Pressure Equalizing Creates Wind
What causes the air to blow from place to place? Is there a giant ogre hiding in a cave somewhere blowing out birthday candles, or is something else at work? As you probably guessed, it is the latter.
Air movement is caused by differences in pressure from one location to another in the Earth’s atmosphere. It is the atmosphere trying to equal out pressures.
Consider a cup of water. What happens when you turn a cup of water upside down on a table? Unless the water is frozen into a solid chunk of ice, the water will flow outward into a puddle. It is impossible to make the liquid water retain the shape of the cup once the cup is removed. This is because without walls holding the water in, the liquid’s natural reaction is to flow as far outward as possible.
Unlike liquids, gases don’t stop once they reach the puddle stage. They keep spreading outward as far as possible. Without walls to hold them in, a gas will continue spreading outward forever, or until they run into other forces.
The only force acting to hold the gases in on Earth is gravity. Thus, all the gases of our atmosphere try to spread out until they are equally dense all around the globe. We call this balance homeostasis. These gases should have reached this state of homeostasis many billions of years ago. So why are things still blowing around today? There are important forces at work that keep the atmosphere from ever being able to stabilize.
The main force responsible for the inability of the gases of the Earth’s atmosphere to stabilize is the unequal heating of one portion of the atmosphere compared to another by the Sun.