Have you ever driven up a hill or mountain, or flown in an airplane, and felt pressure in your ears? This is because of a change in air pressure. Air pressure is simply how much air is pushing down on you and everything else on Earth, but it’s not the same everywhere. Just like the water pressure increases on your body as you swim deeper in a swimming pool, the air pressure increases as you move to a lower elevation. Pressure depends on how much mass is sitting on top of you as shown in this diagram:
Since air pressure is just a measure of how much air is sitting on top of you, the higher you are on the Earth, the less pressure you feel. The average pressure at the sea-level is about 1013 mb (mb is short for millibar, which is a unit of air pressure), or if you’re familiar with putting air into your car’s tires, about 14.7 psi. Let’s compare the air pressure at Pikes Peak, Colorado, where the altitude is about 14,000 feet. At this location, the air pressure is only about 600mb, only 60 percent of that at sea level. This is far lower than the lowest air pressure ever recorded in a hurricane, which was 882mb.
Because pressure decreases with height, there is always low pressure at high elevations and high pressure at the low elevations around the globe. This has created a problem for meteorologists because you can’t tell if there is a low-pressure system, or if a location is just at a high place. Here is an example of a map average air pressure across the United States:
You notice that there is low pressure in the Rocky Mountains, slightly low pressure in the Appalachians, and high pressure over the oceans and along the low elevations of the Mississippi River Valley and Florida. To fix this, a map is needed where all the pressures are at the same elevation, so the term “sea-level pressure” was invented. This is where all the air pressures on the map are adjusted to be the same as if it were at sea level, or an altitude of 0 feet. When mapping sea-level pressure, the only reason the pressure varies from place to place is the result of high and low-pressure systems, which helps meteorologists understand and predict the weather.
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