ANTICYCLONES, TORNADOES, MEASURING WIND

ANTICYCLONES This is the mass of air whose isobars also form an oval or circular 'shape but in which pressure is high at the centre, decreasing towards the outside. Winds in an anticyclone form a clockwise outspiral in the northern hemisphere, whereas they form an anticlockwise outspiral in the southern hemisphere.

TORNADOES Tornadoes are violently rotating storms, characterised by a funnel shaped cloud, in which winds whirl around a small area of extremely low pressure. A
tornado differs from a tropical cyclone in that it forms over the land. The movement of air is, however, similar to that in a depression, moving in spirally from all directions. It is more destructive than a cyclone as the speed of the winds is very high, exceeding 320 km per hour. Tornadoes occur mostly in the Mississippi Valley, and are sometimes known as twisters. They also occur in Australia and occasionally in some mid-latitude places.

MEASURING WIND Description. of wind requires measurement of two qualities: direction and speed. The direction of wind is measured by the wind vane. Wind direction is stated in terms of the direction from which the wind is coming. Wind speed is measured by the anemom­eter. For wind velocity of higher levels, hydrogen-filled pilot balloons called Rawinsondes are released, carrying a target that reflects radar waves and thus can be followed even when the sky is cloudy. Wind velocity is measured on a Beaufort scale, in miles per hour. But the Beaufort scale of numbers has now been largely replaced by a direct statement of wind velocity in knots.