ecules is greatest (around 1,000 millibars at sea level). From there, it declines quickly
with altitude to 500 millibars at around 18,000 feet (5,500 meters). At 40 miles (64.37
kilometers), it will be 1/10,000th of the surface air pressure.
This can also be interpreted another way: For altitudes of less than about 3,000
feet (914.4 meters), the barometric air pressure decreases about 0.01 inches of mer-
cury for each 10 feet (3 meters) of altitude (or a decrease of 1 inch of mercury for each
1,000 foot [304.8 meters] gain in altitude). If millibars are used, it is 1 millibar for
every 26.25 foot (8 meter) altitude gain. That means if a person takes a ride in an ele-
vator, hits the button for the 50th floor—and coincidentally has a barometer in his or
her pocket—the pressure would fall by approximately 0.5 inch (1.27 centimeters) dur-
ing the ascent. This also means that higher-altitude cities have major differences in
barometric readings. For example, the air pressure in almost mile-high Denver, Col-
orado, is only 85 percent that of cities that reside at sea level.How is wind measured?
Wind speed is the measurable motion of air with respect to the surface of the Earth.
It is measured in terms of a unit distance over a unit time, such as miles per hour.
The wind direction is also an indication of the wind’s source. For example, a souther-
ly wind means the wind is blowing toward north—it is coming from a southerly
308 direction.
How is air density related to air pressure?
T
he expression “thin air” is actually a reference to the atmosphere’s density—
or how “thick” the air molecules are near the Earth’s surface. In chemistry
terms, density is merely the mass of anything (including air) divided by the vol-
ume the mass occupies. For example, the density of dry air at sea level is high,
mainly due to the pull of gravity. In metric system terms, sea level density is
about 1.2929 kilograms/meter^3 , or about 1/800th the density of water. But as
altitude increases, the density drops dramatically. Mathematically speaking, the
density of air is proportional to the air pressure and inversely proportional to
temperature. Thus, the higher up one is in the atmosphere, the lower the air
pressure and the lower the air density.There is somewhat of an athletic advantage to higher elevations—at least
for players of such sports as football and baseball. Because the air density is
lower, a ball thrown in high-elevation places like Denver, Colorado, will travel
even farther than a ball thrown in a close-to-sea-level city, such as Miami, Flori-
da. In fact, the air at the Denver stadium allows balls to travel almost 10 per-
cent farther.