Oh, the pressure!

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(Originally written for Allen Publishing’s print products published on January 3, 2019)


Meteorologists talk about barometric pressure or atmospheric pressure when explaining the forecast. We use the terms “low pressure” and “high pressure” to describe the causes for different types of weather. We throw around terms like millibars as if everyone understands what we’re talking about, but do they? Not necessarily.

So, what is pressure in weather terms?

If pressure in general is the force exerted on a unit area, then atmospheric pressure is the force exerted by the atmosphere on a specific area. We use the term barometric pressure almost interchangeably with atmospheric pressure because we use barometers to measure atmospheric pressure.

There are different types of barometers and units used to quantify pressure. The two most common units are inches of mercury and millibars (mb). In a mercury barometer, a column of mercury is used to physically show the amount of pressure on the column. The column will expand or shrink dependent upon the pressure exerted on it by the air surrounding it. The measurement is taken in inches of mercury. Measured this way, the global average air pressure at the surface of the earth is 29.92 inches of mercury.

Millibars are used more often by meteorologists. They are the lines typically seen on weather maps of sea level pressure. It’s not really a metric system unit, although it would be easy to assume that’s the case since in most other situations, the two most common units are based on the British and metric systems. In fact, mercury barometers can give their readings in millimeters of mercury, so they are the metric system version as well as the British system version of pressure reading.

Instead, a millibar is a pressure unit of 1,000 dynes per square centimeter, which is an old way of measuring pressure. Millibars are convenient for reporting atmospheric pressure despite the fact that you don’t see it used for much else. Measured this way, the global average air pressure at the surface of the earth is 1013.25 mb.

When the atmosphere exerts more force per area, we call the pressure “high.” When it exerts less, we call the pressure “low.” Often during the winter, high pressure systems in the mid-latitudes will exert pressure in the 1030 mb to 1040+ mb range. Low pressure systems crossing the continent can bring pressure in the range around 990 mb. By contrast, at her peak intensity, Hurricane Florence’s pressure was measured at 939 mb.

Atmospheric pressure plays a huge role in our weather. High pressure brings clear skies because the air is sinking to ground level. Low pressure brings unsettled weather – possibly even stormy – because the air is rising, which creates clouds and rain. The area directly under the center of a high-pressure system often has calm or very light winds. The area between a system of high pressure and one of low pressure can be quite breezy because the air wants to move from high pressure to low pressure. The closer the two systems are to one another, the windier it is.

In the Northern Hemisphere, the winds around a high-pressure system flow in a clockwise direction away from the center, and the winds around a low-pressure system go counter-clockwise toward the center. Remember, the air wants to go toward where the pressure is lower.

The movement of these systems determines the hour-to-hour and day-to-day forecasts for any given location. They bring our weather.

One thing to note: for this discussion, I’m writing about the weather and the pressure at the earth’s surface. Meteorologists also consider the air pressure at different levels above the ground, which is why you sometimes hear us mentioning an upper level or mid-level low bringing a change in the forecast.

map example of pressure

Credit: National Weather Service. The blue H on the map represents the center of high pressure. The red L represents the center of low pressure. Winds rotate clockwise and outward from the high pressure center and counter-clockwise and inward toward the center of low pressure.