Wind direction and Wind Velocity

Wind direction and wind velocity are two main characteristics of winds. Wind direction is stated in terms of direction from which the wind is blowing and is measured with the help of an instrument called wind vane. Wind velocity is expressed by Beufort Scale. This scale was devised by Beufort in 1805, and it was modid in 1926. It is related to the descriptions of wind effects and estimated velocity at 10 metres above the ground.

Scale Description Wind Speed Observed effects
         
Number   Km/h Mph  
         
0 Calm 0 0 Smoke rises vertically
1 Light air 1-5 1-3 Wind direction shown by smoke draut not by vane
2 Light breeze 6-11 4—7 Wind felt on face, leaves subtle, vane moves
3 Gentle breeze 12-19 8-12 Leaves and small twigs in motion; a ag is extended
4 Moderate breeze 20-28 13-18 Raises dust; small branches move
5 Fresh breeze 29-38 19-24 Small trees sway; small crests on waves or lakes
6 Strong breeze 39-49 25-31 Large branches in motion; wind whistles telephone
wires        
7 Moderate gale 50-61 32-38 Whole trees in motion
8 Fresh gale 62-74 39-46 Branches twigs orees
9 Strong gale 75-88 47-54 Slight structural damage to houses
10 Whole gale 89-102 55-63 Trees uprooted; considerable structured damage
11 Strom 103-117 64-72 Widespread damage
12 Hurricane 118+ 73+ Devastation
         

Cold Front

When a cold air mass forces its way under a mass of warmer air and pushes the latter upward, the front will be called a cold front.

Occluded Front

When the cold front of an atmospheric depression overtakes a warm front and lifts other surface of the earth, a new front is formed which is called an occluded front. Meeting of warm and cold air masses in the temperate zone gives birth to temperate cyclones along the front of warm and cold air masses.

In each case precipitation is likely to occur, because warm air is rising over cold air. The duration and intensity of the precipitation along the two fronts are quite different. The cold front is steep and produces showery, and sometimes violent precipitation for a longer period of time. If the cold front moves faster than the warm front in such a trap, part or all the pocket of warm air may be lied from the surface, thus, producing an occluded front. After occlusion, the air masses lose earlier characters and from new fronts.

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