You see it all the time during the news: a meteorologist pointing at a big blue H for high pressure or a big red L for low pressure. Here is an action shot of me during the noon newscast referring to high pressure over the Midwest.
One of my favorite things to ask kids on a school trip is: “what brings us sunny weather, high or low pressure?” About 80% of the time the kids will say low pressure which is incorrect. The reason why may not be pretty straightforward though.
Think of all of the air above you from the top of your head to space. That’s a lot of air pushing down on you and all of that force is known as pressure. Pressure = force/area. Obviously air pressure drops the higher you get because there is less air (weight) between you and space.
Back at the surface, if the air pressure is actively dropping, that means that air is being removed from the column above you and the weight on you is decreasing. If the air pressure is rising, more air or weight is being added to the column above you.
How do we add or remove air to change the pressure? The answer to that is convergence and divergence. Since a high pressure system has somewhat more atmosphere above it than a low pressure system, we need to get rid of some air. Therefore, air diverges out of high pressure at the surface. Since air cannot come out of the ground to achieve this, the air above a high pressure system sinks until it hits the ground and flows outward.
That air flows from the center of the high to the center of the low. Once the winds converge in the center of the low, they cannot go into the ground so the air must go upwards. As air from the surface rises, it cools until it eventually condenses to form clouds and rain.
You can think of this as a closed loop as wind travels from the center of the high pressure outwards toward the low pressure, then up, etc, etc. What begins this process of rising and sinking air? One factor is temperature: hot air rises and cold air sinks but that is a story for another blog post!
Therefore: high pressure = fair weather, low pressure = sour weather.