How can air be measured?

It might not seem like it, but air has weight. Anything with mass has weight, and we know air has mass because (for example) we can feel it when the wind blows.

The total weight of the atmosphere exerts a pressure of about 14.7 pounds per square inch at sea level. You don't notice this weight, however, because you are used to it.

If you live in Denver, Colorado, which is at an elevation of about 5,000 feet, then about 85 per cent of the atmosphere is above you, resulting in an air pressure of about 12.5 pounds per square inch. At the top of Mount Everest (over 29,000 feet), only 30% of the atmosphere lies above you, leaving an air pressure of only 4.4 pounds per square inch.