REVIEW QUESTIONS
Q1-4.
What are sunspots?
Q1-5.
In the Southern Hemisphere, approximately
what date will the greatest amount of
incoming solar radiation be received?
Q1-6.
What percent of the earth's insolation do land
and water absorb?
Q1-7.What is the effect on a polar air column in
relation to a column of air over the equator?
PRESSURE
LEARNING OBJECTIVE: Describe how
pressure is measured and determine how the
atmosphere is affected by pressure.
DEFINITION AND FORMULA
Pressure is the force per unit area. Atmospheric
pressure is the force per unit area exerted by the
atmosphere in any part of the atmospheric envelope.
Therefore, the greater the force exerted by the air for
any given area, the greater the pressure. Although the
pressure varies on a horizontal plane from day to day,
the greatest pressure variations are with changes in
altitude. Nevertheless, horizontal variations of pressure
are ultimately important in meteorology because the
variations affect weather conditions.
Pressure is one of the most important parameters in
meteorology. Knowledge of the distribution of air and
the resultant variations in air pressure over the earth is
vital in understanding Earth’s fascinating weather
patterns.
Pressure
is
force,
and
force
is
related
to
acceleration and mass by Newton’s second law. This
law states that acceleration of a body is directly
proportional to the force exerted on the body and
inversely proportional to the mass of that body. It may
be expressed as
a
F
m
or F
ma
=
=
“A” is the acceleration, “F” is the force exerted, and
"in" is the mass of the body. This is probably the most
important equation in the mechanics of physics dealing
with force and motion.
NOTE: Be sure to use units of mass and not units of
weight when applying this equation.
STANDARDS OF MEASUREMENT
Atmospheric pressure is normally measured in
meteorology by the use of a mercurial or aneroid
barometer. Pressure is measured in many different
units. One atmosphere of pressure is 29.92 inches of
mercury or 1,013.25 millibars. These measurements
are made under established standard conditions.
STANDARD ATMOSPHERE
The establishment of a standard atmosphere was
necessary to give scientists a yardstick to measure or
compare actual pressure with a known standard. In the
International Civil Aeronautical Organization (ICAO),
the standard atmosphere assumes a mean sea level
temperature of 59°F or 15°C and a standard sea level
pressure of 1,013.25 millibars or 29.92 inches of
mercury. It also has a temperature lapse rate (decrease)
of 3.6°F per 1000 feet or 0.65°C per 100 meters up to 11
kilometers
and
a
tropopause
and
stratosphere
temperature of -56.5°C or -69.7°F.
VERTICAL DISTRIBUTION
Pressure at any point in a column of water, mercury,
or any fluid, depends upon the weight of the column
above that point. Air pressure at any given altitude
within the atmosphere is determined by the weight of
the atmosphere pressing down from above. Therefore,
the pressure decreases with altitude because the weight
of the atmosphere decreases.
It has been found that the pressure decreases by half
for each 18,000-foot (5,400-meter) increase in altitude.
Thus, at 5,400 meters one can expect an average
pressure of about 500 millibars and at 36,000 feet
(10,800 meters) a pressure of only 250 millibars, etc.
Therefore, it may be concluded that atmospheric
pressures are greatest at lower elevations because the
total weight of the atmosphere is greatest at these
points.
There is a change of pressure whenever either the
mass of the atmosphere or the accelerations of the
molecules
within
the
atmosphere
are
changed.
Although
altitude
exerts
the
dominant
control,
temperature and moisture alter pressure at any given
altitude—especially near Earth’s surface where heat
and humidity, are most abundant. The pressure
variations produced by heat and humidity with heat
being the dominant force are responsible for Earth’s
winds through the flow of atmospheric mass from an
area of higher pressure to an area of lower pressure.
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