General Awareness Wind System

Wind System

The pressure difference is the major cause of the genesis of the wind system. The air moves from high pressure to low pressure.

• The slope of pressure from high to low is called as pressure gradient, which is also called as barometric slope.

• The imaginary line joining the points having same pressure is called isobars. The direction of air movement should be perpendicular to the isobars because the direction of pressure gradient is perpendicular to the isobars, but the direction is deviated from the expected one due to coriolis force caused by the rotation of the Earth.

Wind Direction and Related Laws

• The coriolis force generated due to the rotation of Earth acts as a deflective force to the wind direction.

• Because of the 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 referred to as Ferrel's Law. The coriolis force is absent along the equator, but increases progressively towards the poles.

Types of Winds

(i) Permanent Winds

These winds include trade wind, westerlies and polar winds, which are as follow

• Trade Wind : The:;e are steady currents of air blowing from the sub-tropical high pressure belt towards the equatorial low pressure belt. Under the influence of the coriolis forces they flow from the North-East in the Northern hemisphere and from South-East in the Southern hemisphere.

• Westerlies : The permanent winds blowing from the sub-tropical high pressure belt to the sub-polar low pressure belt in both the hemisphere is called westerlies. The general direction of the westerlies is South-West to North-East in the Northern hemisphere and North-West to South-East in the Southern hemisphere.

• Because of the dominance of the land masses in the Northern hemisphere the westerlies become more complex and complicated and become less effective during summer seasons and more vigorous during winter seasons.

• Polar Winds : It blows from polar high pressure belt to sub polar low pressure belt. They are North Easterly in Northern hemisphere and South Easterly in the Southern hemisphere.

(ii) Secondary Wind Movements

Seasonal Winds : The winds, which reverse its direction completely every 6 months is called seasonal winds. The best example is monsoon winds.

`text(Jet Stream)`

• 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.

• The extent of the jet streams narrows down during the summer season because of their Northward shifting while these extend upto 20° North latitude during winter season.

Cyclone and Anti-Cyclone

`text(Cyclone)`

• Cyclones are the centres of low pressure surrounded by closed isobars having increasing pressure outward and closed air circulation from outside towards the central low pressure in such a way that a r blows inward in anticlockwise direction in the Northern hemisphere. Air blows inward in clock wise direction in the Southern hemisphere.

`text(Anti-cyclone)`

• They are the wind System, which has the highest air pressure at the centre and lowest at the outer margins surrounded by circular isobars where wind blows from centre to outward in clockwise direction in Northern hemisphere from centre to outward in anticlockwise direction in Southern hemisphere.
• They are high pressure system and common in sub-tropical belts and practically absent in the equator region. They are generally associated with rainless fair weather and that's why they are called as weatherless phenomena.

`text(Thunderstorms)`

Thunderstorms are local storms characterised by swift upward movement of air and heavy rainfall with cloud thunder and lighting. Structurally, thunderstorms consist of several convective cells, which are characterised by strong updraft of air.

`text(Tornado)`

Tornadoes are very strong tropical cyclones of smaller size. In the Mississippi valley (US), they are called twisters. They are more destructive than cyclones as the speed of winds is very high, exceeding 320 km per hour.

Humidity and Related Aspects

Humidity of air refers to the content of the water vapour present in the air at a particular time and place. Humidity is measured by an instrument called hygrometer. Another instrument used for the same purpose is sling psychometer.

The atmospheric humidity is expressed in a number of ways such as

• Humidity Capacity : The capacity of air of certain volume at certain temperature to retain maximum amount of moisture content.

• Absolute Humidity : The total weight of moisture content per volume of air at definite temperature is called absolute humidity.

• Specific Humidity : The mass of the water vapour in grams contained in a kilogram of air and it represents the actual quantity of moisture present in a definite air.

• Relative Humidity : It is the ratio of the amount of water vapour actually present in the air having definite volume and temperature (i.e. absolute humidity) to the maximum amount the air can hold (i.e. humidity capacity).

Relative Humidity `= text(Absolute Humidity )/text( Humidity Capacity ) xx 100`

• Condensation is the change of physical state of matter from gaseous phase into liquid phase and is the reverse of vaporisation.

• When the relative humidity reaches 100 % the air is completely saturated. The air temperature is said to be as dew point.

• Smog (Smoke + Fog) is a form of fog that occurs in areas, where the air contains a large amount of smoke.

• Fog is made from the droplets of water suspended in the lower layer of the atmosphere. Fog is not considered as a form of
precipitation. Visibility of less than 1 km is the internationally recognised definition of fog.

• Haze is formed by water particles that have condensed in the atmosphere and visibility lie between 1 km to 2 km.

• Frost is the moisture on the ground surface that condenses directly into ice, i.e., when condensation occurs below freezing point.

Precipitation

On the basis of its origin, precipitation may be classified int.o three main types :

`text( Convectional Precipitation)`

• It occurs daily in the afternoon in the equatorial regions. It is of very short duration but occurs in the form of heavy rainfall.

• It occurs through thick, dark and extensive cumulonimbus clouds. It is accompanied by cloud, thunder and lightening.

`text(Orographic Precipitation)`

• The wind ward slope receives the maximum amount of rainfall, whereas the leeward side receives less rainfall.

• The windward slopes of the mountains at the time of rainfall are characterised by cumulus clouds while leeward slope has stratus clouds. It can occur in any season.

`text(Cyclonic Precipitation)`

• Rainfall associated with the temperate cyclone occurs, when two extensive air masses of different physical properties
converge.

• In tropical regions two extensive air masses of similar physical properties converge to form tropical cyclones, wherein lifting of
air is almost vertical and very often associated with convection.

Clouds


 
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