to objects’ temperatures and the transfer of heat between objects of differing tempera-
tures. Called thermodynamics, it is a study that entails a great deal of mathematical
knowledge.
No matter what the type of temperature discussed, the most common apparatus
for measurement is the thermometer. The most familiar thermometers are thin, long,
closed glass tubes containing some type of liquid—most often alcohol or mercury.
When the temperature increases—or the air around the tube heats up—it causes the
liquid to expand, moving it up the tube. Air temperature measurements are most
commonly read in Centigrade or Fahrenheit (for more about Centigrade [Celsius] and
Fahrenheit, see “Mathematics throughout History”).
How are absoluteand relative humiditydetermined?
Like many other facets of meteorology, mathematics comes in handy when determin-
ing absolute and relative humidities. The absolute humidity is the mass of water vapor
divided by the mass of dry air in a specific volume of air at a specific temperature. In
this instance, the warmer the air, the more water vapor it contains.
On the other hand, relative humidity (RH) is the ratio of the absolute humidity to
the highest possible absolute humidity, which in turn depends on the current air tem-
perature. Mathematically, RH is often defined as the ratio of the water vapor density
(mass per unit volume) to the saturation water vapor density, usually expressed as a
percent. The equation for relative humidity is: RH actual water vapor density /
vapor saturation density 100 percent.
More commonly, RH is thought of as the amount of water vapor in the air at a
given temperature in comparison to the amount that the air couldcontain at the same
temperature. For example, if an area is experiencing 100 percent relative humidity,
that usually means the air is saturated with (can’t hold anymore) water vapor. 305
MATH IN THE NATURAL SCIENCES
Why do weather reports sometimes say the humidity is
100 percent when there is no rain or snow falling?A
reading of 100 percent humidity usually means there is a high probability
that rain will be or is occurring, but not always. It might be 100 percent RH
because clouds are forming. If the RH near the ground is much less—for exam-
ple, if a relatively dry air mass is in place—there will be no rain at the surface.
This is why Doppler radar sometimes shows rain or snow in an area when none
is actually reaching the ground.