The atmosphere is made up of molecules which all have weight, so at the surface all those molecules pressing down on us from above exert pressure.
The standard pressure that we feel at sea level is 14.7 pounds per square inch or 1013 millibars if you’re reading a weather map. We don’t feel all that pressure because air doesn’t just press down it presses in on us from all directions.. and because we’re used to it.
Pressure on the surface doesn’t stay constant. Upper level winds and and uneven heating of the atmosphere cause high and low pressure systems to develop, which we can see on a weather map like this one.
Air in a high pressure system is generally subsiding or sinking towards the surface. As the air subsides it compresses, warms up and its relative humidity decreases causing it to “dry out”. Which is why on high pressure days they’re usually aren’t many clouds.
In the northern hemisphere wind blows away from and rotates clockwise around the center of high pressure system. This wind rotation is caused the earths rotation and a thing called the Coriolis effect. In the southern hemisphere the direction of rotation is reversed.
In a low pressure system the air is generally rising. As air rises it cools causing its relativity humidity to increase which then often times causes clouds to from.
Wind blows towards the center and rotates in a counter clockwise direction around low pressure systems.
There is an easy way to remember that called “right hand rule” if you make a fist with your right hand with you thumb pointing the direction the air is moving your fingers will point in the direction of rotation.
So for low pressure the air is rising so our thumb points up and our fingers wrap into a fist in a counterclockwise direction and wind blows counterclockwise around low pressure systems.
So wind blowing from high to low pressure areas is the main cause of large scale winds.
On a pressure map like this one you can see the relative pressure difference by how closely squeezed together the iso bars are. The strongest winds are going to be found when the iso bars are really close together.
Besides generating wind, pressure systems affect thermal activity.
If there is high pressure we know the air is generally going to be sinking that means there are going to be less thermals and any thermals that do form are going to be smaller, more punchy and have sharper edges since they have to overcome the generally sinking air.
On low pressure days were much more likely to find areas of widespread lift and larger, softer thermals.