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 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

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: 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

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

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 steady- state 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