Air Pressure

Air pressure is defined as the pressure exerted by the weight of air on the earth’s surface. As we go up the layers of atmosphere, the pressure falls rapidly. The air pressure is highest at sea level and decreases with height.

Horizontally, the distribution of air pressure is influenced by temperature of air at a given place. In areas where temperature is high the air gets heated and rises. This creates a low-pressure area. Low pressure is associated with cloudy skies and wet weather.

In areas having lower temperature, the air is cold. It is therefore heavy. Heavy air sinks and creates a high pressure area. High pressure is associated with clear and sunny skies.

The air always moves from high pressure areas to low pressure areas.  Horizontal movement of air in response to difference in pressure is termed as wind while vertical or nearly vertical moving air is called air current. Both winds and air currents form the system of circulation in the atmosphere.

Wind

Wind is the horizontal movement of air in response to differences in pressure. The movement of air from high pressure area to low pressure areas is called wind. Winds can be broadly divided into three types:

  1. Planetary or Permanent winds: The trade winds, westerlies and easterlies are the permanent winds. These blow constantly throughout the year in a particular direction.
  2. Seasonal or Periodic winds: These winds change their direction in different seasons.
  3. Local winds: These blow only during a particular period of the day or year in a small area. For example, land and sea breeze.

Pressure Gradient and Winds

There is a close relationship between the pressure and the wind speed. The greater the difference in air pressure between the two points, the steeper is the pressure gradient and greater is the speed of the wind. The gentler the pressure gradient slower is the speed of the wind.

Coriolis Effect and Wind

Winds do not cross the isobars at right angles as the pressure gradient directs them. They get deflected from their original paths. One of the most potent influences on wind direction is the deflection caused by the earth’s rotation on its axis. Demonstrated by Gaspaved de Coriolis in 1844 and known as the Coriolis effect or coriolis force. Coriolis force tend to deflect the winds from there original direction.

In northern hemisphere winds are deflected towards their right, and in the southern hemisphere towards their left. This is known as Farrel’s law. The Coriolis force is absent along the equator but increases progressively towards the poles.

Planetary Wind Systems

Planetary or permanent winds blow from high pressure belts to low pressure belts in the same direction throughout the year. There are three main planetary wind systems on the surface of the earth:

  1. Trade Winds
  2. Westerlies
  3. Polar Winds

Trade Winds or Easterlies

The Trade Winds blow from the sub-tropical high towards the Equatorial low but as they are deflected by the rotating earth, they blow from a north-easterly direction in the Northern Hemisphere (the North-East Trades) and from a South-easterly direction in the Southern Hemisphere (the South-East Trades). As the trade winds tend to blow mainly from the east, they are also known as the Tropical easterlies.

Westerlies

The Westerlies comprise the airflow from the sub-tropical highs to the temperate lows, from about 35° to 60° North and South. These belts also move north and south with the seasonal change of the pressure belts.

Polar Winds (Easterlies)

Cold air tends to move towards equator from the polar highs. As the air moves towards equator, it is deflected to the left, and it results a belt of winds spiraling from the polar region from a South-easterly direction.

Major Local Winds Across Globe

Name Nature of wind Place
Chinook (Snow eaters) Hot, dry wind The Rockies mountains
Foehn Hot, dry wind The Alps
Khamsin Hot, dry wind Egypt
Siroco Hot, moist wind Sahara to the Mediterranean Sea
Solano Hot, moist wind Sahara to the Iberian Peninsula
Harmattan (Guinea Doctor) Hot, dry wind West Africa
Bora Cold, dry wind Blows from Hungary to North Italy
Mistral Cold wind The Alps and France
Punas Cold dry wind Western side of Andes Mountain
Blizzard Cold wind Tundra region
Purga Cold wind Russia
Levanter Cold wind Spain
Norwester Hot wind New Zealand
Santa Ana Hot wind South California
Karaburun (black storm) Hot dusty wind Central Asia
Calima Dust-laden dry wind Saharan Air Layer across the Canary Islands
Elephanta Moist wind in monsoon Malabar coast