by David Murphy
In a way, the weather you experience on this planet can be summed up as the result of the interactions between high pressure systems and low pressure systems.
Every day, the air circulating around the highs slowly flows downhill toward the lows. The lows then raise that air up quickly to the top of the atmosphere, where it can find its way to the top of another high and begin the cycle again. Along the way, the moisture in the air changes back and forth between gas-state water vapor to liquid-state water (and sometimes ice). And we see alternating areas of sunny, dry weather and cloudy, wet weather. The general flow of winds across the face of the earth is also determined by the position of highs and lows, as the circulations around the two are exactly the opposite and very predictable.
As a rule, air always moves in a clockwise direction around the center of a high in the Northern Hemisphere. In the Southern Hemisphere, it's the opposite. This is because of the direction in which the earth rotates on its axis and the way that motion encourages air to move across its spherical surface. Within the tranquil area of a high pressure system, there is very little additional force available to counteract this natural motion and so, the air flows freely and gently at the whim of the earth's movement.
The opposite is true of low pressure centers. Once the air drops down toward the center of a low, other forces (including gravity and air pressure) cause the air to make a turn in toward the center of the low, against it's natural tendency. Again, this turn is counterclockwise in the Northern Hemisphere.
In this way, lows and highs interact with each other constantly, one affecting the position, strength and movement of the other. Additional factors come into play, including temperature, the position of the jetstream and even latitude. But in general, it's the lows and highs that are the major features on any basic weather map---and the basic triggers for whatever weather is to come.