When wind moves with great speed and creates disturbance in the atmosphere, it is called a storm. Storms of summer are mainly caused by the rise in atmospheric temperature. Due to high temperature, air expands and there is a fall in the atmospheric pressure in that area. To balance it, air from colder regions where there is more pressure, rushes to the hotter regions. Carrying with it a large amount of dust and we call it a dust-storm.
Tornadoes, cyclones and hurricanes are some of the dangerous storms in which wind makes fast whirling motion. When the temperature of a small area rises to a high degree and, consequently, the atmospheric pressure falls, air from surrounding areas rushes in to fill this vacuum. Because of its fast velocity, wind’s motion in that area becomes whirling. The hot air goes up fast, forming a funnel-shaped cloud. This is called a tornado. The pressure inside a tornado is so low that any object coming in its way gets sucked in. Wind velocity of a tornado goes up to 200 kilometres per hour.
Hurricanes generally belong to tropical areas. They engulf areas within a radius of 80 to 320 kilometers and the wind-speed goes up to 120 to 200 km per hour. The central portion of the hurricane extending from 5 to 15 kilometers is completely calm. This is called the eye of the hurricane. When this portion reaches a particular area the air there comes to a standstill: This gives the impression that the hurricane is over. But, as soon as it leaves the area, it is followed by strong gusts of wind. Since hurricane also is a kind of whirling storm, it advances with a very fast whirling motion. It is called typhoon in east India and in the areas adjoining the Chinese Sea.
The cyclone is also a very dangerous storm. It devastates very large areas. It is caused by the rush of air towards low-pressure areas resulting in large-scale destruction. The violent cyclone which hit East Pakistan (now Bangladesh) on November 13-14, 1970 claimed lives of one million people.
