Cyclones are the centers of low pressure surrounded by closed isobars having increasing pressure outward and closed air circulation from outside towards the central low pressure in such a way that air blows inward in anticlockwise on northern hemisphere and clockwise in southern hemisphere. They range in shape from circular, elliptical to V shape. From locational viewpoint cyclones are classified into two principal types e.g. i) extra-tropical cyclones/temperate cyclones ii) tropical cyclones.
What Is Temperate Cyclone?
Temperate cyclones are atmospheric disturbances having low pressure in the centers produced in the middle latitudes characterized by converging and rising air, cloudiness and precipitation. They are formed in the regions extending between 350- 65″ latitudes in both hemispheres due to convergence of two contrasting air masses e.g. After their formation temperate cyclones move in easterly direction under the influence of westerly winds and control the weather conditions in the middle latitudes.
Shape, Size and Speed
Temperate cyclones are of different shapes e.g. circular, semi-circular, elliptical, elongated or V, but all of them are characterized by low pressure in their centres and closed isobars. The pressure difference between the centre and periphery is about 10-35 mb. It means that pressure increases from the centre towards outer margin. Average large diameter of an ideal cyclone is about t 900 km while short diameter measures t 000 km. The temperate cyclones move eastward under the influence of westerly winds with average velocity of 32 km per hour in summer and 48 km per hour in winters.
Temperate Cyclone Diagram: A generalized temperate cyclone in northern hemisphere.
Since there is low pressure in the centre of temperate cyclone and air pressure increases outward and hence winds blow from the periphery towards the centre but these winds do not reach the centre straight rather they cut the isobars at the angle of 20° to 400 due to friction and Coriolis force and thus wind direction becomes anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. Since temperate cyclones are formed due to convergence of two contrasting air masses and hence it is natural that there are variations in the nature and direction of winds in different parts of the cyclones.
Different temperatures are noted in different parts of temperate cyclones because of their origin due to convergence of two thermally contrasting air masses. The southern part of cyclone records higher temperature because of the dominance of warm air while the north-eastern, northern and north-western parts record low temperature because of the dominance of cold polar air mass. The western part records lowest temperature.
The areas frequented by temperate cyclones mostly lie in the middle and high latitudes extending between 350-650 latitudes in both the hemispheres. These cyclones move, on an average, in easterly direction.
Cyclones after originating in the north Pacific off the north-east and eastern coasts of Asia move in easterly and north-easterly direction towards the Gulf of Alaska and ultimately merge with Aleutian Lows from where they follow southerly direction and reach as far south as southern California. The cyclones moving inland dissipate and are occluded at the windward western slopes of the Rocky Mountains
Origin of formation
Though the formation and development of temperate cyclones is a quick process but it passes through a series of successive stages. The period of a cyclone from its inception. (cyclogenesis) to its termination (proteolysis or occlusion) is called the ‘life cycle of cyclone’; which is completed through six successive stages.
- First stage: The first stage involves the convergence of two air masses of contrasting physical properties and directions. Initially, the air mass (warm and cold) move parallel to each other and a stationary front is formed. This is called initial stage.
- Second stage: Second stage is also called as ‘incipient stage’, during which the warm and cold air masses penetrate into the territories of each other and thus a wave-like front is formed.
- Third stage: This is the mature stage when the cyclone is fully developed and isobars become almost circular.
- Fourth stage: Warm sector is narrowed in extent due to the advancement of cold front than warm front, as cold front comes nearer to warm front.
- Fifth stage: Starts with the occlusion of cyclone when the advancing cold front finally overtakes the warm front and an occluded front is formed.
- Sixth stage: Warm sector completely disappears, occluded front is eliminated and ultimately cyclone dies’ out.