You didn’t know that air weighed anything? Well, it does, and we call that weight air pressure.
At sea level, the average air pressure, or weight of all the earth’s atmosphere pressing down on one square inch of ground, is 14.7 pounds. This pressure, or weight varies with time and weather.
Fair weather is associated with high pressure (more weight), and bad weather (rain) is associated with low pressure (less weight). The higher you go above sea level, the less air there is directly overhead, therefore the less weight, or pressure.
Three and a half miles above sea level air pressure is cut exactly in half. Seven miles up, air pressure is only one fourth what it is at sea level. Since many jet planes fly at 30,000 or more feet above sea level, you can see the value of pressurized cabins.
The same thing happens with the density of air. At 100 miles above sea level, the air is so thin that it is a more perfect vacuum than anything a scientist can make in a laboratory.