Download November 1998

APPENDIX D
ATMOSPHERIC STABILITY AND CLOUDS
STUDENT HANDOUT
TERMINAL LEARNING OBJECTIVE (TLO): At the completion of this lesson, the student will:
ACTION: Plan a flight mission IFR or VFR in accordance with weather conditions stated in FM 1-230 and
AR 95-1.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
SAFETY REQUIREMENTS: None.
RISK ASSESSMENT LEVEL: Low.
ENVIRONMENTAL CONSIDERATIONS: None.
A. ENABLING LEARNING OBJECTIVE (ELO) #1:
ACTION: Identify the different states in which moisture is found in the atmosphere.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Water states.
(1) Vapor / gas – it is invisible.
(2) Liquid -- can be seen.
(3) Solid -- has structure.
NOTE: If below 0oC and has not solidified, the liquid is supercooled.
b. Examples of water states.
(1) Vapor -- invisible gas.
(2) Liquid - -rain, clouds, fog, dew, etc.
(3) Solid -- ice, snow, hail, frost, etc.
B. ENABLING LEARNING OBJECTIVE (ELO) #2:
ACTION: Identify the changes moisture makes from one water state to another.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Change of moisture state.
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NOTE: Latent or hidden heat—is released or taken in through changes in moisture states.
(1) Evaporation--liquid changes to water vapor. This requires heat; the evaporating water vapor
carries heat with it. This is known as the latent heat of evaporation.
(2) Condensation--water vapor changing to liquid water. Occurs when air is saturated and
impurities are present. Releases the latent heat of condensation.
(3) Fusion (freezing) --the change of state from liquid to solid (releases latent heat).
(4) Sublimation--process when vapor changes directly to ice or ice to vapor. Vapor to ice releases
heat. This is known as the latent heat of sublimation. This process takes place below freezing
temperatures (0o C or 32oF)
(5) Melting--solid water (ice) changes to liquid. Must absorb the heat of melting (approximately 80
calories/gram).
b. Moisture changes readily from one state to another.
C. ENABLING LEARNING OBJECTIVE (ELO) #3:
ACTION: Identify the amount of water vapor contained in a portion of the atmosphere.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Water vapor content.
(1) Saturation--the air has all the water vapor that it can hold at that temperature. Warm air can hold
more water vapor than cold air. The capacity of air to hold water vapor doubles for every 20F increase in
temperature.
(2) Dew point--the temperature to which the air must be cooled for it to become saturated. The
higher the dew point, the more water vapor is present in the air.
NOTE: The difference between temperature and dewpoint is referred to as the “spread”. Condensation
may occur when the spread is 2oC or less.
(3) Relative humidity--the amount of water vapor in the air expressed as a percent, compared to
what it could hold at that temperature.
b. Water vapor content of the air has a direct effect on aircraft performance.
D. ENABLING LEARNING OBJECTIVE (ELO) #4:
ACTION: Identify the adiabatic process.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Definition -- a process in which the temperature of a gas (air) is changed without the addition or
deletion of heat energy.
b. Adiabatic cooling and heating.
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(1) Cooling -- expansion of a gas will cause its temperature to decrease.
(2) Heating -- compression of a gas will cause its temperature to increase.
c. Atmospheric adiabatic processes.
(1) When air rises, it will cool adiabatically because of expansion.
(2) Heating -- when air sinks it will heat adiabatically because of compression.
E. ENABLING LEARNING OBJECTIVE (ELO) #5:
ACTION: Identify the moist and dry adiabatic cooling rates.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Cooling rates.
(1) Dry -- 3C per 1,000 feet occurs when the air is rising and is not saturated (no condensation is
occurring).
(2) Moist -- approximately 1.5C per 1,000 feet occurs after saturation takes place in rising air.
b. The rate at which a parcel of air cools is extremely important in the formation of clouds.
F. ENABLING LEARNING OBJECTIVE (ELO) #6:
ACTION: Identify the points where condensation and free convection take place.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Levels of condensation and free convection.
(1) Lifted Condensation Level (LCL) or Level of Condensation -- an altitude to which the air must be
lifted for condensation to take place. Clouds will form at this altitude. Temperature and dew point are the
same.
(2) Level of Free Convection (LFC) -- an altitude at which the temperature of the air inside the parcel
of air being lifted becomes the same as the surrounding air because of the release of the latent heat of
condensation. Any further lifting will cause the air inside the parcel to become warmer than the
surrounding air and have its own lifting force. Thunderstorms may begin to form at this point.
b. These two levels have direct bearing on the altitude and types of clouds that will form.
G. ENABLING LEARNING OBJECTIVE (ELO) #7:
ACTION: Identify factors that determine atmosphere stability.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
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a. Stability conditions (three).
(1) Stable -- air that resists vertical movement.
(2) Unstable -- air that starts moving vertically and will continue without any outside force.
(3) Conditionally unstable (convectively unstable) -- air that may be stable under one set of
conditions and unstable under another.
b. Stability factors.
(1) The Observed Lapse Rate (OLR) is the best way to determine atmospheric stability.
(a) OLR less than the moist adiabatic cooling rate (1.5o), air will be absolutely stable.
(B) OLR greater than the dry adiabatic cooling rate (3.0o), air will be absolutely unstable.
(c) OLR between the dry and moist adiabatic cooling rates (1.5o – 3.0o), air will be conditionally
unstable.
(2) Moisture content -- air with high moisture content tends to be more unstable than dry air, if lifting
action occurs.
H. ENABLING LEARNING OBJECTIVE (ELO) #8:
ACTION: Identify how clouds are formed.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Definition -- extremely small condensed water vapor droplets suspended in the atmosphere (less
than .002 inches).
b. Formation of clouds -- formation is the direct result of saturation. Saturation is generally the result of
cooling.
c. Cooling process-(1) Adiabatic -- most common, vertical development.
(2) Nocturnal -- low clouds and fog.
(3) Advection -- warm, moist air moving over a colder surface usually producing low clouds.
I. ENABLING LEARNING OBJECTIVE (ELO) #9:
ACTION: Identify the basic types of clouds.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Categories of clouds.
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(1) Cumulus -- clouds with vertical development formed by rising air. Tops may reach over 60,000
feet.
(2) Stratus -- clouds with horizontal development, generally formed in stable air.
b. Each of these two cloud types has distinctive characteristics that affect flight operations.
J. ENABLING LEARNING OBJECTIVE (ELO) #10:
ACTION: Identify the classification of clouds in terms of the cloud’s altitude.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Classification and altitudes.
(1) Names used to describe various types of clouds.
(a) Nimbus -- rain cloud.
(b) Fraction -- fragmented clouds.
(c) Alto -- middle level clouds.
(2) Altitude of cloud type. Cloud bases reported to the nearest 100 feet AGL.
(a) Low -- bases up to 6,500 feet.
(B) Middle -- bases 6,500 feet to 20,000 feet.
(c) High -- bases over 20,000 feet.
b. Clouds with vertical development--may extend from near the surface to over 60,000 feet.
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K. ENABLING LEARNING OBJECTIVE (ELO) #11:
ACTION: Identify cloud types by assigned abbreviations.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Abbreviations.
(1) Cumulus -- Cu.
(2) Cumulonimbus -- Cb.
(3) Stratus -- St.
(4) Stratocumulus -- Sc.
(5) Nimbostratus -- Ns.
(6) Altostratus -- As.
(7) Altocumulus -- Ac.
(8) Cirrus -- Ci
(9) Cirrostratus -- Cs.
(10) Cirrocumulus -- Cc.
b. The above list covers only the most common types.
L. ENABLING LEARNING OBJECTIVE (ELO) #12:
ACTION: Identify the flight conditions that would be encountered in the two basic cloud types.
CONDITION: In a classroom environment.
STANDARD: IAW FM 1-230.
a. Cumulus and cumulonimbus clouds-- may be extremely violent, thunderstorms, microbursts, heavy
precipitation, possible hail, possible tornadoes, icing, gusty surface winds; good visibility outside of clouds
and precipitation.
b. Stratus clouds -- may extend over large areas causing low ceilings, poor visibility, light steady
precipitation, little or no turbulence, icing, and light surface winds.
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