Module 3 | Scenario 2: Atmospheric Pressure and Altitude

Key Concepts

a. What is pressure?

Pressure is a measure of the weight of the atmosphere. As you move to higher altitudes, there is less atmosphere above you, and the pressure decreases. Conversely, if you move to lower altitudes, there is more atmosphere above you, and the pressure increases.  

b. Measuring pressure

Pressure was once measured using a device known as a Torricellian barometer, which was invented by Evangelista Torricelli in the 1600s. As illustrated in Figure 1, a tube with a sealed end was filled with mercury and placed upside down in a bowl filled with mercury. The pressure of the atmosphere thus affects the mercury in the bowl, but not the tube. As a result, the mercury in the tube would rise until the weight of the mercury in the tube equaled the weight of the atmosphere and mercury out of the tube. 

Figure 1. Schematic of a Torricellian barometer. Source.

If you were to transport such a barometer to a higher altitude, the mercury in the tube would lower as the atmospheric pressure and weight decrease.  

The pressure used to be measured using barometers of this type, which is why you sometimes hear pressure reported in inches of mercury or just inches for short.  

However, meteorologists don't use inches of mercury anymore. In the United States, they use a metric unit called a millibar (abbreviated mb).

Averaged globally, the pressure at sea level is 29.92 inches of mercury or 1,013.25 millibar.  

c. Pressure and altitude

Figure 2 illustrates the pressure at the base and top of several ski areas or mountains. Alyeska in Alaska is located very near sea level with base and top lift elevations of 80 and 840 meters, respectively. Based on the globally averaged pressure distribution, this equates to pressures of 1,004 and 917 millibar, respectively.  

Figure 2. Elevations and pressures at selected ski resorts and mountains.

Other resorts depicted and presented from left to right in Figure 2 in terms of increasing elevation are Snoqualmie Pass, Washington; Squaw Valley, California; Snowbird, Utah; and Breckenridge, Colorado. Breckenridge has the highest lift in the United States, reaching an altitude of 3,962 meters (12,840 feet), where the pressure is about 623 millibar. 

Chamonix, France has the largest lift-served vertical drop in the world, 2,807 meters (9,263 feet), although you need a guide to ski it. If you do, the pressure on your ski from top to bottom will increase from about 629 millibar to 895 millibar. Relative to the pressure at the top, this is a 42% increase!

The world's highest ski area is Jade Dragon/Snow Mountain in China. At the reported top elevation of 4,700 meters (15,420 feet), the pressure is about 562 millibar, or about 55% of sea level pressure.  

Finally, the world's longest theoretical ski descent is on Mt. St. Elias in Alaska, where it is theoretically possible to ski from the 5,488 meters (18,808 ft) summit to Icy Bay at sea level. In doing so, you would descend through approximately half the atmosphere, beginning at about 506 millibar and ending at 1013 millibar. Sound like fun? Probably not. I don't think it's ever been skied in one continuous descent.  

d. Pressure-level maps

Instead of using maps at constant altitudes, meteorologists use maps at constant pressures. This is because many meteorological concepts are better understood using pressure than altitude. For example, they often look at 850-millibar, 700-millibar, or 500-millibar maps. These maps might include analyses of temperature, wind, or other variables. For example, Figure 3 is an analysis of 700-millibar temperature (color contours every 2˚C) and wind vectors overlaid on satellite and radar imagery. As can be inferred from Figure 2 above, 700-millibar is at about 3,000 meters above sea level, or 10,000 feet. On this day, a weak cold front was moving into northern Utah. Note how the temperatures are lower immediately to the northwest.  

Figure 3. An analysis of 700-millibar temperature (color contours every 2˚C) and wind vectors overlaid on satellite and radar imagery.

For example, meteorologists often use 700-millibar maps to examine the weather in the central Wasatch since it is near 3,000 meters and just below the highest peaks, which reach a bit over 3,000 meters. For instance, the top of the Snowbird tram is 3,353 meters. Thus, 700-millibar is roughly crest level, meaning it is near the elevation of the highest peaks.