Fronts do not form everywhere. Their development is confined to certain well-defined zones. Fronts usually develop in those areas of the world where air masses have strong temperature contrasts. Therefore, all the major zones of frontogenesis are the regions of transition between the principal air mass sources.
The most important frontal zone is the Atlantic Polar Front which has its full development during the winter. The polar continental and the tropical maritime air masses by coming into close proximity create the polar fronts.
In winter there is a sharp contrast between the physical properties of these air masses. The Atlantic polar front extends up to Europe in the east. There is a wide variation is the position of this front. The West Indies and Portugal form the southern limit, whereas the Great Lakes and Iceland define the northern limit of this front.
Cyclones which are produced along this front account for the widespread precipitation over a large area extending from the eastern part of North America, through the North Atlantic to the northwestern part of Europe.
However, during the summer this front is less developed than in winter, because the temperature contrast between the air masses is kept at its minimum.
Next important front is the Atlantic-Arctic Front. Its formation takes place along the boundaries of the Arctic source region and the maritime polar air masses.
When relatively warmer maritime air meets the extremely cold Arctic air, strong contrasts in the air mass properties are produced. During winter all the storms produced on this front move out from Iceland to the Barents Sea via northern Norway.
The Arctic Front changes its location according to the season. In summer this frontal zone develops along the Arctic coasts of Siberia and North America. In winter over North America it develops on the border between the polar continental air and Pacific maritime air.
The third important zone of frontal wave development is known as the Mediterranean front. It is located over the Mediterranean-Caspian Sea region in winter. Its development takes place on the border between the cool polar continental European air and the winter air masses of North Africa.
The Mediterranean Basin presents ideal conditions to initiate the process of frontogenesis. Winter storms that develop here move towards the east and northeast; some move towards southern Russia, while others travel towards the Near East giving welcome winter precipitation over the arid regions.
Sometimes the frontal weather disturbances developing over the Mediterranean Basin travel up to Pakistan and north-west India. Remember that winter precipitation of the northern part of Indian subcontinent is produced by these winter cyclones.
Another frontal zone is known as the Pacific-Arctic Front. This zone extends from the Rocky Mountains to the Great Lakes region. The position of this front is highly variable, but it is true that in winter there is a general equator-ward shift.
The winter disturbances that develop along this front may move out to such long distances as Texas and northern Mexico. With the outbreak of the Arctic air in the rear of such storms, the most dreaded cold wave sweeps across the entire region of the Southern United States.
In winter there are two Pacific polar fronts-one near the coast of Asia, and the other near the coast of North America. The frontal storms that develop on these fronts control the weather of the entire region extending from the Gulf of Alaska to Southern California and Western Mexico.
The Pacific polar fronts owe their existence to the presence of two high pressure areas on the North Pacific. The winter rainfall along the Pacific coast of North America is produced by the storms developed on these fronts.
During summer the air mass contrasts are much weaker so that the permanent fronts are present only around the Arctic region. On the Atlantic side, the cold Arctic air sometimes travels far southward.
On the Pacific side of North America, the powerful subtropical high-pressure cell does not allow the formation of polar fronts in summer.
The polar fronts over the western Atlantic and Pacific develop 10° further north in summer that in winter. The summer polar fronts develop over Eurasia and middle North America.
In the southern hemisphere the average position of the Polar Front is about 45°S in January. In July there are two Polar Fronts – one originates over South America and the other at 170°W.
Depict the mean positions of different frontal zones at the two extreme seasons. These figures also present a generalized picture of the distribution of principal air masses over the surface of the earth. However, these air masses and frontal zones change their positions with the changing seasons.