Temperate Cyclones

A temperate cyclone is referred as mid-latitude depressions, extra-tropical cyclones, frontal depressions and wave cyclones. Temperate cyclones are active above mid-latitudinal region between 35o to 65o latitudes in both hemispheres. The direction of movement is blow from west to east and more pronounced in the winter seasons. It is in these latitude zones the polar and tropical air masses meet and form fronts. Most of these cyclones form at wavelike twist or perturbation on these fronts. On weather map, cyclones are shown as a low pressure area enclosed by a number of isobars circular or elliptical in shape. It is also referred as cyclone or depression. When these isobars take an elongated shape, the pressure system is called as trough. These cyclones are mainly observed in Atlantic Ocean and northwest Europe. These cyclones have characteristics to develop over both oceanic and land surface. Much of the highly variable and cloudy weather come across in the temperate zone. Since mid-latitude is an area of convergence of different air masses, it leads to the formation of fronts as well as the cyclonic conditions are bound to happen

Fronts

They have similarity in terms of air pressure, horizontal temperature and moisture distribution. Based on temperature characteristics, air masses are of two types – warm and cold. Both of the air masses march forward and both of them converge. The convergence is known as fronts

Chritchfield defined front as “sloping boundary surface between contrasting air masses”. Strahler and Strahler have defined the front as “sharply defined boundary between itself and a neighbouring air mass”. According to Oliver and Hidor front is “identified as a zone of transition between airs of different properties”. In a very simple term, front is a narrow zone between two air masses like a frontier when the armies of two countries come to fight each other. As mentioned before, air masses are of two types – warm and cold, two types of fronts with distinct characteristics are formed in the process of marching forward and getting interacted with each other. Latter on the intermingling of the two gives birth to another one known as – occluded front.

Warm Front: In the northern hemisphere, the warm front (Figure 4) develops to the southeast; to the right side of the moving direction of warm air mass. In southern hemisphere, it develops to the southeast; to the left side of the moving direction of warm air mass. The warm front development is caused and modified due to the interaction of the air masses. On the warm front, the effect of warm air mass is pronounced, and hence, it is named so.

Cold Front: In the northern hemisphere, the cold front (Figure 4) develops to the northwest; to the right side of the moving direction of cold air mass. In southern hemisphere, it develops to the southwest; to the left side of the moving direction of cold air mass. The development is caused and modified due to the interaction of the air masses. On the cold front, the effect of cold air mass is marked, and hence, it is named as cold front.

Occluded Front: Occlusion means the blocking or sealing the existence and with respect to fronts. It signifies the removal of the fronts from the ground. Occlusion of the fronts occurs when the cold air mass occupy the ground space surrounding the hairpin turned clubbed warm and cold

Characteristics of Temperate Cyclones

Size and Shape

The temperate cyclones are asymmetrical and shaped like an inverted ‘V’. They stretch over 500 to 600 km. They may spread over 2500 km over North America (Polar Vortex). They have a height of 8 to 11 km.

Wind Velocity And Strength

The wind strength is more in eastern and southern portions, more over North America compared to Europe. The wind velocity increases with the approach but decreases after the cyclone has passed. The temperate cyclones are formed over a very large areas. The development of low pressure is not very intense. The isobars are relatively wideapart and therefore, the pressure gradient is smaller and hence, the velocity is moderate around 30 to 40 km per kour.

Frequency: Since the temperate cyclones are formed due to fornt formmation, they are occuring throughout the year. Therefore, they are not confined to any particular season but are slightly their frequency is less during summer in comparison to winter.

Structure

The north-western sector is the cold sector and the north-eastern sector is the warm sector (Because cold air masses in north and warm air masses in south push against each other and rotate anti-clockwise in northern hemisphere).

Areal Coverage: The temperate cyclones develop over a very large areas occupying more than 1500 or even sometime more than 2000 km in diameter. Its vertical dimensions reaches upto tropopause limit

Location:

Temperate cyclones exhibts wide variety of characteristics. They are formed between to 35˚ to 65˚ north and south latitudes in the sub-polar frontal zone and this is a result of interaction between cold polar mass and warm tropical air masss in the westerly wind belt. This type of cyclone is more pronounced in northern hemisphere due to greater temperature contrast created by land-water distribution. Climatologies suggest that, globally, mid-latitude cyclones predominantly form in one of two locations: downwind of major mountain ranges and near the eastern coastlines of continents.

Formation: The formation of temperatte cyclone is predominaently controlled and dominated by the formation of the fronts after coming into the contacts of two different air masses warm and cold. Thermal contrast of the two types of air masses is the genesis for the development of low pressure center and finally the creation of temperate/ extra-tropical cyclone. Their occurrence are possible over large arreas characterized by land or water or at the joining places of the two.

Air Pressure: As mentioned before, the pressure gradient in temperate cyclone is gentle as the isobars are widespread. Hence, the intensity of the winds in this type of cyclones are moderate. Generally the isobaric difference varies between 10 to 15 mb.

Orientation And Movement Jet stream plays a major role in temperate cyclonogeneis. Jet streams also influence the path of temperate cyclonesDirection of Movement: Since, the temperate cyclones are developed in the mid-latitudes areas where the westerlies are prominently blowing, the direction is, in general, from west to east. Due to Coriolis effects, the direction is not straight from west to east but follow the path from southwest to northeast in northern hemisphere. In southern hemisphere, it is from northwest to southeast direction. Therefore, they move from mid-latitude areas to high latitude areas. The direction of the tracks of this type of cyclone is governed by the surface roughness and the characteristics of the areas from where they pass. In general, it is curvilinear direction that they follow. With the passage of the cyclonic tracks, they get reduced, fronts weakened and finally they dissipate. In case the storm front is directed southwards, the center moves quite deep southwards-even up to the Mediterranean region [sometimes causing the Mediterranean cyclones or Western Disturbances (They are very important as they bring rains to North-West India – Punjab, Haryana