Pressure<span> with Height: </span>pressure<span> decreases with </span>increasing altitude<span>. The </span>pressure<span> at any level in the </span>atmosphere<span> may be interpreted as the total weight of the </span>air<span> above a unit area at any </span>elevation<span>. At higher elevations, there are fewer </span>air<span> molecules above a given surface than a similar surface at lower levels.</span>
<span>The
higher the altitude, the lower the atmospheric pressure. Lower pressure
in turn causes water to evaporate more quickly, and water actually
boils at a lower temperature. The percentage of oxygen in the air at two
miles (3.2 km.) is the same as at sea level (21%). However, the air
pressure is 30% lower at the higher altitude due to the fact that the
atmosphere is less dense--that is, the air molecules are farther apart.
The important effect of this decrease in pressure is this: in a given
volume of air, there are fewer molecules present. This is really just
another way of saying that the pressure is lower. This is called Boyle's
law.</span>
