As in case of other materials, the air in the earth's atmosphere is built of molecules that move about incessantly. Greater is the air pressure where denser concentration of molecules is found. Since air molecules are governed by gravity, the greatest air pressure is found nearest the earth's surface and with altitude that decreases. The atmospheric pressure at sea level is about 14 and a half pounds on each square inch about which are normally not much conscious.
If air pressure is measured in hectopascals or millibars, normal air pressure ranges from 980 to 1040 hectopascals while on local conditions that is very much dependent.
At different speeds these molecules incessantly move while the speeds depend on temperature, lower the temperature slower is the peed and for higher temperature faster is the speed. The molecules speed up as a mass of air is heated and that results in expansion of air. The density of the molecules decreases as expansion takes place and thereby the air mass becomes lighter. The air mass that has become lighter starts rising now. Convection is the term for the entire process.
All through the world convection is going on constantly in the atmosphere. However neither regular nor uniform is the process. It depends on many local factors whether convection will take place, and if it does at all, how much that will be.
As air rises it cools. Therefore in due course the rising warm air masses get cooled down and to the ground that its starts sinking back. With the rise of air, low pressure results. High pressure is the result where it is sinking. As equilibrium is the goal of atmosphere, air moves from high pressure areas to low pressure ones. Wind is this very movement of air. More the difference between the high pressure and low pressure zones, more forceful winds would be the result.
An elliptical orbit the earth or the third rock from the sun follows and to complete that the time it takes is one year. The time to complete one orbit marks stretch of the earth's year. Since elliptical is the earth's orbit, in certain parts of the year it really comes closer to the sun. However that does not cause seasons even though more solar heat the earth receives then.
The earth's movement is not limited to its travel through space in its own orbit, but it spins also on its axis like a top. In every 24-hour one spin it completes. Alteration between day and night is caused by the earth's rotation. Weather conditions differ between day and night as is reflected in high and low temperatures and the effect of that is felt on weather patterns too.
On its axis the planet earth is tilted about 23 degrees and that causes really the change of seasons. Due to tilting different parts of the earth come closer to the sun as around the sun the earth travels in its orbit. Therefore during certain parts of the year different parts of the earth receive different amounts of heat.
In December the Northern Hemisphere receives less light since the North Pole is tilted away from the sun then. Thus the days become shorter there while the temperatures go down.
But as the earth moves on its orbit, North Pole comes close to the sun due to earth's tilt. Till June 21 it continues to come closer to the sun and then the summer reaches its maximum. On that very day the sun is just above the Tropic of Cancer. Around the sun the earth continues to move until December 21 when the winter reaches its peak and the cycle is complete.
Just opposite are the seasons in the Southern Hemisphere even though the same cycle it experiences. In December the Southern Hemisphere is the closest to the sun and therefore it is summertime there. Quite logically June21 is the date marked for winter peak in the Southern Hemisphere.
The middle and high latitudes only, i.e., above 23.5 degrees north and below 23.5 degrees south, enjoy four seasons precisely. Normally for the tropics only wet and dry seasons are the two seasonal variations.
