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|
|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|
|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|
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.
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.