How High and Low Air Pressure Drives Your Daily Weather

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Low Pressure = Bad Weather, High Pressure = Good Weather

Highs are generally associated with clear conditions, and lows, with unsettled weather. NOAA NWS NCEP WPC

Regardless of whether you analyze weather maps on a daily basis or simply watch them on the 6 o'clock news, two things are likely true: you know that the blue H's and red L's stand for high and low pressure; and you know that anytime high pressure moves in you can count on blue skies, whereas when it comes to low pressure, you can expect rain.

While this relationship between air pressure and weather conditions may be common knowledge, the reason why low pressure is linked to cloudy, rainy (and snowy) weather, and why high pressure is linked to clear conditions, isn't as easily understood. By the end of this slideshow, it will be!

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It's All About Airflow

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The reason why lows bring unsettled weather, and highs, fair weather, has to do with how air behaves and moves around each one. It's a scientific rule-of-thumb that air flows from areas of higher pressure toward areas of lower pressure. Well, whenever a low or high-pressure feature moves into an area, you can expect air to move in the horizontal, across. These horizontal motions actually generate vertical motion of air overhead too -- and it is this vertical motion of air that initiates weather making.

Let's take a look at the surface winds around and resulting upward movement of air aloft from a low and high center.

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Low Pressure Promotes Rising Airflow

Air at the Surface "Piles Up"

Let's consider a surface low-pressure system. From our knowledge of winds, we know that air flows from areas of higher pressure toward areas of lower pressure, so we expect air from surrounding areas to be directed inward toward the low center. This inward-directed airflow is opposed by Earth's rotation, the Coriolis force, which deflects it to the right in the Northern Hemisphere. The resulting, deviated wind blows counterclockwise around the center of low pressure. This net inward spiraling. Here's how...

As more and more air converges (comes together) in this way at the low's location to help "fill it in", it can't move downward because of the ground below, so it piles upward; it must rise in height to allow for more air now occupying. (This process creates a "taller" and much heavier column of air.) As it rises, the water vapor within it cools and condenses, producing clouds and eventually precipitation -- the very reason why low-pressure centers are related to unstable conditions and stormy weather!

Air Overhead Rises

Once the air reaches the upper atmosphere, it diverges (spreads out). Because this spreading out of air aloft eventually works its way back to the surface and is included again in the inflow of air that this action actually helps to continuously "feed" the surface low-pressure center.

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High Pressure Promotes Sinking Airflow

Air at the Surface Spreads Away

In contrast to low-pressure systems, which have.... high-pressure systems have more air pressure than their surroundings. As a result, they're constantly pushing air away from them into areas that have lower pressure. This leads to diverging winds (winds that spread out) at the surface.  spirals outward around the surface high-pressure center in a clockwise direction in the Northern Hemisphere (due to the Earth's rotation and friction).

Air Overhead Sinks

As air near the surface spreads away from the high, air from above sinks to replace it. In general, sinking air dries out an air mass. Anytime air descends, it compresses and warms. And since warm air can "hold" more water vapor, clouds moisture tend to evaporate. Thus, clear, cloudless, sunny skies, light winds, and generally fair weather are typical anytime a high-pressure system dominates a region.