A cyclone is a low pressure area surrounded by high pressure areas from all sides. It is circular or elliptical in shape. Winds move from all sides to the central low. They assume anti-clockwise direction in the northern hemisphere and clock-wise direction in the southern hemisphere due to coriolis effect. Depending upon their area of origin, their nature and the weather associated with them, the cyclones are divided into following two broad categories:
(i) Temperate cyclones, or (ii) Tropical cyclones.
Also known as extra-tropical, or wave cyclones or depressions, these cyclones originate in the temperate zone extending from 35′ to 65′ north and south latitudes. Several theories have been propounded to explain the origin of temperate cyclones but the most accepted theory is the Polar Front theory. It was put forward by Vilhelm Bjerknes and his son Jacob Bjerknes, the two famous meteorologists of Norway near the end of World War I. According to this theory the temperate cyclones come tinto being due to collision of two air masses of contrasting characteristics of temperature and humidity.
They meet each other at about 60′ latitude. Here they do not merge with each other readily, rather they from a front known as Polar Front. The cold air mass pushes the warm air mass upwards and a void is created due to decrease in pressure. The air from the surrounding areas rushes in to ll this void and a cyclone is formed.
A temperate cyclone assumes various shapes such as circular, semi-circular, elongated or V-shaped during the course of its growth and development. Most temperate cyclones are elliptical shaped on full growth. The average radius of a tempe rate cyclone varies from 400 m to 800 km; but several cyclones extend over several thousand kilometres. In some extreme cases the diameter of a temperate cyclone may vary from 150 km to 3000 km. The average wind velocity in winter is 40 to 60 km per hour which is reduced to 15-20 km per hour in summer. Generally the temperate cyclones move from west to east under the inuence of westerly winds. Their average speed of easterly movement is 32 km per hour in summers and 49 km per hour winters.
Temperate cyclones are associated with varying weather. Before the arrival of the cyclone, there are cirrostratus clouds. Sun looks more pale and there is Lunar Halo (circle around the moon). The arrival of the cyclone is indicated by rise in temperature and fall in atmospheric pressure. Wind direction becomes variable. Light rainfall continues for about 24 hours. The rain stops at the arrival of warm front. Pressure becomes stable and cloud cover becomes thin. The temperature starts falling after the warm front has crossed. This is an indication of the arrival of cold front. Again there are clouds in the sky and it starts raining.
Sometimes it is sky and it starts raining. Sometimes it is associated with hail and lightning. The sky becomes clear after the cold front crosses. Most of the temperate cyclones originate in the North Atlantic Ocean and North Pacific Ocean where they affect large parts of the U.S.A., Canada, west European countries like Britain, Norway, Sweden and vast areas of China and Japan. They also originate in the Mediterranean Sea and after crossing the Middle East countries reach Pakistan and North West India. The light winter rainfall caused by these cyclones in Punjab, Haryana, and western parts of Uttar Pradesh is extremely useful to rabi crops, especially wheat.
These cyclones originate in the tropical zone and de velop under the following conditions:
The central low pressure is known as ‘Eye of the Cyclone’. It is a region of calm with subsiding air having lowest pressure and highest temperature. Surrounding this area there is a zone of strong winds with clouds extending vertically. Surrounding the ‘eye’ is the ‘eye wall’. This is marked by a strong spiralling ascent of air to greater height reaching the tropopause. The wind reaches maximum velocity in this region, reaching as high as 250 km per hour. Torrential rain occurs here. The diameter of the storm over the Bay of Bengal, Arabian Sea and India Ocean is between 600-1200 km. The cyclone creates storm surges and they inundate the coastal low lands. The storm peters out on the land. It should be noted that there are no cyclones at the equator because coriolis force is nil there. Hence most of the tropical cyclones originate from 5′ to 30′ latitudes.
The chief areas of activity for the tropical cyclone are the following: