Lesson 2-3
Aviation Weather
Lesson Overview
Causes of atmospheric instability
 Types and causes of turbulence
 How types of severe weather affect aviation

Chapter 2, Lesson 3
Quick Write
What is the story behind Colonel Duckworth and
his AT-6 trainer aircraft--why did he decide to
fly into a hurricane…twice? What happened?
Chapter 2, Lesson 3
Courtesy of the National Museum of the
USAF
Causes of Atmospheric
Instability


Small vertical movements grow until
they produce turbulent airflow and air
circulation
Why does air cool as it rises and warm
as it falls




Adiabatic means no heat is gained or lost
in a system
Adiabatic cooling and heating take place in
upward and downward moving air
When air rises, volume increases while
pressure and temperature decreases
When air descends the opposite is true
Chapter 2, Lesson 3
Reproduced from NOAA/National Weather Service
Causes of Atmospheric
Instability, cont.

Water vapor is less dense than air



As air becomes drier, air becomes denser and sinks
Lapse Rate: The rate at which temperature
decreases with increasing altitude



Moisture decreases air density, causes air to rise
It is different between moist and dry air
Moist air cools at a slower rate than dry air
Combination of moisture and temperature
determines air stability and weather


Cool, dry air is very stable and resists vertical movement
Warm, moist air produces the greatest instability
Turbulence

What is it?
 Produced by an unstable atmosphere
 A sign of rapidly rising and falling currents of air
 Can occur anywhere from ground to tropopause
Thermal Turbulence




Rise of warm air, taking place on a
local scale
Plowed ground, rocks, sand, and
barren land emit a large amount
of heat
Water, trees, and other growing
things absorb heat
These reactions to sun’s energy
results in uneven heating of the
air, creates small areas called
convective currents
Convective Currents



Create bumpy, turbulent air
On low-altitude flight, pilots may run into updrafts over
pavement or barren places, and downdrafts over water
or forests
To avoid these turbulent conditions, they can fly at
higher altitude, even above cumulus clouds
Mechanical Turbulence



Generated by resistance of an object moving over another
As air moves over Earth’s surface, friction that develops
between air and surface modifies the air’s movement
Large objects—mountains to man-made structures such as
buildings—generate mechanical turbulence
Wind Shear


Abrupt, dramatic change in wind speed and/or direction
Low-level are linked with thunderstorms, and
temperature inversions, with strong upper-level winds


Extremely dangerous due to proximity of ground
High-Level begins around
18,000 feet and can
produce clear air
turbulence (CAT)

Most CAT takes place
around, but not in, jet
streams
Thunderstorms




Convection triggers thunderstorms
Water vapors rising with warm air
condense into clouds
Convection process then continues
within the clouds
At the tropopause the unstable
atmosphere stabilizes
Chapter 2, Lesson 3
Reproduced from NOAA/Earth System Research
Laboratory
Thunderstorms

To form, three things are needed:





As warm moist air rises, it condenses into clouds
Condensation give off heat, further warming the air


Enough water vapor
An unstable lapse rate
Lifting action to power up the storm
Process can continue until clouds reaches tropopause
As raindrops form and begin to fall, the air compresses
and cools, slowing convection until the storm and
clouds break up
Chapter 2, Lesson 3
Reproduced from US Department of
Transportation/Federal Aviation Administration
Thunderstorm Stages

Cumulus Stage



Mature Stage




Lifting action, vertically growing clouds
Strong updrafts stop moisture from falling
Most violent time for the cloud
Rain begins to fall, causing cooler downdrafts that cause
violent wind sheers with warmer updrafts
As cooler downdrafts hit surface, they move horizontally in
front of storm producing gust fronts
Dissipating Stage

Downdrafts spread out and replace warm updrafts that were
feeding the storm
Thunderstorms
Air Mass vs Steady-State

Air mass Thunderstorm



Occur in unstable air and only last an hour or two
Usually caused by warm surface temperatures
Steady-state Thunderstorm


More threatening than air mass thunderstorm
Caused by:






Fronts
Big winds that run into one another
A long stretch of low pressure called a trough
Often form into narrow band of active thunderstorms called
squall lines
Updrafts grow stronger
Can last much longer than 2 hours
Hazards to Aircraft



Look for squall lines, tornadoes, turbulence, icing,
hail, lightning, and poor visibility
Light aircraft won’t be able to fly over
thunderstorms
Air Force instructs pilots to fly 20 miles from the
storm’s edge
Squall Lines



Develop in moist,
unstable air, on or in
advance of a cold front
Often contain steadystate thunderstorms
Form quickly, are
strongest in the late
afternoon and early
evening
Tornadoes
Materialize out of the most violent thunderstorms
 Can sharpen into a powerful vortex that reaches
from the ground into the clouds
 Funnel-shaped cloud that stretches earthward
from a cumulonimbus base
 200+ mph winds

Turbulence



Pilots encounter strongest turbulence inside a cloud
Turbulence can occur outside of thunderstorm clouds
 Several thousand feet above
 20 miles sideways
Gust fronts can be up to 15 miles ahead of any storm
Icing—Hail

Icing, is a product of updrafts


In a thunderstorm, the updraft lifts water vapor above
freezing level, the water supercools
Hail, another form of supercooled water

Can be just as dangerous to aircraft as turbulence
Lightning—Poor Visibility

Lightning—hazard that’s most closely associated with
thunderstorms


Can puncture an aircraft’s skin, damage its communications
and navigational equipment
Visibility is near zero within a thunderstorm cloud
Activity:
AtmosModeler Lab
Temperature and Altitude

Use the AtmosModeler interactive simulation
software to investigate how changes in
altitude affect temperature

Gather and record data on temperature and
altitude then use that information to make
calculations
Chapter 2, Lesson 3
Next….


Done – aviation
weather
Next – weather
forecasting
Chapter 2, Lesson 3
Courtesy of NASA/George Shelton