Atmospheric Pressure and Winds
The air, that is a mixture of several gases, exerts pressure through its weight. Air pressure or atmospheric pressure is defined as total weight of a mass of column of air above per unit area at sea level.
Distribution of Atmospheric Pressure
Vertical distribution : The density ,of air and the atmospheric pressure is high at the lower layer of the atmosphere. Atmospheric pressure decreases with increasing height. Though there is no direct relation between the increasing height and the rate of decrease in atmospheric pressures, yet in the troposphere the rate of decrease in air pressure is 34 mb for every 300 m of height.
Horizontal Distribution and Pressure Belts
- Equatorial low pressure belt (10°N-10°S) : This is a belt of very low atmospheric pressure. The equatorial low pressure belt is thermally induced, because the ground surface is intensely heated during the day due to almost vertical sun rays and thus the lower most layers of air, coming in contact with the heated land, also gets warmed. In this zone, there is almost no horizontal movement of air. The air in this belt rises up. This belt represents the zone of convergence of N-E and S-E Trade winds. Because of frequent calm conditions, this belt is called a 'Belt of calm' or 'doldrums'.
- Sub-Tropical High Pressure Belt (231/2° - 35° in both the hemispheres) : This belt owes its origin to the rotation of the earth and sinking and settling down of winds. It is. thus, mainly dynamically induced. The air rising at the equatorial belt starts blowing poleward. But, these winds get deflected towards east due to rotation of the earth. This phenomenon was first discovered by the French scientist Coriolis, hence this force exerted by the rotation of the earth is called coriolis force. The quantity of the force keeps increasing with increasing distances from the equatorial belt. This zone of high pressure is called 'Horse Latitude' because of prevalence of frequent calms. In ancient times, the merchants carrying horses in their ships had to throw out some of the horses while passing throuth this zone of calm in order to lighten their ships. This is why this zone is called 'Horse Latitude'.
- Sub-Polor Law Pressure Belt (45° - 661/2° in the hemispheres): This belt is also dynamically induced. In fact, the surface wind speads outward from this zone due to rotation of the earth and low pressure is caused. In this zone, the air coming from the sub¬tropical and polar high-pressure zones converge and rise up, creating a zone of low pressure. This zone is characterized cyclonic storms.
- Polar High Pressure Belt (Near the North and the South' Poles) : High pressure persists at the poles throughout the year because of prevalence of very low temperature all the year. The air subsides in this zone due to intensive cooling and results into high density.
This horizontally moving air is called wind. Blowing winds are an effort on the part of nature to balance the pressure differences at various places. The air blowing almost vertically is called air current.
The direction of surface winds is usually controlled by the pressure gradient and rotation of the earth. Because of the rotation of the earth along its axis, the winds are deflected does not blow at right angle of the isobars. The force which deflects the direction of winds is called Coriolis force. Because of Coriolis force, all winds are deflected to the right clockwise in the Northern Hemisphere, while they are deflected to the left anti-clockwise in the Southern Hemisphere with respect to the rotating earth. Since this phenomenon was firstly proved by a French scientist Ferrel, it is called Ferrel's Law.
Types of Winds
Based on their characteristic, winds are classified as:
- Prevailing or Permanent or Planetary winds
- Seasonal winds
- Local winds
Permanent or planetary winds
The winds blowing almost in the same direction throughout the year are called permanent or planetary winds. Trade winds, Westerlies and Polar winds are included under it.
Trade winds : These are the permanent winds blowing in both the hemispheres from the subtropical high-pressure belts to the equatorial low pressure belt. The word "Trade' is derived from a German word which means 'a fixed path' or 'track'. Hence, trade winds are the winds having fixed paths. These winds have north-eastely direction in the Northern Hemisphere, while they have south-easterly direction in the Southern Hemisphere. Near the equator, these converge and rise causing convectional rainfall in the equatorial region.
Westerlies : These are the permanent winds blowing from the sub-tropical high pressure, belts to the sub-polar low pressure belts in both hemispheres. The general direction of Westelies is South West to North East in the N. Hemisphere and North West to South East Southern Hemisphere. These winds are developed in the 40°-65°latitudes. In this Southern Hemisphere, because of lack masses and dominance of ocean, their velocity so high that they are called Roaring Forties (40°S), Furious Fifties (50°S) and Shriekig Sixties (60°S). These names are given by the sailors who were being effected by those westerlies.
Polar winds: Winds blowing from the high-pressure belts to the sub-polar low belts, in both the hemispheres, are called polar winds. These winds are north-easterly in the Hemisphere and south-easterly in the Hemisphere. Due to very low temperature humidity bearing capacity of polar winds is very low. When these winds meet the Westerlies sub-polar regions, polar fronts are develop, temperate cyclones are generated.
The winds which change their direction of blowing with the changing seasons are called seasonal winds. They are also called temporary winds, Monsoon winds, sea breeze, land breeze, mountain breeze and valley breeze are included under it.
Monsoon winds : The entire surface winds, which change their directions with changing seasons are called monsoon winds. These winds blow from sea to land in summer and from land to sea in winter.
Land and Sea Breezes : Land is heated more quickly than'the adjacent sea during the day time. As a result, warm air rises over the land creating low pressure area there. However, there is prevailing high pressure over the adjacent sea. As a result, the pressure gradient causes the air to blow from the high pressure to the low pressure areas i.e. from the sea to the land. This is called sea breeze.
Mountain and Valley Breezes : The slope:, and valley floors in the mountainous regions are more heated through insolation during daytime. "Consequently, the warm air moves upslope. This upward moving breeze during daytime is called Valley Breeze. Valley breezes reach mountain peaks and many a time yield precipitation. In the night, the upper part of the mountain cools quickly and starts falling down along the slope of the mountain. This is called Mountain Breeze.
These winds blow due to local variation in the temperature and pressure, and influence a very small area. Some important local winds are :
- Chinook : Chinook means the 'snow-eater' (adopted from the language of Red Indians). This is the hot and dry wind blowing along the eastern slope of the Rockies and covers an area from the southern part of Colorado in the south to British Columbia in Canada in the North.
- Foehn : This is similar to Chinook and blows along the northern slope of the Alps. It affects the Switzerland most.
- Sirocco : This is a warm, dry and dusty wind which blows in northerly direction from the Sahara Desert and after crossing over the Mediterranean Sea reaches Italy, Spain etc., where it is also known as Blood rain because of its redish sand brought alongwith it from Sahara desert. There are different local names for Sirocco in Africa e.g. 'Khamsin' in Egypt, 'Gibli' in Libya and "Chilli' in Tunisia, ijn Spain and Canary, and Madeira islands, it is known as 'Leveche' and 'Leste' respectively.
- Black Roller : These are the warm and dry dusty winds, blowing in the great plains of North America.
- Yoma : This is the warm and dry wind like 'Santa Ana', blowing in Japan. Temporal: This is the monsoon wind blowing in the Central America.
- Simoom : This is the warm and dry wind blowing in the Arabian Desert. It causes storms and obstructs visibility.
- Samoon: This is the wind blowing in the Kurdistan region of Iran and Iraq and has the characteristics similar to Foehn.
- Karaburan: These are the dust laden fast blowing winds in the Tarim Basin in the Central Asia.
- Harmattan : This is the warm and dry wind blowing from north-east and east to the west in the Sahara desert. The weather becomes suddenly dry and pleasant in the western coast of Africa, at the arrival of Harmattan. Therefore, it is called 'Doctor' in the New Guniea.
- Brick fielder: This is the warm and dry wind blowing in the Victoria province of Australia.
- Norwester : This is the warm, and dry wind blowing in northern New Zealand.
- Loo : This is a hot and dry wind blowing in the northern India from the north west and west to the east. It is sometimes called 'heat wave'.
- Santa Ana : This is the warm and dry wind blowing in California (USA).
- Zonda : This is a warm wind blowing in Argentina and Uruguay, from the Andes to the plains. This is also called 'cool Foehn'.
- Mistral : This is the cold local wind blowing in Spain and France from north-west to south¬east direction. While blowing through the narrow valley of the Rhone River.
- Blizzard : It is a violent stormy cold polar wind laden with dry snow and is prevalent in north and south Polar Regions. These winds affect Canada and USA.
- Pampero : These are the cold polar winds blowing very fast in the pampas region of South America.