Probing Pressure

Read...

Of all the atmospheric variables depicted on the station model, the most difficult to directly detect using our human senses is air pressure. Oh yes, you probably already know that changes in air pressure are at work whenever your ears pop on a drive up or down a mountain or during take-off and landing of an airplane. Also, some people with arthritis or bursitis experience discomfort when air pressure decreases as a storm approaches. But, with few exceptions, most folks can't physically sense that air molecules exert a whopping pressure on them. Even if you have a barometer in your home, this instrument only "passively" measures the pressure exerted by air molecules.

Okay, how much force do air molecules exert? In high school, you probably learned that pressure results when a force is exerted on an object (picture a boxer's jab against a punching bag). A large force exerted on a small area (such as a hockey player's weight pressing down on a tiny area of the ice in contact with his weight-bearing skates) results in a very large pressure. You also probably remember that, at sea level, the average air pressure is approximately 14 pounds per square inch! That amounts to a total force of over 1.5 tons on top of third base in the "hot corner" of the baseball diamond in Yankee Stadium (in case you're interested in making the calculation yourself, the dimensions of a baseball bag are 15 inches by 15 inches, which equals an area of 225 square inches).

For the record, pounds per square inch are not the preferred units for pressure in this course and in several other circles. For example, many home barometers measure pressure in inches of mercury, which are based, not surprisingly, on the mercury barometer. After air was evacuated from a glass tube, the open end of the tube was immersed in a reservoir of mercury, allowing air pressure to force mercury to rise in the glass tube. At sea level, the standard height of the mercury column is 29.92 inches, which converts to 76 centimeters. Although inches of mercury are perfectly acceptable in the context of high- and low-pressure systems you routinely see on the evening news, professional weather forecasters prefer another measure of air pressure -- millibars. Let's investigate.

On this image from space, a large shield of clouds marks the domain of a moderately strong low-pressure system off the Middle Atlantic Seaboard on January 13, 2002, while high pressure fosters mainly clear skies over the Gulf States.

Credit: NOAA

You will later learn that evolving horizontal patterns of air pressure are crucial to weather forecasting, which is one of the reasons why forecasters pay such close attention to centers of highest and lowest pressure on weather maps. On the image of clouds from space shown on the left, please note the expansive shield of clouds associated with a moderately strong low-pressure system off the Middle Atlantic Seaboard. In contrast, notice the relatively clear skies associated with high pressure over the western Gulf States. We will discuss the general weather patterns associated with highs and lows later in the course.

Because air pressure plays such an important role in determining the type of weather we might experience, it is no surprise that it has a place on the station model. So let's discuss how to decode the three numbers in the upper-hand corner of any station model that represent air pressure.

Look at the current map of surface observations and check out the upper right-hand corner of one of the station models. The three digits in the upper-right corner of a station model represent sea-level pressure expressed in millibars (mb). Before you can understand how to decode these digits, you must have a sense for the range of sea-level pressures observed in the atmosphere (you'll see why in a minute).

For starters, the average sea-level pressure (computed over the entire earth over a long period of time) is roughly 1011 mb. Weather associated with average sea-level pressure can be rather tame, so let's entertain the notion of a low-pressure system. For the record, a representative value for sea-level pressure at the center of a formidable low-pressure system (an "L" on a weather map) that can cause, for example, heavy snow during winter might be in the neighborhood of 960 to 980 mb.

This artificial trace of sea-level pressure (formally called a barograph trace) gives you a sense of the range in barometric readings associated with notorious low- and high-pressure systems in recent history. For sake of comparison, the barograph trace includes markers for average sea-level pressure and typical values for generically strong high- and low-pressure systems. In case you're wondering, a barograph is a recording aneroid barometer invented by Lucien Vidie, a French engineer, in 1843. Check out a photograph of a barograph in action.

Credit: David Babb

On the flip side, a representative value for sea-level pressure at the center of a strong high-pressure system that comes down from Canada in winter might be in the neighborhood of 1035 to 1050 mb. As a general guideline, nearly all sea-level pressures lie between 960 millibars and 1050 millibars, with most pressure readings falling between 980 and 1040 mb. Narrowing down the field even further, most sea-level pressures tend to cluster closer to 1013 mb.

There are exceptions, of course. Hurricanes, the king of low-pressure systems on Planet Earth, can achieve very low-pressure readings at their center (the eye). Consider Hurricane Linda, which, on September 12, 1997, became the most powerful hurricane ever observed in the eastern Pacific Ocean. The lowest sea-level pressure retrieved by aircraft reconnaissance was 902 mb, as winds gusted over 200 miles an hour just outside the storm's eye. Needless to say, you will rarely, if at all, encounter pressure readings this low when dealing with most mundane station models.

On the other extreme, the king of high-pressure systems that occasionally form over Siberia during the throes of Arctic winter can attain maximum sea-level pressure readings above 1060 mb. You will rarely, if at all, encounter such high-pressure readings; although, in fairness, very high barometric readings are possible in Alaska whenever high-pressure systems slide across the Bering Sea from Siberia.

Okay, let's return to decoding sea-level pressure on the station model. The three numbers in the upper-right-hand corner of the station model represent the last three digits of the station's sea-level pressure, which is always expressed to the nearest tenth of a millibar. Thus, to decode the pressure reading, you must first add a decimal in front of the right-most digit. Then you need to place either a "9" or a "10" in front of the three digits. How do you make such a decision? Just keep in mind that most values of sea-level pressure lie between 980 and 1040 mb (with a few occasional outliers between 950 and 980 mb and 1040 and 1050 mb, and even fewer outliers lower than 950 mb and higher than 1050 mb).

Based on statistical distributions of sea-level pressure, here are general guidelines for decoding pressure on surface station models: If the station-model code for sea-level pressure is less than "500", add a "10" (for example, "032" = 1003.2 mb. If the station-model pressure is greater than "500" add a "9" (for example, "968" = 996.8 mb). Caveat: Beware of these guidelines whenever you're dealing with a strong low pressure (such as a hurricane) or a burly high-pressure system from the Arctic. In such cases, you'll discover that the guidelines will typically break down. Again, always decode pressure shown on station models with the prevailing weather pattern in mind.