2st PART

THE COMPOSITION OF THE ATMOSPHERE ATMOSPHERICAL PRESSION TRACK OF WINDS GENERAL CIRCULATION DIFFERENT TYPES OF CLOUDS DIFFERENT TYPES OF OPTICAL PHENOMENA JET-STREAM

Generale Circulation

The worldwide system of winds which transports warm air from the equator, where sunlight is most important, to the higher latitudes is called the general circulation of the atmosphere, causing climate zones of the Earth.

The general circulation of air is broken into three cells in each hemisphere :

- The Hadley cell,
- The Ferrel cell,
- The Polar cell.

Diagram of wind circulation in 3D with the cells and the names of the winds
- Trades winds ;
- Westerlies ;
- Polar front ;
- Hadley cell.

Sunlight is strongest near the equator. The heated air rises at this point and extends north and south. After the air which is cooled as is heavier, it fulls down back to the Earth's surface in the subtropical climate zone between latitudes 25° and 40°. This cool air descending stabilizes the atmosphere, preventing clouds formation and precipitation. Consequently, several climates of the desert in the world can be found in the subtropical climate zone. Surface air from subtropical regions returns to Ecuador to replace the rising air, thus completing the cycle of air circulation in the Hadley cell.

Although the physical reality of Hadley cells has been questioned, they provide an excellent way to describe how heat is transfered through the Earth by air movement. The general circulation is in use to transfer thermal energy from warm equatorial regions to temperate and colder polar regions. Without such latitudinal redistribution of heat, Ecuador would be much hotter and the poles would be much colder.

Without rotation of the Earth, the air would travel from north to south directly through the difference of temperature between the low and high latitudes. The effect of the Coriolis force due to the rotation of the Earth, however, swings the winds to their right in the Northern Hemisphere, and to their left in the Southern Hemisphere. So the movement of the air towards the Equator, a flow of Northeast in the southern hemisphere and Southeast in the Northern hemisphere create the trade winds in the tropical regions. The air that travels toward the poles constitute he westerlies associated with the belt of cyclonic systems of low pressure at about 50° to 60° north and south. In general, where the air goes down, high pressures develop, for example at subtropical latitudes or near the poles. Where air rises, air pressure is low, as in Ecuador and in the middle latitudes where thunderstorms or frontal systems develop.

Diagram of the horizontal circulation of the atmosphere in Juily : (yellow) anticyclone ; (blue) depression Click here to enlarge

Diagram of the horizontal circulation of the atmosphere in January: (yellow) anticyclone ; (blue) depression Click here to enlarge

The Intertropical Convergence Zone (ITCZ) moves with the inclination of the axis of
the Earth according to the seasons. Convective rains which go with the passage of
the Intertropical Convergence Zone are the fundamental source of precipitation
in locations from 10 to 23,50° North and South latitude. The animation above
shows the seasonal movement of the global circulation of the Earth.

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