but meteorologists do not make this distinction. The
heat of sublimation equals the heat of fusion plus the
heat of vaporization for a substance. The calories
required for water to sublime are: 80 + 597.3 = 677.3, if
the vapor has a temperature of 0°C.
In the sublimation process of vapor passing directly
into the solid form without going through the liquid
phase, the calories released are the same as those for the
sublimation of a solid to a gas. Sublimation of water
vapor to ice frequently takes place in the atmosphere
when supercooled water vapor crystallizes directly into
ice crystals and forms cirriform clouds.
REVIEW QUESTIONS
Q2-4. What are the two basic particles that make up
the composition of matter?
Q2-5.
What is the correct formula for density?
Q2-6.
What is fusion?
GAS LAWS
LEARNING OBJECTIVE: Recognize how
pressure, temperature, and density affect the
atmosphere and describe how the gas laws are
applied in meteorology.
Since the atmosphere is a mixture of gases, its
behavior
is
governed
by
well-defined
laws.
Understanding the gas laws enables you to see that the
behavior of any gas depends upon the variations in
temperature, pressure, and density.
To assist in comparing different gases and in
measuring changes of gases it is necessary to have a
standard or constant to measure these changes against.
The standard used for gases are: a pressure of 760
millimeters
of
mercury
(1,013.25
mb)
and
a
temperature of 0°C. These figures are sometimes
referred to as Standard Temperature and Pressure
(STP).
KINETIC THEORY OF GASES
The Kinetic theory of gases refers to the motions of
gases. Gases consist of molecules that have no inherent
tendency to stay in one place as do the molecules of a
solid. Instead, the molecules of gas, since they are
smaller than the space between them, are free to move
about. The motion is in straight lines until the lines
collide with each other or with other obstructions,
making their overall motion random. When a gas is
enclosed, its pressure depends on the number of times
the molecules strike the surrounding walls. The number
of blows that the molecules strike per second against
the walls remains constant as long as the temperature
and the volume remain constant.
If the volume (the space occupied by the gas) is
decreased, the number of blows against the wall is
increased, thereby increasing the pressure if the
temperature remains constant. Temperature is a
measure of the molecular activity of the gas molecules
and a measure of the internal energy of a gas. When the
temperature is increased, there is a corresponding
increase in the speed of the molecules; they strike the
walls at a faster rate, thereby increasing the pressure
provided the volume remains constant.
Therefore,
there is a close relationship of volume, pressure, and
density of gases.
BOYLE’S LAW
Boyle’s law states that the volume of a gas is
inversely proportional to its pressure, provided the
temperature remains constant. This means that if the
volume is halved, the pressure is doubled. An example
of Boyle’s law is a tire pump. As the volume of the
pump’s cylinder is decreased by pushing the handle
down, the pressure at the nozzle is increased. Another
way of putting it is, as you increase the pressure in the
cylinder by pushing down the handle, you also decrease
the volume of the cylinder.
The formula for Boyle’s law is as follows:
VP = V’P’
V = initial volume
P = initial pressure
V’ = new volume
P’ = new pressure
For example, assume 20 cm3 of gas has a pressure
of 1,000 mb. If the pressure is increased to 1,015 mb
and the temperature remains constant, what will be the
new volume? Applying the formula, we have
V = 20 cm3
P = 1000 mb
V’ = Unknown in cm3
P’ = 1015 mb
V P = V’ P’
20 1,000 = V’ 1,015
20,000 = V’ 1,015
V’ =20000
1015
,
,
V’ = 19.71 cm3
2-8
