Pressure difference is the major cause of the genesis of the wind system.
Slope of pressure from high to low is called as Pressure gradient, also called Barometric slope.
Imaginary lines joining the points having same pressure is called Isobars. The direction of air movement should be perpendicular to isobars as direction of Pressure gradient is perpendicular to isobars, but it is deviated from the expected one due to Coriolis force caused by the rotation of the earth.
Wind direction and the related laws –
Because of Coriolis force all the winds are deflected to the right in the northern hemisphere while they are deflected to the left in the southern hemisphere, with respect to the rotating earth. This is reoffered to as Farrel’s law.
The Coriolis force is absent along the equator but increases progressively towards the poles.
Types of winds –
Permanent winds.
Trade winds.
Steady current of air blowing from sub-tropic high pressure belt towards the equatorial low pressure belt.
Under the influence of the Coriolis force they flow from the north-east in the northern hemisphere and from south-east in the southern hemisphere.
Westerlies.
Blows from sub tropic high pressure belt towards the sub polar low pressure belt in both the hemispheres.
General direction is south-west to north-west in northern hemisphere and north-west to south-east in the southern hemisphere.
They are best developed between 400-650 S latitude. These latitudes are often called Roaring forties, Furious fifties and Shrieking sixties.
Polar winds.
Blows from polar high pressure belt towards the sub polar low pressure belt.
Are north easterly in northern hemisphere and south easterly in southern hemisphere.
Seasonal winds.
Reverses its direction completely in duration of every six months.
Example: Monsoon winds etc.
Local winds.
Is horizontal movement of winds on small scale.
Movement is mainly caused by the local factors such as water body, mountain etc.
They may be hot, cold, dry or warm.
Example: Land breeze (warm), Sea breeze (cold), Chinook (warm), Loo (hot/dry), Kalbaisakhi/Bardoli chheerha (hot) etc.
Cyclones –
Low pressure surrounded by closed isobars having increased pressure outward and closed air circulation from outside towards the central low pressure.
Air blows inward in anticlockwise direction in the northern hemisphere.
Air blows inward in clockwise direction in the southern hemisphere.
Cyclone are mainly of two types:
Tropical cyclone.
Have thermal origin.
They acquire whirling motion due to coriolis force.
Wind speed varies from 0 km/h to 1200 km/h.
Temperate cyclone.
Develop over mid latitudes, range 350 to 650 N and S.
They are also called Extra tropical or Wave cyclones.
High pressure at centre and low pressure at the outer margins surrounded by circular isobars where wind blows.
Wind blows center to outward in clockwise direction in the northern hemisphere.
Wind blows centre to outward in anticlockwise direction in southern hemisphere.
Are high pressure systems, common in sub-tropical belt and practically absent in the equator region.
Generally associated with rainless fair weather and that’s why they are called as Weatherless phenomena.
Thunderstorms –
Are local storms, characterized by swift upward movement of air and heavy rainfall with loud thunder and lightning.
Structurally a thunderstorm consists of several convective cells which are characterized by strong updraft of air.
Tornado –
Very strong tropical cyclones of smaller size.
In Mississippi valley (USA) they are called twisters.
More destructive than cyclones.
Sometimes speed exceeds 320 km/h.
Jet streams –
The strong and rapidly moving circumpolar westerly air circulation in a narrow belt of a few 100 km width in the upper limit of troposphere is called Jet stream.
Their circulation path is wavy and meandering.
Extent of jet streams narrows down during the summer season because of their northward shifting while these extends up to 200 North latitude during winter season.