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Pilot's Handbook of Aeronautical Knowledge
Weather Theory
Atmospheric Stability

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




The adiabatic process takes place in all upward and
downward moving air. When air rises into an area of lower
pressure, it expands to a larger volume. As the molecules
of air expand, the temperature of the air lowers. As a result,
when a parcel of air rises, pressure decreases, volume
increases, and temperature decreases. When air descends,
the opposite is true. The rate at which temperature decreases
with an increase in altitude is referred to as its lapse rate.
As air ascends through the atmosphere, the average rate of
temperature change is 2 °C (3.5 °F) per 1,000 feet.

Since water vapor is lighter than air, moisture decreases air
density, causing it to rise. Conversely, as moisture decreases,
air becomes denser and tends to sink. Since moist air cools at a
slower rate, it is generally less stable than dry air since the moist
air must rise higher before its temperature cools to that of the
surrounding air. The dry adiabatic lapse rate (unsaturated air)
is 3 °C (5.4 °F) per 1,000 feet. The moist adiabatic lapse rate
varies from 1.1 °C to 2.8 °C (2 °F to 5 °F) per 1,000 feet.

The combination of moisture and temperature determine the
stability of the air and the resulting weather. Cool, dry air
is very stable and resists vertical movement, which leads to
good and generally clear weather. The greatest instability
occurs when the air is moist and warm, as it is in the tropical
regions in the summer. Typically, thunderstorms appear on
a daily basis in these regions due to the instability of the
surrounding air.

As air rises and expands in the atmosphere, the temperature
decreases. There is an atmospheric anomaly that can occur;
however, that changes this typical pattern of atmospheric
behavior. When the temperature of the air rises with altitude, a
temperature inversion exists. Inversion layers are commonly
shallow layers of smooth, stable air close to the ground. The
temperature of the air increases with altitude to a certain
point, which is the top of the inversion. The air at the top
of the layer acts as a lid, keeping weather and pollutants
trapped below. If the relative humidity of the air is high, it
can contribute to the formation of clouds, fog, haze, or smoke,
resulting in diminished visibility in the inversion layer.

Surface based temperature inversions occur on clear, cool
nights when the air close to the ground is cooled by the
lowering temperature of the ground. The air within a few
hundred feet of the surface becomes cooler than the air above
it. Frontal inversions occur when warm air spreads over a
layer of cooler air, or cooler air is forced under a layer of
warmer air.

Moisture and Temperature
The atmosphere, by nature, contains moisture in the form
of water vapor. The amount of moisture present in the
atmosphere is dependent upon the temperature of the air.
Every 20 °F increase in temperature doubles the amount of
moisture the air can hold. Conversely, a decrease of 20 °F
cuts the capacity in half.

Water is present in the atmosphere in three states: liquid,
solid, and gaseous. All three forms can readily change to
another, and all are present within the temperature ranges of
the atmosphere. As water changes from one state to another,
an exchange of heat takes place. These changes occur through
the processes of evaporation, sublimation, condensation,
deposition, melting, or freezing. However, water vapor
is added into the atmosphere only by the processes of
evaporation and sublimation.

Evaporation is the changing of liquid water to water vapor.
As water vapor forms, it absorbs heat from the nearest
available source. This heat exchange is known as the latent
heat of evaporation. A good example is the evaporation of
human perspiration. The net effect is a cooling sensation
as heat is extracted from the body. Similarly, sublimation
is the changing of ice directly to water vapor, completely
bypassing the liquid stage. Though dry ice is not made of
water, but rather carbon dioxide, it demonstrates the principle
of sublimation, when a solid turns directly into vapor.

Relative Humidity
Humidity refers to the amount of water vapor present in the
atmosphere at a given time. Relative humidity is the actual
amount of moisture in the air compared to the total amount of
moisture the air could hold at that temperature. For example,
if the current relative humidity is 65 percent, the air is
holding 65 percent of the total amount of moisture that it is
capable of holding at that temperature and pressure. While
much of the western United States rarely sees days of high
humidity, relative humidity readings of 75 to 90 percent are
not uncommon in the southern United States during warmer
months. [Figure 11-20]

Temperature/Dew Point Relationship
The relationship between dew point and temperature defines
the concept of relative humidity. The dew point, given in
degrees, is the temperature at which the air can hold no
more moisture. When the temperature of the air is reduced
to the dew point, the air is completely saturated and moisture
begins to condense out of the air in the form of fog, dew,
frost, clouds, rain, hail, or snow.