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Weather Theory
6 April 2005
Heating of the Earth
 All weather on earth is caused by
differential (unequal) heating of the
earth’s surface!
 Large-scale atmospheric circulation
(warm air rises; cooler air settles)
 Add rotational effects of the Coriolis
Force and winds are deflected to the
right (in the Northern Hemisphere)
 Coriolis force causes wind to flow
parallel to isobars.
Circulation and Wind
 Air circulates clockwise around a high and
counterclockwise around a low.
 When isobars are closer together, wind speed increases.
 Surface friction causes winds to not exactly parallel the
isobars but rather move outward from high pressure
areas.
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(e.g., more favorable winds will be found south of highpressure areas when flying west.)
Temperature
 Standard Temperature is 15 Celsius
 Temperature decreases an average of 2
degrees per 1000 feet. (Standard lapse rate)
 When temperature increases with altitude,
there is a temperature inversion.
 The most frequent ground-based TI is
produced by radiation on a clear, still night.
Moisture
 Key Terms
 Relative humidity: percentage of air’s
maximum saturation
 Dewpoint: temperature to which air must
be cooled to become saturated (100% RH)
 Adding and removing moisture:
 Evaporation
 Sublimation
Airmasses and Fronts
 Air masses are formed when a large body of air sits
on top of a region for a long time, taking on its
moisture and temperature contents.
 Two airmasses are separated by a front.
 Ridges are lines of high pressure
 Troughs are lines of low pressure.
Fronts
 Frontal passage is denoted by:
 Change in temperature
 Decrease in pressure followed by an
increase
 A shift in wind direction, speed, or both
Atmospheric Stability
 Stability is the resistance
of the atmosphere to
vertical motion


A stable atmosphere
resists vertical motion
An unstable atmosphere
allows convective
currents to develop.
 Stable air is often
associated with a
temperature inversion
Stable Air
Unstable Air
Stratiform
clouds
Cumuliform
clouds/fog
Smooth air
Rough air
Continuous Showery
precipitation precipitation
Poor
visibility
Good
visibility
Clouds
 Two general types:
 Cumuliform
 stratiform
Clouds
 Cumuliform
 Form in unstable air
 Are vertically developed (billowy)
 Showery precipitation
 Stratiform
 Form in unstable air
 Flat/even
 Continuous light rain
 Cirrus
 High clouds composed of ice crystals
Clouds
 Bases of cumuliform clouds can be
determined by:
 Subtracting temperature (Celsius) from
dewpoint and dividing by 2.5 (temperature per
1000 feet that temp. and dewpoint converge)
 This is the height (in thousands of feet) of the
base of the clouds
Turbulence
 Found in convective currents (e.g. when
cumulus clouds are present)
 Strongest in areas with large updrafts
(towering cumulus/cumulonimbus)
 When severe turbulence is encountered,
reduce airplane’s airspeed to
maneuvering speed and attempt to
maintain a level flight attitude
Thunderstorms
 Three prerequisites:
 Water vapor
 Unstable lapse rate (unstable air)
 Lifting action
 Three stages:
 Cumulus
 Mature
 Dissipating
Thunderstorms
Lightning always present
Hail frequently created in up/down drafts
Significant windshear turbulence
Embedded thunderstorms are those that are
within large cloud areas and cannot be seen
 Squall-line thunderstorms create the most
hazardous conditions and develop ahead of
cold fronts
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Wind Shear
 Any change in direction/speed in a short
distance
 Low-level wind shear most hazardous
 Temperature inversions
 Around thunderstorms
 Found when wind speed between 2000’ and 4000’ is
at least 25 kts
Wind shear
 Two hazardous conditions:
 Loss of tailwind
 Airspeed initially increases
 Aircraft pitches up
 Altitude increases, causing a requirement for
less power
 Loss of headwind
 Airspeed decreases (dangerous if flying slowly)
 Aircraft pitches down and begins to sink
Icing
 Two types:
 Clear ice – hard /glossy, cumuliform clouds
 Rime ice – stratiform clouds/small drops
 Occurs when water contacts airplane
parts that are below freezing
 Requires visible moisture
 Ice pellets indicate freezing rain above
(most dangerous)
Icing

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Dangerous!
Increases weight
Increases drag
Decreases lift
Decreases thrust
Increases stalling speed
Fog
 Radiation fog
 Terrestrial radiation cools the ground, cooling air
touching ground
 Advection fog
 Warm, moist air moves over cold ground
 Upslope fog
 Moist, stable air is cooled to dewpoint
 Preciptation fog
 Caused by evaporating precipitation
 Steam fog
 Cold, dry air moves over relatively warm water
Frost
 Ice deposits formed by sublimation on a
surface
 Formed when collecting surface is at or below
the freezing point
 Frost causes loss of aerodynamic efficiency
due to early flow separation
 Do not take off with frost on your airplane!
Next Week…
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