General Circulation of the Atmosphere

Since the atmosphere is heated from above by direct solar radiation, it heats the Earth’s surface unevenly. The tropics are hot, but are hotter further south due to proximity to the equator.

The extra heat makes the air rise there. The hot air moves away from the equatorial regions, generally northward or southward toward the Tropic of Cancer and Tropic of Capricorn.

The pattern of planetary winds largely depends on:

1. latitudinal variation of atmospheric heating;
2. emergence of pressure belts;
3. the migration of belts following apparent path of the sun;
4. the distribution of continents and oceans;
5. the rotation of earth.

The pattern of the movement of the planetary winds is called the general circulation of the atmosphere. The general circulation of the atmosphere also sets in motion the ocean water circulation which influences the earth’s climate.

The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created.

Another reason for sinking is the cooling of air when it reaches 30o N and S latitudes. Down below near the land surface the air flows towards the equator as the easterlies.

The easterlies from either side of the equator converge in the Inter Tropical Convergence Zone (ITCZ). Such circulations from the surface upwards and vice-versa are called cells. Such a cell in the tropics is called Hadley Cell.

At the surface these winds are called westerlies and the cell is known as the Ferrel cell. Such appearance of warm water off the coast of Peru is known as the El Nino.

The El Nino event is closely associated with the pressure changes in the Central Pacific and Australia. This change in pressure condition over Pacific is known as the southern oscillation.

The combined phenomenon of southern oscillation and El Nino is known as ENSO.

In the years when the ENSO is strong, large-scale variations in weather occur over the world.

The arid west coast of South America receives heavy rainfall, drought occurs in Australia and sometimes in India and floods in China. This phenomenon is closely monitored and is used for long range forecasting in major parts of the world.

Seasonal Wind

The pattern of wind circulation is modified in different seasons due to the shifting of regions of maximum heating, pressure and wind belts.

The most important factor influencing the weather of any region is its latitude. The Earth’s rotation draws air with it as it proceeds toward the equator.

It is therefore natural that air pressure zones, which create a pattern of movement of air masses, should extend from the equator toward either pole.

The most pronounced effect of such a shift is noticed in the monsoons, especially over southeast Asia. The other local deviations from the general circulation system are as follows.

Local Winds

The effect of earth surfaces heating and cooling on wind circulation patterns is one of the most basic elements of meteorology.

Surface winds are created by these thermal differences, not by the general movement of large masses of atmosphere or by differences in humidity or pressure.

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

Land and Sea Breezes

Land and sea breezes are wind and weather phenomena associated with coastal areas. A land breeze is a breeze blowing from land out toward a body of water.

Thus, 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.

Mountain and Valley Winds

In mountainous regions, during the day the slopes get heated up and air moves upslope and to fill the resulting gap the air from the valley blows up the valley. This wind is known as the valley breeze.

During the night the slopes get cooled and the dense air descends into the valley as the mountain wind.

The cool air, of the high plateaus and ice fields draining into the valley is called katabatic wind.

Air Masses

Air mass is a large volume of air in the atmosphere that is mostly uniform in temperature and moisture. It can be classified according to latitude and longitude, and whether it is moving or stationary.

Air masses differ in their horizontal temperature and humidity distribution. They can be cold or warm, and high or low relative humidity.

When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains.

The air with distinctive characteristics in terms of temperature and humidity is called an airmass.

It is defined as a large body of air having little horizontal variation in temperature and moisture. The homogenous surfaces, over which air masses form, are called the source regions.

The air masses are classified according to the source regions. There are five major source regions. These are:

1. Warm tropical and subtropical oceans;
2. The subtropical hot deserts;
3. The relatively cold high latitude oceans;
4. The very cold snow covered continents in high latitudes;
5. Permanently ice covered continents in the Arctic and Antarctica.

Fronts

When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis. There are four types of fronts:

• Cold
• Warm
• Stationary
• Occluded

Extra Tropical Cyclones

Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes.

Extratropical cyclones are responsible for the majority of weather throughout the world. When these systems emerge into warmer waters they are capable of reviving due to the release of latent heat. I

The systems developing in the mid and high latitude, beyond the tropics are called the middle latitude or extra tropical cyclones.

Extra tropical cyclones form along the polar front. Initially, the front is stationary. In the northern hemisphere, warm air blows from the south and cold air from the north of the front.

The cold front approaches the warm air from behind and pushes the warm air up. As a result, cumulus clouds develop along the cold front. The cold front moves faster than the warm front ultimately overtaking the warm front. The warm air is completely lifted up and the front is occluded and the cyclone dissipates.

Tropical Cyclones

Are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges.

One of the most devastating natural calamities. They are known as Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the Western Pacific and South China Sea, and Willy- willies in the Western Australia.

Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are:

1. Large sea surface with temperature higher than 27° C;
2. Presence of the Coriolis force;
3. Small variations in the vertical wind speed;
4. A pre-existing weak- low-pressure area or low-level-cyclonic circulation;
5. Upper divergence above the sea level system.

The energy that intensifies the storm, comes from the condensation process in the towering cumulonimbus clouds, surrounding the centre of the storm. With continuous supply of moisture from the sea, the storm is further snengthened. On reaching the land the moisture supply is cut off and the storm dissipates.