High pressure will be settling in for the weekend, and if you like sunshine, that’s a good thing. On the other hand, if you are concerned over the fact that most of North Carolina has slipped back into a moderate drought, it’s a bad thing. You’re probably well aware after a lifetime of watching the weather forecast that high pressure means fair weather and low pressure typically means storms, but do you know just what causes high and low pressure?
Air flows along the surface of the earth in all directions. We feel the airflow as wind. When winds that are moving in opposite directions meet, we have what’s called a convergence zone because the air converges in one place. This build up of extra mass has to go somewhere. It can’t go downward, obviously, because the earth is solid, so it must go up.
As the air moves away from the surface of the earth, it takes with it the extra mass - and then some. This leaves an area of lower pressure at the surface. Air that moves upward also takes energy and moisture with it, and what goes up must come down. If there is enough moisture rising, clouds will form, and, eventually, that moisture returns to the earth as rain.
Meanwhile, the air could rise until it hits the top of the troposphere, which is the layer of the atmosphere that we live in. The point where the air can rise no more is called the tropopause. The air now moves through the upper atmosphere as wind until it meets air moving in the opposite direction - and then must go somewhere, again. This time, it must come down. We call this motion sinking air, and when it reaches the surface bringing its additional mass with it, an area of high pressure is created.
In order for clouds to form, the air must be able to rise. So with sinking air, we have fair skies and high pressure.