Atmospheric Circulation

Atmospheric pressure

• Atmospheric pressure also determines when the air will rise or sink.
• The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure.
• Unit: Milibar.
• Sea level average Atmospheric pressure: 1,013.2 milibar.
• Measured by Mercury barometer or the Aneroid barometer.
• The pressure decreases with height.
• Isobars are lines connecting places having equal pressure.

Vertical variation of pressure

• Pressure decreases rapidly with height. The decrease amounts to about 1 mb for each 10 m increase in elevation.

Standard Pressure and Temperature at Selected Levels

Level	Pressure  in mb	Temperature  °C
Sea Level	1,013.25	15.2
1 km	898.78	8.7
5 km	540.48	-17.3
10 km	265.00	- 49.7

• The vertical pressure gradient force is much larger than that of the horizontal pressure gradient.

Horizontal distribution of pressure

• Studied by Isobars

World Distribution of Sea Level Pressure

• Equatorial low - Near the equator the sea level pressure is low.
• Subtropical highs - 30° N and 30° S are found the high-pressure areas.
• Sub polar lows - pole wards along 60° N and 60° S, the low-pressure belts.
• Polar high - the pressure is high at Poles.
• Pressure Belt not permanent in nature.
• Pressure in Northern Hemisphere
• Winter move Southward.
• Summer move Northward.

Forces Affecting the Velocity and Direction of Wind

1. Gravitational force act downward.
2. Pressure Gradient Force.
• Strong where the isobars are close to each other.
• Weak where the isobars are apart.
• 
3. Frictional Force.
• Greater at Surface (up to elevation of 1 – 3 km), Minimal at Sea.
4. Coriolis Force (Due to Rotation of Earth).
• Rotation of the earth about its axis affects the direction of the wind. Right direction in the northern hemisphere. Left in the southern hemisphere.
• Coriolis force is directly proportional to the angle of latitude.
• Maximum at the poles and is absent (Zero) at the equator. At equator, wind blows perpendicular to the isobars. (reason why tropical cyclones are not formed near the equator).
• The Coriolis force acts perpendicular to the pressure gradient force. The pressure gradient force is perpendicular to an isobar.
• Higher the pressure gradient force → Higher velocity of the wind → larger is the deflection.
• As a result of these two forces operating perpendicular to each other, in the low-pressure areas the wind blows around it.
• 

Pressure and Wind

• Geostrophic Wind - Isobars are straight and when there is no friction, Pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar.
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• The wind circulation around a low is called cyclonic circulation. Around a high it is called anti cyclonic circulation.
• 
• Over low pressure area - air will converge and rise.
• Over high pressure area - air will subside from above and diverge at the surface.
• Rising of Air due to convergence, Convection Currents, Orographic uplift & uplift along fronts – also essential for formation of Cloud.
• 

General circulation of the atmosphere

1. The pattern of planetary winds largely depends on:
2. Latitudinal variation of atmospheric heating.
3. Emergence of pressure belts.
4. The migration of belts following apparent path of the sun.
5. The distribution of continents and oceans.
6. The rotation of earth.

Hadley Cell - Easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ). Such circulations from the surface upwards and vice-versa

Ferrel Cell or Mid-Latitude Cell - Middle latitudes the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high.

Polar Cell - At polar latitudes the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies.

Seasonal Wind

• Monsoons, especially over southeast Asia.

Local Winds

• Differences in the heating and cooling of earth surfaces and the cycles those develop daily or annually can create several common, local or regional winds.

Land and Sea Breezes

• During the day the land heats up faster and becomes warmer than the sea. Over the land the air rises giving rise to a low pressure area, whereas the sea is relatively cool and the pressure over sea is relatively high. Pressure gradient from sea to land is created and the wind blows from the sea to the land as the sea breeze.
• In the night the reversal of condition takes place. The land loses heat faster and is cooler than the sea. The pressure gradient is from the land to the sea and hence land breeze results.

Valley & Mountain Breeze

Air Masses

- An air mass is a large body of air with generally uniform temperature and humidity. The area over which an air mass originates is what provides its characteristics. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below. As such, air masses are associated with high pressure systems.

- Each of the two divisions are then divided based upon the temperature content of the surface over which they originate.

• Arctic air masses, designated by the letter 'A', are very cold as they originate over the Arctic or Antarctic regions.
• Polar air masses, designated by the letter 'P', are not as cold as Arctic air masses as they originate over the higher latitudes of both land and sea.
• Tropical air masses, designated by the letter 'T', are warm/hot as they originate over the lower latitudes of both land and sea

Fronts

• Fronts are identified by change of temperature based upon their motion. With a cold front, a colder air mass is replacing a warmer air mass. A warm front is the opposite affect in that warm air replaces cold air. There is also a stationary front, which, as the name implies, means the boundary between two air masses does not move.
• Cold Front

• Warm Front