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Role of air (atmosphere)

Life on earth is totally dependent on air. It also has an important role in making weather.
Why does the wind blow, and what makes it stop? How do clouds form and when does it rain or snow? Why is it colder on top of a mountain than in the valley below?

To understand weather, we need to first of all know a lot more about air – its distribution, temperature, pressure, humidity and movement. Extending for hundreds of kilometres above the earth’s surface, air becomes thinner and thinner as one goes higher up as for instance on mountain tops.

Let us focus our attention on some of air’s properties. The first being temperature. When the ground (land and water) gets heated by the sun, air just above the surface gets heated up. Hence air that is closest to earth is the warmest. But because the ground is unevenly heated by the sun, the air’s temperature is also not uniform. The temperature of air above land will be much higher than that above water during daytime, and at night it will be the opposite – air above land will be cooler than air above water. Of course if the water body is small, as for example a small pond, not much of difference can be noticed. It is only when we consider large water bodies, like the sea, that the temperature difference become appreciable. When warm air comes in contact with cold air, there is always an exchange of heat until the two air masses acquire uniform temperatures. Also warm air is lighter than cold air and hence in the process of achieving uniform temperature, air keeps circulating, warm air rising up and cold air sinking down. This air movement causes wind, for, wind is just moving air. Depending on the amount of air that is participating in such circulations, we can have a very faint breeze, or a strong wind. In summary, land is unevenly heated, hence air is unevenly heated, and in an effort to equalise temperatures, air moves causing winds or breezes.

If there is a temperature difference between different air masses, then these air masses will also have different pressures. For warm air expands and hence it occupies more space and hence it will have a lower pressure than cold air which occupies much less space. Air movement will always be aimed at equalising temperatures and consequently pressures. That is the reason you hear about storms associated with low pressure areas moving in towards high pressure regions.

As you can see, the sun and the earth are really the cause of winds. Without the sun, the air would not be heated, and part of the air must be heated to make it flow and circulate. Heat and air, working together, make winds.

Recalling that it is really the earth that heats up air and not the sun directly, it is now possible for us to understand why it is warmer close to the earth surface than above it. But what about mountains and hills? Why is it less warm on hilltops and mountain tops? Does the surface not get warmed up there; after all it also receives the same amount of sunlight?

Remember that air gets thinner and thinner as you go higher and higher. Thicker the air warmer it gets quicker. On mountaintops, the air is thin and hence it takes longer for it to get warm.

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