High & low pressure defined: How air moves at the surface

Weather Glossary

ROCHESTER, NY (WROC) – The News 8 weather team often refers to “high pressure” and “low pressure”, but what does this mean? Let us take a deeper dive into how pressure works and how this drives weather.

First, we need to remember that we live in an atmosphere. Air molecules exert a certain pressure on us and the surroundings. That pressure decreases with height until there is barely any pressure and we enter into space.

Image taken from NOAA. Millibars (mb) is the common way pressure is measured and is also more formally hectopascals (hPa). Inches of mercury is also another way pressure is measured.

Average surface pressure is 1013 mb (hPa) and this can vary significantly, down to 940mb, how low Hurricane Sandy reached, and up to a deep Canadian 1040 mb (hPa) high. These numbers can go even lower or higher depending on the air mass. Remaining at the surface, pressure differences develop within different air masses that have different temperature and moisture compositions. Colder air is more dense than warmer air, so it will inevitably have a higher pressure and be more compressed.

Colder, more dense air exists to our north. Warmer, less dense air exists to our south. Where these two meet can be where fronts develop around low and high pressure systems.

Now we will look down at earth, something you normally see when a meteorologist shows you an analysis map. When it comes to higher pressure at the surface, air wants to naturally flow back to equilibrium. That would mean that air is flowing away from high pressure at the surface. DO NOT FORGET THAT THE ATMOSPHERE IS 3D! If air is flowing away from high pressure, it is replaced by air above it, so we have sinking air within an area of high pressure. That will prevent from cloud formation and you can expect a lack of precipitation and even blue skies.

In this scenario, you would have clear skies over the “H” and precipitation forming around the “L”

Lower pressure next, the more exciting of the two if you like active weather. Air is flowing toward lower pressure in its never-ending quest for equilibrium. As air flows toward the low, it cannot simply just crash into itself. It must rise as it moves inward. That rising motion will lead to clouds and eventually precipitation!

Low means H2O —— High means the air is dry

Above is a nice way to remember the difference between the two. Generally low pressure means there will be some type of precipitation around and high pressure generally means you will not see any precipitation. These are over generalizations, but you get the idea. Below is a picture representation of how air flows in all three dimensions of the atmosphere.

Sinking motion results in clearing skies and rising motion results in expansion, condensation, cloud formation, and precipitation.

You have probably noticed how these two graphics show some rotation, and you will very rarely see wind flow directly from high to low pressure. The two main forces (besides the pressure gradient force) are the Coriolis force and friction. The Coriolis force is because of the rotation of the earth. That spins the wind direction and we get a counterclockwise (cyclonic) flow around low pressure and clockwise flow (anticyclonic) flow around high pressure in the northern hemisphere. The opposite exists in the southern hemisphere.

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