Probing Pressure

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman


After completing this section, you should be able to describe atmospheric pressure, the typical units of pressure that meteorologists use, and the typical range of sea-level pressures observed on earth. By applying this knowledge, along with the guidance in this section, you should be able to decode sea-level pressure from the station model.


"Pressure...pushing down on me, pressing down on you..."

Those lyrics come from the song "Under Pressure" by Queen (featuring David Bowie) from 1981. As we start our investigations of pressure, we have to start with the basics. For starters, what is pressure? On that matter, Queen basically nailed it. It's a force that pushes down on me and you (and everything else), although pressure isn't easy for us to "feel" with our human senses except under certain circumstances. You've probably noticed the impacts of pressure if your ears have popped while driving up or down a mountain, or if you experience discomfort when air pressure decreases as a storm approaches (as many folks with arthritis or bursitis experience).

Meteorologists are concerned about atmospheric pressure, which is the pressure exerted by air molecules, and you may recall from a high school science class that pressure is defined as a force per unit area. In a more practical sense, the pressure exerted by air molecules at a weather station is approximately the weight of the air in a column that extends from a fixed area on the ground to the top of the atmosphere. At sea level, the weight of a column of air on one square inch of area is roughly 14.7 pounds, resulting in an air pressure of 14.7 pounds per square inch. For perspective, that amounts to a total force of more than two tons on just the area covered by a single base on a baseball field (an 18-inch by 18-inch area). Surprised?

Meteorologists typically don't work with pressure in pounds per square inch, however. For example, many home barometers (instruments for measuring atmospheric pressure) measure pressure in inches of mercury, which are based on the mercury barometer. Mercury barometers measured pressure after air was evacuated from a glass tube, and 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 (76 centimeters). More commonly, meteorologists often work with pressure in units of millibars (abbreviated "mb"). For reference, an atmospheric pressure of 14.7 pounds per square inch (when the height of a mercury barometer would be 29.92 inches) is equal to about 1013 millibars.

A satellite image that illustrates the relationship between clouds and surface pressure.
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, while high pressure fosters mainly clear skies over the Gulf States.
Credit: NOAA

The connection between surface pressure and the weight of a column of air that extends above the surface has many important consequences. For starters, processes that reduce the weight of an air column also act to decrease the surface pressure. On the other hand, processes that add weight to air columns act to increase surface pressure. 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 (typically marked by a blue "H" and a red "L", respectively). In a very general sense, low-pressure systems tend to bring inclement weather (clouds and precipitation), while high pressure systems tend to bring "fair" weather (sunshine and relatively calm conditions).

The bottom line here is that when you hear meteorologists refer to a "low pressure system," they are really talking about is a "lightweight." In other words, the air column above the center of a low weighs less than any of the surrounding air columns. On the flip side, a high pressure system is a "heavyweight" because the air column above the center of the high weighs more than any of the surrounding air columns. Now, I should point out that the difference in pressure between a run-of-the mill high-pressure system and a pretty strong low-pressure system is only about five percent. In the image on the right, for example, the difference between the labeled high and low is only 32 millibars (1018 millibars - 986 millibars), so the difference was even less than five percent in this case. Still, these differences have very important consequences for the weather, as you'll learn!

To give you an idea of the range of sea-level pressures across the world, the average sea-level pressure (computed over the entire earth over a long period of time) is roughly 1013 mb. A very strong high pressure system in the winter may measure around 1050 millibars. On the other hand, a representative value for sea-level pressure at the center of a fierce low-pressure system that can cause, for example, heavy snow during winter might be in the neighborhood of 960 to 980 mb.

An artificial barograph trace showing typical and extreme sea-level pressure values.
This artificial trace of sea-level pressure (formally called a barograph trace) gives you a sense of the range in sea-level pressure readings associated with notorious low- and high-pressure systems. 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

As a general guideline, nearly all sea-level pressures lie between 950 millibars and 1050 millibars, with most pressure readings falling between 980 and 1040 millibars. Narrowing down the field even further, sea-level pressures often tend to cluster closer to 1013 mb.

There are exceptions, of course. The bottom of the observed range of sea-level pressures is populated by the "kings" of all low-pressure systems on our planet -- hurricanes (called "typhoons" in some parts of the world). Very intense hurricanes can have sea-level pressures down near 900 millibars. In 2017, for example, at its peak intensity, Hurricane Maria had a minimum sea-level pressure of 908 millibars. The storm later went on to devastate Puerto Rico, and its fierce winds completely destroyed the island's NEXRAD Doppler radar (this short video highlights Maria's damage to Puerto Rico, and includes some stunning images of the damage to the radar, if you're interested). A handful of hurricanes and typhoons globally have even had sea-level pressures drop a bit below 900 millibars. On the other extreme, the kings of high-pressure systems that occasionally form over Siberia during the throes of Arctic winter can attain maximum sea-level pressure readings above 1050 or even 1060 millibars.

Ultimately, the pressures associated with very intense hurricanes and very strong high-pressure systems in the winter are pretty rare, so we can use the general guideline above (that nearly all sea-level pressures lie between 950 millibars and 1050 millibars) to help us interpret pressure data from various maps. With that in mind, let's turn to this section's Key Skill -- decoding sea-level pressure on the station model.

Key Skill...

See image caption.
A sample station model with sea-level pressure and the three-hour pressure tendency highlighted.
Credit: David Babb

Because air pressure plays such an important role in determining the type of weather we might experience, it's no surprise that it has a place on the station model. But, interpreting pressure on a station model is not quite as straightforward as the other variables we've covered. To see the pressure information displayed on a station model, check out the image on the right. The three digits listed in the upper right on the station model represent the sea-level pressure, while the two digits below represent the three-hour pressure tendency (change in pressure over the previous three hours), which is not always reported. For now, we're going to focus on the sea-level pressure value in the upper right (we'll deal with pressure tendency later on).

The three digits in the upper-right-hand corner of the station model represent the last three digits of the station's sea-level pressure, 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 decide whether a "9" or a "10" should go in front of the three digits? This is where knowing the typical range of sea-level pressures is helpful. Remember that nearly all values of sea-level pressure are between 950 millibars and 1050 millibars (unless you're dealing with an intense hurricane or an extremely strong Arctic high in winter). So, in the example on the right, we must need a "10" in front of the "046" to give 1004.6 millibars. Placing a "9" in front would have given 904.6 millibars, which wouldn't make sense (unless an extremely intense hurricane was right near the station).

Based on statistical distributions of sea-level pressure, if the three digits you see on the station model are less than "500," you'll typically place a "10" in front of them, while if the three digits are greater than "500," you'll typically place a "9" in front of them. In most cases, you want to choose whichever will give you a sea-level pressure between 950 mb and 1050 mb. As mentioned above, some exceptions exist, but the exceptions are rare. Still, if you are dealing with a strong hurricane or a burly high-pressure system from the Arctic, these guidelines might break down, so forecasters must be aware of the general weather pattern when decoding pressure.

I recommend practicing with the interactive station model tool below. The tool defaults to a sea-level pressure of 1004.6 millibars ("046"), but you can change the value in the "Current Conditions" panel on the right. For example, type in pressures of 999.6 mb, 986.2 mb, and 1028.9 mb and see how they appear on the station model. Practice decoding some random 3-digit coded pressures (decode "953", "069", and "395", for example) and check your answers with the tool by typing your answer into the "Current Conditions" panel and see if the station model displays the 3-digit code that you started with.

Quiz Yourself...

Ready to check your skill at decoding pressures from a station model? Use the quiz below to practice. If you can get at least 9 out of 10 on the quiz, you've likely got the hang of it! Make sure to note if "special circumstances" apply in each question, and good luck! You're welcome to try as many times as you would like.

Explore Further...

In our discussion of pressure, I repeatedly referred to "sea-level pressure," even though most land areas on earth do not lie at sea level. Why make that distinction? Well, in order to analyze the horizontal patterns of surface air pressure that govern weather, meteorologists require a "level playing field," and that's why they're interested in "sea-level pressure."

The skyline of Denver, Colorado, with the Rocky Mountains in the background.
Given that Denver, Colorado, lies an altitude of roughly 5300 feet, the surface pressure often flirts with 850 mb, even on days when skies are clear.

What do I mean by that? To illustrate, I kept tabs on pressure readings with the barometer on my cellphone during a trip into the Rocky Mountains, just west of Denver, Colorado, including a trip up the highest paved road in North America to Mount Blue Sky (formerly Mount Evans). Upon reaching the summit, the barometer app on my phone read 613.07 hectopascals (equal to 613.07 millibars), and this wasn't a faulty observation! This chart of mean station pressure for the United States shows very low pressures in the Rocky Mountains (less than 780 millibars in some areas), on average. Is there some kind of monster low-pressure system permanently parked in the Rockies? Of course not! The station pressures are always low there because of the high elevations in the Rockies (we'll explore this relationship later). The dramatic variation in station pressure based on elevation makes it virtually impossible for meteorologists to use station pressure to track centers of high and low pressure. Regardless of the strength and position of various high- and low-pressure systems, the map of station pressure would always show the lowest pressures in the highest-elevation regions. So, in order to level the playing field, meteorologists adjust station pressure to sea level.

Meteorologists "correct" the station pressure to sea level by estimating the weight of an imaginary column of air that extends from the station to sea level. The surface temperature at the location is used to compute a representative density of the imaginary column, which when combined with the station altitude is then converted to a column weight. In turn, this estimated weight of the imaginary air column converted into a pressure adjustment that gets added to the observed station pressure. This results in the adjusted sea-level pressure that you see displayed on the station model. This schematic of the adjustment process may help you visualize how it's done.