How is barometric(or air) pressure
measured?
Barometric (or air) pressure, named after
the instrument used to measure this
pressure, is caused by the weight of the
atmosphere pressing down on the land,
ocean, and air below, with gravity creat-
ing the downward force. Because pres-
sure is dependent on the amount of air
above a certain point, pressures are great-
est at the surface and less at higher alti-
tudes. On the average, at sea level, the air
has a pressure of 14.7 pounds per square
inch (a one-inch square has 14.7 pounds
of air pressure on each side).
The United States National Weather
Service does not measure pressure in
pounds per square inch, but in terms of
inches of mercury—or how high the
pressure pushes mercury in a sealed tube.
Air pressure aloft is reported in millibars
(or hectopascals [hPa], a term most often used by scientists to measure air pressure).
Most of us are very familiar with air pressure as it changes with the weather. For
example, the use of the terms “high pressure system” and “low pressure systems” are
often indicators of the types of weather fronts traveling through a region. In general,
falling air pressure (seen on a barometer) means that clouds and precipitation are
more likely; rising air pressure means that clear weather is more likely. In addition,
many people experience “personal” changes in air pressure. For example, the discom-
fort or even pain felt in a person’s ears as they ascend or descend in an airplane, a large
hill, or even in an elevator is evidence of changing air pressure.
How are millibars convertedto inches of mercury?
Mathematically, the conversion is simple. The air pressure at sea level is 29.92 inches
of mercury, or 1,013.2 millibars. For example, if you see air pressure of 1,016 millibars
on a weather map, convert to inches of mercury by multiplying by 29.92, and then
divide by 1,013.2. The result is 30.00 inches of mercury.
How much does air pressure decreasewith altitude?
It takes mathematics to figure out how much air pressure decreases with altitude.
Close to the surface, and due to the pull of gravity, the air pressure exerted by air mol- 307
MATH IN THE NATURAL SCIENCES
A barometer is a type of gauge that measures atmos-
pheric pressure, which makes it useful in detecting
high and low pressure systems that can predict
changes in the weather. Taxi/Getty Images.