What's the difference between high and low pressure?

Dateline article| David Murphy|

by David Murphy

If you like your weather nice and sunny, high pressure is generally your friend. If your garden needs rain, low pressure is what you're looking for.

An area of high barometric pressure (known simply as a High in weather lingo) is a stable air mass that is not likely to produce precipitation. In fact, it's shown on weather maps with a big blue H; blue for blue skies. Why do highs equal good weather? It's because air in the vicinity of a high is gently sinking toward the surface. Sinking air warms the lower it gets---and warmer air has a much greater ability to store water in its gas state. This means clouds (which are made of liquid water) are absorbed into the warming air and disappear (becoming invisible water vapor). And of course, the same thing happens to any precipitation; it dries up and dissappears.

On the other hand, air with a lower barometric pressure is usually more unstable and more likely to produce clouds and rain. In fact, lows are shown on weather maps as a big red L; red for trouble! Air within a low is generally rising and cooling, which means it can not hold onto as much gas-state water (water vapor). And so, much of its water vapor condenses and becomes clouds, as well as rain or snow.

Most important is whether the barometric pressure is changing over a given point. Meteorologists keep an eye on lowering pressure---and how fast it's lowering; useful information to have when you're trying to plot the direction of an approaching storm and the likelihood of precipitation. In fact, computer models rely on barometric pressure readings taken over a large area to help them forecast the movement and strength of storms.

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