Origin and structure:
Anticyclones are high pressure systems around which the wind blows clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere. There are various types of anticyclones such as the cold-core anticyclones of the high latitudes and the warm-core anticyclones of lower latitudes.
According to some other meteorologists there is a third category of anticyclones which are described as the sluggish systems filling the spaces between moving temperate cyclones.
Cold anticyclones of the middle latitudes are also called ‘polar outbreak highs’. Sometimes the last member of a cyclone family draws cold air masses from the sub-polar regions in its rear part. These rapidly moving anticyclones move southward towards the lower latitudes.
This produces the cold waves so often experienced in the southern parts of the temperate regions. When they enter into the subtropical regions, they undergo a gradual transformation and ultimately become warm anticyclones.
In North America they originate in northern Canada and, move southward and southeastward across the central eastern United States. In Asia they originate in the eastern part of Siberia and move towards China and Japan.
The exact mechanism of the formation of anticyclones is still not clear. But the most probable cause of their formation seems to be the radiational cooling of the layers of atmosphere lying close to the snow-covered surface.
According to Trewartha, the southward surge of extremely cold and dense polar air is caused by the combined effect of an upper-air long wave and an expulsion of cold Arctic Basin air aloft.
Since there is subsidence within these anticyclones, there is subsidence inversion produced in the atmosphere which results in atmospheric stability. Under certain conditions, an anticyclone may undergo distinct development and may become intense.
These developments are invariably associated with intense cyclonic activity in the neighbouring areas. Even then the anticyclones never develop such intensities as are acquired by well-developed cyclones.
However, it is to be remembered that the individual anticyclones are made up of different types of air masses at different times. Therefore the weather associated with them always shows different characteristics.
But one characteristic is always shared by all the anticyclones, i.e. they are never affected by advection from extraneous sources. The weather produced by any anticyclone is very much regional and diurnal in character.
Anticyclone circulation is characterized by subsidence and surface divergence. Anticyclone wind system is not so well developed as is the case with a cyclonic circulation. In the eastern part of a moving anticyclone, there are north-westerly winds, while on the westward side or on the rear the southeasterly winds prevail.
Pressure gradient is never so steep and the wind-velocities are never as high as in a cyclone. On the other hand, anticyclones have always high winds. The front part of cold-core anticyclones is always marked by cold waves and blizzards in the middle latitudes.
Unlike cyclones, the anticyclones are always associated with scanty rainfall. Subsidence and divergent wind system within an anticyclone do not favour condensation and cloud formation.
But in case the southeasterly air is moist, there may be some precipitation on the rear of an anticyclone.
On occasions, the cold northwesterly winds may produce snowfall. Whatever uncertainty in weather is produced, it is because the anticyclones are often capped by cyclonic circulation aloft.
The surface temperature conditions in an anticyclone depend upon temperature of the air masses involved, humidity of air, and season of the year.
In winter, the cold anticyclones originating in the snow-covered sub-polar legions always bring with them very low temperatures and blizzards which render the winter chill unbearable. The middle-latitude anticyclones always produce the lowest temperatures of the season.
In summer, the stagnant type of warm anticyclones associated with the air of subtropical or tropical origin produce extremely high temperatures, called ‘heat waves’. Clear weather allows the maximum receipts of solar radiation during the day. Tropical air masses carry heat to the north as the high pressure system moves into the sub-polar regions.
Since the anticyclone conditions favour clear weather, the diurnal range of temperature is bound to be large.
The regions of origin and paths of movement of the cold and warm anticyclones are different. Sub-polar regions give birth to cold anticyclones which always move towards the south.
The warm anticyclones generally move from west to east. However, the source regions as well as the tracks followed by anticyclones tend to shift towards the north in summer and south in winter.
In winter, there are two regions of high frequency of cold anticyclones: the extensive plateau of the Rocky Mountains in northwestern Canada and east central Siberia. The states of Nevada, Utah, and Idaho have the largest number of anticyclones.
The area extending from Alaska to the Great Plains has high frequency of high pressure systems. These anticyclones are cold and shallow highs comprising the polar continental air (cP).
The cold Canadian anticyclones, which travel from their centers of origin to the Middle Atlantic States, bring with them cold waves, blizzards or snow storms and lowest temperatures to the Mississippi Valley. Some of these anticyclones push their way to the Gulf States where they are called ‘norther’, the most dreaded weather phenomenon.
Cold anticyclones originating in east central Siberia travel towards northern China and reach the mouth of the Yangtze River. They sometimes cross over to Japan.
In Europe, there are only a few cold anticyclones moving southward from the Peninsula of Scandinavia.
In summer, as stated earlier, the storm paths and their centers of maximum frequency shift towards the north. The Great Lake region has a maximum of anticyclone frequency in summer, whereas the cold anticyclones of the winter months avoid this region because of intense cyclonic development there.
In eastern Asia too, the number of anticyclones is greatly reduced. Some of the feeble anticyclones move across China in summer. Now, the subtropical anticyclones follow a more northerly route.
Their frequency over the eastern Pacific is high, and there are only a few high pressure systems in the western part.
This is so because in this part of the Pacific the summer monsoon dominates the weather drama. A number of warm subtropical anticyclones pass over the Mediterranean Sea.
It may be noted that in winter the anticyclones form with a great regularity in the higher latitudes of the continents. Since these anticyclones result from radiational cooling of the earth’s surface, they are also called thermal anticyclones.
In the upper troposphere they either disappear or shift towards the lower latitudes. On the contrary, the anticyclones that form over the ocean surface have warm air in their upper parts. Such anticyclones are called dynamic anticyclones. These anticyclones maintain their vigour up to considerable heights.
The warm season anticyclones are invariably associated with clear weather, but some of the winter highs produce cumulus or cumulonimbus clouds. In winter, the anticyclonic conditions are conducive to the formation of radiation fogs during night.
On the rear of these anticyclones warm and moist air currents from over the oceans produce advection fogs. On certain occasions, the day-time surface heating causes the dissipation of fog at the surface, but the remaining fog still persists at higher levels.
When looked at from the surface of the earth, these upper-level fogs appear as low stratus clouds. These foggy layers do not allow the light rays to reach the surface in toto.
In the middle-latitude regions this phenomenon of subdued day-light is termed the ‘anticyclonic gloom’.