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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.

4. A little about trade winds

Winds "happen" when pressure differences makes warm air rises and cold air sink and replace the warm air. The spherical shape of the earth controls all of the large scale movements.

Not surprisingly, the greatest upward movement [warm air] is around the equator since that part receives the most direct sunlight making the air warmer than other parts. Cooler air comes in from the poles where not as much sunlight is received and replaces the warm air. This creates the ever famous trade winds.

As the warm air rises and moves towards the poles, it starts to cool [because of higher elevation], and then it starts to sink. Soon enough, it becomes part of the cold air mass. And the cycle continues. Of course, this is a simplification. Factors such as season, geography of areas, coriollis effect complicate this scenario.

Sometimes, high altitude winds start to blow strongly from west to east. These are called jet streams. They are usually as high as 35 kilometers from the ground. These air drastically effect the weather over the land.

Take the jet stream over North America for example. It separates the humid and moist air of the Gulf Mexico from the cold, dry air of Canada. However, if the jet stream moves south, then it brings with the cold, dry air greatly effecting northern USA by creating winter conditions.

In the summer time, it moves northward which pulls warm, moist gulf air into the states.