So, in order to level the playing field, meteorologists adjust station pressure to sea level.īy adjusting to sea level, meteorologists are essentially pretending that high-elevation locations (like Denver) are located at sea level, and as such, they adjust all barometer readings to what they would be if they were located at sea level. Regardless of the strength and position of various high- and low-pressure systems, the map of station pressure would always look something like the one above (lowest pressures in the highest-elevation regions). 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. 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. The first thing you might notice on the map is the area of very low pressures in the Rocky Mountains (less than 780 millibars in some areas). To see what I mean, check out the map of long-term average surface pressure (called station pressure) across the United States below. For meteorologists, surface pressure's dependence on elevation presents a bit of a problem. So, the fact that pressure decreases with increasing height explains why the surface pressure at high elevation locations (like Denver) is much lower than at sea level. In fact, pressure at the top of the troposphere is typically less than 300 millibars (less than 30 percent of the pressure at sea level). The reason why that's the case is fairly intuitive: the higher the altitude, the less air (and weight) there is above you in an atmospheric column. The pressure on your barometer would be so low because pressure decreases with increasing height everywhere in the atmosphere. That's much lower than the typical range of sea-level pressures we talked about in the previous section. ![]() To see what I mean, consider this: if you brought a barometer with you on a trip to Denver, Colorado (elevation about one mile above sea level), it would regularly measure a pressure of about 850 millibars. ![]() But, most of the United States (and the rest of the world's land masses) aren't at sea level, so 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." ![]() In the last section, our discussion of pressure was focused on the pressure at sea level.
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