From Charles’ law we learned that when the
temperature increases, the volume increases and the
density decreases. Therefore, the thickness of a layer of
air is greater when the temperature increases. To find
the height of a pressure surface in the atmosphere (such
as in working up an adiabatic chart), these two variables
(temperature and density) must be taken into
consideration. By working upward through the
atmosphere, the height of that pressure surface can be
computed by adding thicknesses together. A good tool
for determining height and thickness of layers is the
Skew-T Log P diagram, located in AWS/TR-79/006.
Since there are occasions when Skew-Ts are not
available, a simplified version of the hypsometric
formula is presented here. This formula for computing
the thickness of a layer is accurate within 2 percent;
therefore, it is suitable for all calculations that the
Aerographer’s Mate would make on a daily basis.
The thickness of a layer can be determined by the
following formula:
Z = (49,080 + 107t) Po
P
Po
P
-
+
Z = altitude difference in feet (unknown
thickness of layer)
49,080 = A constant (representing gravitation and
height of the D-mb level above the
surface)
107 = A constant (representing density and
mean virtual temperature)
t = mean temperature in degrees Fahrenheit
Po = pressure at the bottom point of the layer
P = pressure at the top point of the layer
For example, let us assume that a layer of air
between 800 and 700 millibars has a mean temperature
of 30°F. Applying the formula, we have
Z = (49,080 + 107 × 30) 800
700
800
700
-
+
Z = (49,080 + 3,210) 100
1
500
,
Z = (52,290) 1
15
Z = 3,486 feet (1,063 meters)
(1 meter = 3.28 feet)
REVIEW QUESTIONS
Q2-7. What three things does the behavior of gases
depend on?
Q2-8.
According to Boyle's Law, how is volume and
pressure related?
Q2-9.
According
to
Charles'
Law,
how
is
temperature and pressure related?
Q2-10.
What is the formula for the Universal Gas
Law?
ATMOSPHERIC ENERGY
LEARNING OBJECTIVE: Describe the
adiabatic process and determine how stability
and instability affect the atmosphere.
There are two basic kinds of atmospheric energy
important to AGs—kinetic and potential. Kinetic
energy is energy that performs work due to present
motion while potential energy is energy that is stored
for later action. Kinetic energy is discussed first in
relation to its effect on the behavior of gases.
According to the kinetic theory of gases, the
temperature of a gas is dependent upon the rate at which
the molecules are moving about and is proportional to
the kinetic energy of the moving molecules. The kinetic
energy of the moving molecules of a gas is the internal
energy of the gas; it follows that an increase in
temperature is accompanied by an increase in the
internal energy of the gas. Likewise, an increase in the
internal energy results in an increase in the temperature
of the gas. This relationship, between heat and energy,
is called thermodynamics.
An increase in the temperature of a gas or in its
internal energy can be produced by the addition of heat
or by performing work on the gas. A combination of
these can also produce an increase in temperature or
internal energy. This is in accordance with the first law
of thermodynamics.
FIRST LAW OF THERMODYNAMICS
This law states that the quantity of energy supplied
to any system in the form of heat is equal to work done
by the system plus the change in internal energy of the
system. In the application of the first law of
thermodynamics to a gas, it may be said that the two
main forms of energy are internal energy and work
energy. Internal energy is manifested as sensible heat or
simply temperature. Work energy is manifested as
pressure changes in the gas. In other words, work is
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