Every night on the forecast I give the barometer reading, which is a measure of pressure. Atmospheric pressure, in its simplest terms is equal to the weight of the air above the point at which you measure. At sea level, there’s about 14.7 pounds per square inch of air pressure. So in other words, at sea level if you created a column of air one square inch in cross sectional area, and extended it all the way up to the top of the atmosphere, it would weight on average about 14.7 pounds. But how high would that column be? There are really several different ways to answer.

As you go higher in elevation, and the pressure of the atmosphere lowers, it gradually becomes thinner until any air molecules become so far apart as to be essentially non-existent. If you really want to go to the extreme, the top of the outer layer of the atmosphere (called the exosphere) is about 6,000 miles above the surface of the earth. But that’s really much higher than any “air” exists. A better “top” would be what’s called the Kármán Line.

Named after Theodore Von Kármán, a Hungarian physicist who made a lot of contributions to the fields of aeronautics (think airplanes) and astronautics (think spaceships). He spearheaded a study that tried to determine an altitude where planes could no longer fly… or at least would have to go so fast as to essentially reach escape velocity. He came up with a number that rounded off to 100 kilometers (or 62 miles) above sea level. That figure has almost been universally accepted as the “start of space.” So by this definition, the atmosphere is 62 miles high.

Works for me.

Below that boundary you essentially have all the air around the earth. So here’s an interesting question: At what altitude are you halfway through the atmosphere?

Seems like a pretty simple answer at first blush. If the atmosphere is 62 miles thick, then the halfway point must be 31 miles up. But if you went up that high, you’d still be above over 99% of the air around the earth. Because air compresses, there’s a lot more packed into the lowest part of the atmosphere, and you really don’t have to go all that far before you reach the halfway point.

The weight of the atmosphere creates a pressure of about 1,000 millibars at its base (sea level). So the halfway point should be the altitude at which you have half that pressure (500 millibars). That occurs roughly at an altitude of 18,000 feet here at our latitude. It’s lower on average at the poles, and higher at the equator. You noticed that I said “roughly.” Depending on the weather patterns, that elevation of the 500 millibar surface will rise and fall. The change in the density of air isn’t constant around the globe at any time. When I talk about high and low pressure centers, those affect how fast you reach the “halfway point.” In fact, most of the computer models that I look at to try and predict the weather use this to make what are essentially topographic maps of the various pressure levels. The lower the 500 millibar elevation, the “deeper” the low… the higher the heights, the stronger the high.