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Introduction

1. A little about the atmosphere and pressure

Though it may seem as if air does not have any weight, it does. Air is made of bodies that have a certain air pressure. And if you have taken a look at a weather map, you will probably remember numerous wavy line, often in a circular pattern. These lines are called isobars and they represent areas with the same air pressure.

You are probably wondering why atmospheric pressure is different in different places. This is so because the temperature as you move from place to place. Because of the temperature differences, warm air rises because it is lighter than cold air. As the warm air rises, the pressure is lowered at the surface, somewhat like a 'sucking' action. Suddenly, cold air moves in these 'empty' spots to fill the void, creating pressure differences.

2. A little about the circulation of water

Water evaporates from plants, animals, oceans and other bodies of water forms water vapor in the sky. This water vapor then rises high into the sky because it is lighter than air and the it cools and condenses to form clouds. If it condenses even more and the conditions are right, then that water falls as rain to the earth.

Want to know why water condenses? Since air cools as it rises, it soon becomes saturated at a certain height. For instance, if air is about 70 o F, then it can hold four times as much water as air which is at 35 o F.

Different cloud formation can sometimes give away the forecast for the near future.

3. A little about warm and cold fronts

A front is the interface where cold air meets its opponent, warm air. Warm air being the less dense soon "takes over" the cold front by rising over it. When the warm air rises, it goes under the same metamorphosis as water when it rises; it forms water vapor, then clouds, and then, sometimes, rain.

If you remember seeing spiral patterns on weather maps, they occur where at the "battlefield", that is, the front. This spiraling motion appears because of the rotation of the earth; if the earth was flat, then the interface would be a straight line. However, because of the coriollis effect [what scientists call this effect], things move west to east in the northern hemisphere and east to west in the sothern hemisphere.