Besides the horizontal velocity of rotation and movement along the ground, the air in the revolving cloud has a large vertical velocity also. The air really has an upward spiralling motion. Estimates based on the weight of the objects lifted suggest upward velocities of one hundred to two hundred kilometres per hour. The pressure inside the tornado cloud is very low; it can be as much as thirty percent less than the atmospheric pressure prevailing outside the tornado cloud. This decrease of pressure as the tornado cloud passes over a building has a devastating effect. The outward force exerted by the air inside a building over which a tornado passes can be as much as three tons per square metre of the ceiling and the side wall. It is a suction effect of the tornado. If the tornado grazes one side of a wall, the wall will be pushed sideways into the cloud. The tornado passes over a particular object or a building in a minute or two. Within this short time, fantastic destruction is inflicted, buildings are destroyed. Individuals and vehicles on its path are lifted up by the vertical currents and left to fall to the ground safely or perched on tree tops or buildings. Trees are uprooted if directly on its path. Tornadoes occur frequently in temperate latitudes, especially over regions where there are no high east-west mountain ranges. They occur very rarely in India of hot weather and then only in the north. The east-west alignment of the Great Himalayan Range acts as a deterrent to the formation of tornadoes in the subcontinent by preventing the inflow of very cold air from the north. A tornado hit the north-west of the Indian state of Assam in the year 1963 which resulted in the killing of one hundred and thirty nine people and made three thousand seven hundred and sixty homeless in thirty three villages. This violent tornado travelled in the southern direction. In the year 1978, at least one hundred and fifty people lost their lives due to a tornado which mainly the villages of Keonjhargarh and Purunabandha in Orissa. In the year 1993 a tornado struck the state of West Bengal in India and in the month of March, 2009 a violent tornado hit the Rajkanika block of Kendrapada district in Orissa state which resulted in severe casualties. |