1. Lesson 2-3 Aviation Weather

2. Lesson Overview Causes of atmospheric instability
Types and causes of turbulence
How types of severe weather affect aviation
Chapter 2, Lesson 3

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

4. 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

5. Causes of Atmospheric Instability, cont.
Water vapor is less dense than air
Moisture decreases air density, causes air to rise
As air becomes drier, air becomes denser and sinks
Lapse Rate: The rate at which temperature decreases with increasing altitude
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

6. 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

7. 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

8. 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

9. 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

10. 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 ,000 feet and can produce clear air turbulence (CAT)
Most CAT takes place around, but not in, jet streams

11. 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

12. Thunderstorms To form, three things are needed:
Enough water vapor
An unstable lapse rate
Lifting action to power up the storm
As warm moist air rises, it condenses into clouds
Condensation give off heat, further warming the air
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

13. Thunderstorm Stages Cumulus Stage Mature Stage Dissipating Stage
Lifting action, vertically growing clouds
Strong updrafts stop moisture from falling
Mature Stage
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
To form, three things are needed:
Enough water vapor
An unstable lapse rate
Lifting action to power up the storm
As warm moist air rises, it condenses into clouds
Condensation give off heat, further warming the air
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

14. 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

15. 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

16. Squall Lines
Develop in moist, unstable air, on or in advance of a cold front
Often contain steady- state thunderstorms
Form quickly, are strongest in the late afternoon and early evening

17. 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

18. 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

19. 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

20. 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

21. 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

22. Next…. Done – aviation weather Next – weather forecasting
Chapter 2, Lesson 3
Courtesy of NASA/George Shelton