Weather Theory
Chapter 12
Introduction
Weather is an important factor that influences aircraft
performance and flying safety. It is the state of the atmosphere
at a given time and place with respect to variables, such as
temperature (heat or cold), moisture (wetness or dryness),
wind velocity (calm or storm), visibility (clearness or
cloudiness), and barometric pressure (high or low). The
term “weather” can also apply to adverse or destructive
atmospheric conditions, such as high winds.
This chapter explains basic weather theory and offers pilots
background knowledge of weather principles. It is designed
to help them gain a good understanding of how weather
affects daily flying activities. Understanding the theories
behind weather helps a pilot make sound weather decisions
based on the reports and forecasts obtained from a Flight
Service Station (FSS) weather specialist and other aviation
weather services.
Be it a local flight or a long cross-country flight, decisions
based on weather can dramatically affect the safety of the flight.
21%21%
78%78% NitrogenNitrogen
OxygenOxygen
1%
Figure 12-1. Composition of the atmosphere.
Troposphere Stratosphere
Mesosphere
Thermosphere
280,000 feet
160,000 feet
20,000 feet
Figure 12-2. Layers of the atmosphere.
Atmosphere
The atmosphere is a blanket of air made up of a mixture of
gases that surrounds the Earth and reaches almost 350 miles
from the surface of the Earth. This mixture is in constant
motion. If the atmosphere were visible, it might look like
an ocean with swirls and eddies, rising and falling air, and
waves that travel for great distances.
Life on Earth is supported by the atmosphere, solar energy,
and the planet’s magnetic fields. The atmosphere absorbs
energy from the sun, recycles water and other chemicals, and
works with the electrical and magnetic forces to provide a
moderate climate. The atmosphere also protects life on Earth
from high energy radiation and the frigid vacuum of space.
Composition of the Atmosphere
In any given volume of air, nitrogen accounts for 78 percent
of the gases that comprise the atmosphere, while oxygen
makes up 21 percent. Argon, carbon dioxide, and traces
of other gases make up the remaining one percent. This
volume of air also contains some water vapor, varying from
zero to about five percent by volume. This small amount of
water vapor is responsible for major changes in the weather.
[Figure 12-1]
The envelope of gases surrounding the Earth changes
from the ground up. Four distinct layers or spheres of the
atmosphere have been identified using thermal characteristics
(temperature changes), chemical composition, movement,
and density. [Figure 12-2]
The first layer, known as the troposphere, extends from 6
to 20 kilometers (km) (4 to 12 miles) over the northern and
southern poles and up to 48,000 feet (14.5 km) over the
equatorial regions. The vast majority of weather, clouds,
storms, and temperature variances occur within this first
layer of the atmosphere. Inside the troposphere, the average
temperature decreases at a rate of about 2 °Celsius (C) every
Figure 12-3. Circulation pattern in a static environment.
1,000 feet of altitude gain, and the pressure decreases at a rate
of about one inch per 1,000 feet of altitude gain.
At the top of the troposphere is a boundary known as the
tropopause, which traps moisture and the associated weather
in the troposphere. The altitude of the tropopause varies with
latitude and with the season of the year; therefore, it takes
on an elliptical shape as opposed to round. Location of the
tropopause is important because it is commonly associated with
the location of the jet stream and possible clear air turbulence.
Above the tropopause are three more atmospheric levels. The
first is the stratosphere, which extends from the tropopause to
a height of about 160,000 feet (50 km). Little weather exists
in this layer and the air remains stable, although certain types
of clouds occasionally extend in it. Above the stratosphere
are the mesosphere and thermosphere, which have little
influence over weather.
Atmospheric Circulation
As noted earlier, the atmosphere is in constant motion.
Certain factors combine to set the atmosphere in motion, but a
major factor is the uneven heating of the Earth’s surface. This
heating upsets the equilibrium of the atmosphere, creating
changes in air movement and atmospheric pressure. The
movement of air around the surface of the Earth is called
atmospheric circulation.
Heating of the Earth’s surface is accomplished by several
processes, but in the simple convection-only model used for
this discussion, the Earth is warmed by energy radiating from
the sun. The process causes a circular motion that results
when warm air rises and is replaced by cooler air.
Warm air rises because heat causes air molecules to spread
apart. As the air expands, it becomes less dense and lighter
than the surrounding air. As air cools, the molecules pack
together more closely, becoming denser and heavier than
warm air. As a result, cool, heavy air tends to sink and replace
warmer, rising air.
Because the Earth has a curved surface that rotates on a tilted
axis while orbiting the sun, the equatorial regions of the Earth
receive a greater amount of heat from the sun than the polar
regions. The amount of solar energy that heats the Earth
depends on the time of year and the latitude of the specific
region. All of these factors affect the length of time and the
angle at which sunlight strikes the surface.
Solar heating causes higher temperatures in equatorial areas,
which causes the air to be less dense and rise. As the warm
air flows toward the poles, it cools, becoming denser and
sinks back toward the surface. [Figure 12-3]
Atmospheric Pressure
The unequal heating of the Earth’s surface not only modifies
air density and creates circulation patterns; it also causes
changes in air pressure or the force exerted by the weight
of air molecules. Although air molecules are invisible, they
still have weight and take up space.
Imagine a sealed column of air that has a footprint of one
square inch and is 350 miles high. It would take 14.7 pounds
of effort to lift that column. This represents the air’s weight;
if the column is shortened, the pressure exerted at the bottom
(and its weight) would be less.
The weight of the shortened column of air at 18,000 feet is
approximately 7.4 pounds; almost 50 percent that at sea level.
For instance, if a bathroom scale (calibrated for sea level)
were raised to 18,000 feet, the column of air weighing 14.7
pounds at sea level would be 18,000 feet shorter and would
weigh approximately 7.3 pounds (50 percent) less than at
sea level. [Figure 12-4]
The actual pressure at a given place and time differs with
altitude, temperature, and density of the air. These conditions
also affect aircraft performance, especially with regard to
takeoff, rate of climb, and landings.
Coriolis Force
In general atmospheric circulation theory, areas of low
pressure exist over the equatorial regions and areas of high
pressure exist over the polar regions due to a difference in
temperature. The resulting low pressure allows the high-
pressure air at the poles to flow along the planet’s surface
toward the equator. While this pattern of air circulation is
14.7
lb
1
1 Square Inch 1 Square Inch
7.4 lb
18,000 feet18,000 feet
1
1 Square Inch 1 Square Inch
Sea level
Figure 12-4. Atmosphere weights.
Figure 12-5. Three-cell circulation pattern due to the rotation of
the Earth.
correct in theory, the circulation of air is modified by several
forces, the most important of which is the rotation of the Earth.
The force created by the rotation of the Earth is known as
the Coriolis force. This force is not perceptible to humans as
they walk around because humans move slowly and travel
relatively short distances compared to the size and rotation
rate of the Earth. However, the Coriolis force significantly
affects motion over large distances, such as an air mass or
body of water.
The Coriolis force deflects air to the right in the Northern
Hemisphere, causing it to follow a curved path instead of a
straight line. The amount of deflection differs depending on
the latitude. It is greatest at the poles and diminishes to zero
at the equator. The magnitude of Coriolis force also differs
with the speed of the moving body—the greater the speed,
the greater the deviation. In the Northern Hemisphere, the
rotation of the Earth deflects moving air to the right and
changes the general circulation pattern of the air.
The Coriolis force causes the general flow to break up into
three distinct cells in each hemisphere. [Figure 12-5] In
the Northern Hemisphere, the warm air at the equator rises
upward from the surface, travels northward, and is deflected
eastward by the rotation of the Earth. By the time it has
traveled one-third of the distance from the equator to the
North Pole, it is no longer moving northward, but eastward.
This air cools and sinks in a belt-like area at about 30°
latitude, creating an area of high pressure as it sinks toward
the surface. Then, it flows southward along the surface back
toward the equator. Coriolis force bends the flow to the right,
thus creating the northeasterly trade winds that prevail from
30° latitude to the equator. Similar forces create circulation
cells that encircle the Earth between 30° and 60° latitude and
between 60° and the poles. This circulation pattern results in
the prevailing upper level westerly winds in the conterminous
United States.
Circulation patterns are further complicated by seasonal
changes, differences between the surfaces of continents and
oceans, and other factors such as frictional forces caused
by the topography of the Earth’s surface that modify the
movement of the air in the atmosphere. For example, within
2,000 feet of the ground, the friction between the surface and
the atmosphere slows the moving air. The wind is diverted
from its path because of the frictional force. Thus, the wind
direction at the surface varies somewhat from the wind
direction just a few thousand feet above the Earth.
Measurement of Atmosphere Pressure
Atmospheric pressure historically was measured in inches of
mercury ("Hg) by a mercurial barometer. [Figure 12-6] The
barometer measures the height of a column of mercury inside a
glass tube. A section of the mercury is exposed to the pressure
of the atmosphere, which exerts a force on the mercury. An
increase in pressure forces the mercury to rise inside the tube.
When the pressure drops, mercury drains out of the tube
decreasing the height of the column. This type of barometer is
typically used in a laboratory or weather observation station,
is not easily transported, and difficult to read.
29.92 "Hg = 1,013.2 mb (hPa) = 14.7 lb/in2
Sea level
29.92" (760 mm)
Height of mercury Atmospheric pressure
At sea level in a standard
atmosphere, the weight
of the atmosphere
(14.7 lb/in2) supports
a column of mercury
29.92 inches high.
Figure 12-6. Although mercurial barometers are no longer used
in the U. S., they are still a good historical reference for where the
altimeter setting came from (inches of mercury).
Lower
Higher
Atmospheric pressure
Sealed aneroid cell
Sealed aneroid cell
Sealed aneroid cell
Figure 12-7. Aneroid barometer.
An aneroid barometer is the standard instrument used
to measure pressure; it is easier to read and transport.
[Figure 12-7] The aneroid barometer contains a closed vessel
called an aneroid cell that contracts or expands with changes
in pressure. The aneroid cell attaches to a pressure indicator
with a mechanical linkage to provide pressure readings. The
pressure sensing part of an aircraft altimeter is essentially
an aneroid barometer. It is important to note that due to
the linkage mechanism of an aneroid barometer, it is not as
accurate as a mercurial barometer.
To provide a common reference, the International Standard
Atmosphere (ISA) has been established. These standard
conditions are the basis for certain flight instruments and
most aircraft performance data. Standard sea level pressure
is defined as 29.92 "Hg and a standard temperature of 59 °F
(15 °C). Atmospheric pressure is also reported in millibars
(mb), with 1 "Hg equal to approximately 34 mb. Standard sea
level pressure is 1,013.2 mb. Typical mb pressure readings
range from 950.0 to 1,040.0 mb. Surface charts, high and low
pressure centers, and hurricane data are reported using mb.
Since weather stations are located around the globe, all local
barometric pressure readings are converted to a sea level
pressure to provide a standard for records and reports. To
achieve this, each station converts its barometric pressure by
adding approximately 1 "Hg for every 1,000 feet of elevation.
For example, a station at 5,000 feet above sea level, with a
reading of 24.92 "Hg, reports a sea level pressure reading of
29.92 "Hg. [Figure 12-8] Using common sea level pressure
readings helps ensure aircraft altimeters are set correctly,
based on the current pressure readings.
By tracking barometric pressure trends across a large area,
weather forecasters can more accurately predict movement
of pressure systems and the associated weather. For example,
tracking a pattern of rising pressure at a single weather station
generally indicates the approach of fair weather. Conversely,
decreasing or rapidly falling pressure usually indicates
approaching bad weather and, possibly, severe storms.
Altitude and Atmospheric Pressure
As altitude increases, atmospheric pressure decreases. On
average, with every 1,000 feet of increase in altitude, the
atmospheric pressure decreases 1 "Hg. As pressure decreases,
the air becomes less dense or thinner. This is the equivalent of
being at a higher altitude and is referred to as density altitude.
As pressure decreases, density altitude increases and has a
pronounced effect on aircraft performance.
Differences in air density caused by changes in temperature
result in a change in pressure. This, in turn, creates motion in
the atmosphere, both vertically and horizontally, in the form
of currents and wind. The atmosphere is almost constantly in
motion as it strives to reach equilibrium. These never-ending
air movements set up chain reactions that cause a continuing
variety in the weather.
Standard Atmosphere
Denver 29.92 "Hg
24.92 "Hg
Station Pressure
Denver
29.92 "Hg
Station Pressure
New Orleans
New Orleans 29.92 "Hg
Figure 12-8. Station pressure is converted to and reported in sea level pressure.
1,590 feet
745 feet
Pressure Altitude: Sea level
Pressure Altitude: 8,000 feet
TAKEOFF DISTANCE
MAXIMUM WEIGHT 2,400 LB
Pressure
altitude
(feet)
S.L.
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
Ground
roll
(feet)
745
815
895
980
1,075
1,185
1,305
1,440
1,590
Total feet
to clear
50 foot obstacle
1,320
1,445
1,585
1,740
1,920
2,125
2,360
2,635
2,960
0 °C
Figure 12-9. Takeoff distances increase with increased altitude.
Altitude and Flight
Altitude affects every aspect of flight from aircraft
performance to human performance. At higher altitudes,
with a decreased atmospheric pressure, takeoff and landing
distances are increased, while climb rates decrease.
When an aircraft takes off, lift is created by the flow of air
around the wings. If the air is thin, more speed is required
to obtain enough lift for takeoff; therefore, the ground run
is longer. An aircraft that requires 745 feet of ground run at
sea level requires more than double that at a pressure altitude
of 8,000 feet. [Figure 12-9]. It is also true that at higher
altitudes, due to the decreased density of the air, aircraft
engines and propellers are less efficient. This leads to reduced
rates of climb and a greater ground run for obstacle clearance.
Altitude and the Human Body
As discussed earlier, nitrogen and other trace gases make
up 79 percent of the atmosphere, while the remaining 21