The limits of measurement, however, are 1050 millimars as the highest possible amount, and 960 millibars as the lowest. Mars has a measurement of about 50.5 millibars, while Venus exhibits twenty times Earth's pressure, enough to crush a human being. Different air contents in each, however, make them both unliveable. This is measured in units of millibars by a device called a barometer. At sea surface elevation, the average atmospheric pressure is around 1000 millibars.
Pressure decreases with altitude.
The jet stream is usually found at the 500 millibar level. Since the atmosphere is composed of gases, which are very compressible, the 500 millibar level is around 18,000 feet altitude, rather than half way to the top of the atmosphere. In fact, most of the gases comprising the atmosphere are below 36,000 feet. Surface pressures vary, however, and this is what causes wind. Air at a given elevation will move automatically to fill vacuums.