The general global distribution of the cyclone tracks. A glance at the map shows that the general direction of movement of temperate cyclones is from west to east with frequent trends towards the southeast to northeast.
In other words, the mid-latitude cyclones are subjected to the general westerly flow of atmosphere in the temperate zone. Even though there is no definite path which most of the cyclones follow, it is at least true that there are certain tracks which are most commonly followed.
The heavy concentration of storm tracks in the vicinity of the Aleutian and Icelandic lows is the most important feature of the world distribution of the paths followed by the middle-latitude cyclones.
However, each individual storm follows its independent path. These storm tracks, as they are called, vary with the changing season. Like the latitudinal shifting of the wind and pressure belts, there is a definite seasonal shifting of the paths of cyclones.
During winter months the opposing air masses have greater contrasts in their physical properties, so the winter cyclones that develop in the middle-latitude zone are greater in number and are more intense.
This accounts for the greater changeability of weather in the temperate zone during the winter months than summer season, when atmospheric circulation becomes sluggish.
Since most of the cyclones and anticyclones in the westerly wind belt move from west to east, people in those regions look to the west for the coming weather. The weatherman for the same reason takes into consideration the weather conditions to the west, rather than to the east of his observatory.
It is to be pointed out that the flow aloft is also an important factor so far as it determines how rapidly the pressure systems advance and what direction they follow.
On the average, cyclones may cover a distance of about 1000 km per day. However, the rate of movement of an individual storm varies from 500 to 2000 km per day. Cyclones invariably move towards higher latitudes, whereas the anticyclones originating in middle latitudes mainly travel equator-ward to the subtropical region.
Most of the temperate cyclones originating in the North Pacific off the eastern coast of Asia move northward toward the Gulf of Alaska, where they merge with the Aleutian low. The winter storms follow a more southerly route in the Pacific and move as far south as Southern California.
Most of the Pacific weather disturbances dissipate on reaching the windward slope of the Rockies. However, some of these storms regenerate on the eastern side of these high mountain chains.
The most favourite areas for the rejuvenation of winter storms are Colorado and Alberta. Cyclones forming in Canada move southward towards the Great Lakes region and then turn towards northeast and move out into Atlantic Ocean. The Great Lakes region is the stormiest region in North America.
In North America there are three predominant cyclone tracks which are frequented by extra-tropical cyclones;
(1) The cyclone path along the boundary of Canada and the United States.
(2) Cyclones moving from western Canada or the North Pacific move southeastward into the Mississippi Valley and thence northeastward to the Great Lakes, the New England States, or the St. Lawrence Valley.
(3) Cyclones originating in the south- western region travel towards east to the Mississippi Valley and then northeastward to New England.
A few storms originating in North America are so developed that they make trans-Atlantic journey to the continent of Europe. There are several storms of high intensity that move across the British Isles and enter into Russia.
As pointed out earlier, many of the Pacific storms become occluded by the time they reach the west coast of North America and are unable to cross the mountains. But some of them cross the Rockies and re-develop on their leeward side.
There are at least four source regions in North America where the temperate cyclones originate and develop:-
(1) The area lying to the east of the Sierra Nevada range where weak winter depressions from.
(2) Another favoured location for cyclone regeneration is over eastern Colorado. Storms developed in this region are called Colorado Lows. Some of these storms are very severe and increase in intensity as they move towards the Great Lakes region. They produce heavy rain and snow in the region.
(3) The third area of storm development lies to the east of the Canadian Rockies. Storms of this area are called Alberta lows which are very severe. These storms are associated with blizzards and cold wave.
(4) The Great Lakes region is the breeding place for numerous winter storms because of steep temperature gradient there. These storms generally unite with the Alberta storms and increase their intensity.
The Gulf of Mexico is another region where a number of storms originate. They move northward on the west side of the Appalachians. These storms move northward with the Gulf Stream and ultimately merge with the Icelandic low.
A zone extending from Iceland to the Barents Sea is also a favoured area for the origin and development of a number of mid-latitude cyclones. These disturbances have their trajectory eastward over northern Europe and even Siberia.
In continental Europe the largest number of winter cyclones form over the Baltic Sea. Heat given off by the large expanse of warmer sea surface stimulates the formation of cyclones here.
The sharp contrast between the temperature of the Baltic Sea and the adjoining land areas leads to the formation of storms in this region. Under similar conditions numerous storms form over the Mediterranean basin.
They move northeastward reaching Soviet Russia or travel to the east as far as northern India. Most of the winter rainfall of the Middle East countries and northern India is produced by the passage of these storms.
In summer all the paths of the cyclone shift towards north with the result that there are no temperate cyclones found anywhere in the subtropical region and the warm temperate zone. A high concentration of storms is witnessed in the higher latitudes of the Bering Strait and the American and Russian sub-Arctic and Arctic regions.
In the southern hemisphere, the Antarctic frontal zone and the associated storms occur all the year round around the Antarctic ice pack.