmosphere above the surface, which then results in higher surface barometric pressures. The pressure difference between the high pressure area and its lower-pressure surroundings causes a wind to develop flowing from higher to lower pressure. But because of the rotation of the Earth, the wind is deflected to the right (in the Northern Hemisphere) which then causes the wind to flow in a clockwise direction around the high pressure zone. |
In an anticyclone, air masses drop extensively. At the same time, the air warms itself up, so that no condensation and consequently no cloud formation can take place. Near to the ground, the air flows out of the anticyclone in the direction of depression – it diverges. Hence, there is no formation of fronts in altitude. During the subsidence of the air masses, an inversion forms. That is where the clouds are dissolved.
An anticyclone is builded quiet slowly. The forces of circulation in the subtropic areas lead to stable anticyclones.
Because of the differences in the origin or development, the anticyclones are divided into three categories:
A cold anticyclone originates if air cools off, for example, in winter above a cool land mass (e.g., Central Asian high). Then the air has a bigger density and exerts a higher pressure on the base. In the middle latitudes, it can also originate in the form of flat wedges in the back of cyclones as a ridge of high pressure.
A dynamic anticyclone is generated by the Rossby-waves (Polar front, Jet Stream). The dynamic Azores anticyclone exerts, on this occasion, a big influence on the weather of Central Europe.
A high anticyclone is an anticyclone which appears at big heights and is thus shown in high weather maps. It is always connected with a ground low-pressure area, because with the warming of the surfaces, the vertical pressure gradient is lowered and reflects itself the relative atmospheric pressure reduction on the ground with increasing height in a pressure relatively higher to the horizontal surroundings. Hence, one can derive the other way around a height low-pressure area also from a ground anticyclone (also thermal anticyclone).
Picture Credit : Google