Earth’s atmosphere is in perpetual motion movement which is striving to eradicate the constant differences by temperature and pressure between different parts of the globe. It is this motion that produces the winds and storms with which we are all familiar. This circulation plays a basic part in maintaining a steady state in the atmosphere and generating the climatic zones that characterize the earth. So far we have considered the upward movements which transfer energy from the surface to the atmosphere.
The basic force causing atmospheric motion is the pressure gradient; this gradient arises from the unequal heating of the atmosphere by solar radiation. At the equator- the “firebox” of the circulation, as it has been called- solar radiation is converted into heat. The air expands and rises and flows out towards the poles. Cool, demure air from the poles returns to replace it. At depth, the flow is reversed. So long as this unequal heating is continued, the cellular flow is maintained.
The air circulation within the tropics consists of two cells. Air blows in towards the low-pressure belts of the equator (the equatorial trough) across the subtropical seas. The equatorial air then diverges and flows polewards, so the potential energy is exported to higher latitude.
|There are four main surface wind belts that can be distinguished, viz–|
(I) The equatorial trough, which is a shallow trough of low pressure generally situated near the equator.
(II) The trade winds, which lies between the equatorial trough and the subtropical highs. This zone occupies nearly half the globe, much of it ocean, within that area the steady trade wind provides a stable and relatively constant climate.
(III) The Westerlies lies in the mid-latitudes and produces cyclones and monsoon.
(IV) Polar Easterlies, which are linked with shallow polar anticyclones.
The surface wind of any location can be depicted by “windrose” which represent wind speed, direction, percentage of the calm period over the season in any location. There are a variety of weather events that occur every year. These are floods, tornadoes, hurricanes, blizzards or even heatwaves. It is to be further noted that weather is a description of physical conditions in the atmosphere (humidity, temperature, pressure, wind and precipitation), then climate is the pattern of weather in a region over long time periods. The interactions of atmospheric systems are so complex that climatic conditions are never exactly the same at any given location from one time to the next. While it is possible to discern patterns of average conditions over a season, year, decade or century, complex fluctuations and cycles within cycles make generalizations difficult and forecasting hazardous. We always wonder whether anomalies in local weather patterns represent normal variations, a unique abnormality, or the beginning of a shift to a new region. When climatic change is gradual, species may have time to adapt or migrate to more suitable locations. Where the climatic change is relatively abrupt, many organisms are unable to respond before conditions exceed their tolerance limits, the whole community may be destroyed and if the climatic change is widespread, many species may become extinct.