| We know that if the atmosphere did not exist, | | | | between the high pressure and low pressure |
| the earth would go without weather. Imagine for | | | | zones, more forceful winds would be the result. |
| a moment for argument's sake that the earth | | | | An elliptical orbit the earth or the third rock from |
| has an atmosphere indeed, as it is today, though | | | | the sun follows and to complete that the time it |
| the sun is absent. Absence of sun would mean | | | | takes is one year. The time to complete one orbit |
| absence of weather and that is not only because | | | | marks stretch of the earth's year. Since elliptical is |
| a frozen solid form the atmosphere will take. | | | | the earth's orbit, in certain parts of the year it |
| Actually the interaction between heat of the sun | | | | really comes closer to the sun. However that |
| and our atmosphere builds air masses, while | | | | does not cause seasons even though more solar |
| circulation patterns appear due to that. Pressure | | | | heat the earth receives then. |
| differences motivate air movements and wind is | | | | The earth's movement is not limited to its travel |
| the end result of the same. | | | | through space in its own orbit, but it spins also on |
| As in case of other materials, the air in the earth's | | | | its axis like a top. In every 24-hour one spin it |
| atmosphere is built of molecules that move about | | | | completes. Alteration between day and night is |
| incessantly. Greater is the air pressure where | | | | caused by the earth's rotation. Weather conditions |
| denser concentration of molecules is found. Since | | | | differ between day and night as is reflected in |
| air molecules are governed by gravity, the | | | | high and low temperatures and the effect of that |
| greatest air pressure is found nearest the earth's | | | | is felt on weather patterns too. |
| surface and with altitude that decreases. The | | | | On its axis the planet earth is tilted about 23 |
| atmospheric pressure at sea level is about 14 and | | | | degrees and that causes really the change of |
| a half pounds on each square inch about which | | | | seasons. Due to tilting different parts of the earth |
| are normally not much conscious. | | | | come closer to the sun as around the sun the |
| If air pressure is measured in hectopascals or | | | | earth travels in its orbit. Therefore during certain |
| millibars, normal air pressure ranges from 980 to | | | | parts of the year different parts of the earth |
| 1040 hectopascals while on local conditions that is | | | | receive different amounts of heat. |
| very much dependent. | | | | In December the Northern Hemisphere receives |
| At different speeds these molecules incessantly | | | | less light since the North Pole is tilted away from |
| move while the speeds depend on temperature, | | | | the sun then. Thus the days become shorter |
| lower the temperature slower is the peed and for | | | | there while the temperatures go down. |
| higher temperature faster is the speed. The | | | | But as the earth moves on its orbit, North Pole |
| molecules speed up as a mass of air is heated | | | | comes close to the sun due to earth's tilt. Till June |
| and that results in expansion of air. The density of | | | | 21 it continues to come closer to the sun and |
| the molecules decreases as expansion takes place | | | | then the summer reaches its maximum. On that |
| and thereby the air mass becomes lighter. The air | | | | very day the sun is just above the Tropic of |
| mass that has become lighter starts rising now. | | | | Cancer. Around the sun the earth continues to |
| Convection is the term for the entire process. | | | | move until December 21 when the winter reaches |
| All through the world convection is going on | | | | its peak and the cycle is complete. |
| constantly in the atmosphere. However neither | | | | Just opposite are the seasons in the Southern |
| regular nor uniform is the process. It depends on | | | | Hemisphere even though the same cycle it |
| many local factors whether convection will take | | | | experiences. In December the Southern |
| place, and if it does at all, how much that will be. | | | | Hemisphere is the closest to the sun and |
| As air rises it cools. Therefore in due course the | | | | therefore it is summertime there. Quite logically |
| rising warm air masses get cooled down and to | | | | June21 is the date marked for winter peak in the |
| the ground that its starts sinking back. With the | | | | Southern Hemisphere. |
| rise of air, low pressure results. High pressure is | | | | The middle and high latitudes only, i.e., above 23.5 |
| the result where it is sinking. As equilibrium is the | | | | degrees north and below 23.5 degrees south, |
| goal of atmosphere, air moves from high | | | | enjoy four seasons precisely. Normally for the |
| pressure areas to low pressure ones. Wind is this | | | | tropics only wet and dry seasons are the two |
| very movement of air. More the difference | | | | seasonal variations. |