Aviation Weather Fundamentals2/15/2010

Todd Deckardtwdeckard@ground-effect.com

CFI-wannabe

Lesson Plan: WX Fundamentals• Objective

– Learner should have a very basic knowledge of weather effects, reading a surface depiction chart, and gathering a synoptic picture of the current conditions and forecast.

• Content– Highs, Lows, Fronts, Wind, Temperature, Precipitation, Visibility – Weather Map Symbols

• Schedule– 02: introduction– 18 slides x 60 seconds (skip stability and visibility slides if necessary)

• Equipment– Powerpoint, Projector

• Elements– Basic weather vocabulary, weather chart symbols, strategy for getting The Big Picture

• Instructors Actions– Tell “there I was…” stories while slamming slide deck

• Students Actions– Answer overhead questions, take home quiz, pick a hypothetical flight and “try it” each day

• Completion Standards– No one sleeping

Weather

• Fundamentals• The Weather Map• The Weather Synopsis

Conversation about the weather is the last refuge of the un-imaginative Oscar Wilde

Fundamentals

1. High and Low Pressure2. Fronts3. Wind4. Temperature5. Stability6. Visibility

• The Sun heats the earths surface un-evenly. The difference in temperature upsets any equilibrium in the atmosphere creating changes in pressure and resulting in air movement. This movement of air is called circulation.

• The air which has been warmed is less dense and will rise• The air which is cooler will sink.

• Air flows from High to Low Pressure

High and Low Pressure

The winds and the waves are always on the side of the ablest navigators.— Edward Gibbon, 'The History of the Decline and Fall of the Roman Empire.'

• The rotation of the earth about its axis gives rise to the coriolis effect.

• Imagine you and your co-pilot are on opposite sides of a merry-go-round trying to throw a ball back and forth.

• It flies straight as seen from the playground but you both would think the ball curved.

• The earth is spinning like the merry go-round.

• Circulating air will curl clockwise in the Northern Hemisphere as it flows from High to Low

"I just whip out my blue card with a hole in it and read what it says: 'When color of card matches color of sky, FLY!'" — Gordon Baxter

Coriolis Effect

Circulation

• Question: if you stand with the wind at your back and stretch out your arm which hand points to the low?

Fronts

• Colliding air masses produce Fronts

Cold Northern AirAdvancing

Warm Southern AirRetreating

Warm Front

• Warm air slides up and over the heavy cold air• Water vapor condenses as air is forced up and chilled

by the colder altitude

Flight toward a warm front, from the cold side, is characterized by gradually lowering stratiform clouds then light continuous precipitation, lowering ceilings and visibility.

Hint: sunshines

Cold Front• A cold front is the boundary between warm and cool air when the cool air

is advancing to replace the warm air. As a cold front moves into an area, the heavier, cool air pushes under the lighter, warm air it's replacing.

• If the rising air is humid, water vapor in it will condense into clouds or rain• Precipitation ahead of a cold front can be continuous, but behind the

front, becomes showery. The visibility outside of showers improves..

Cold fronts are recognized in flight usually by a line of Towering Cumulous, Cumulo-Nimbus or Alto-Cumulous clouds.

Hint: icicles

Wind

• Isobars are lines of equal pressure• Wind flows roughly parallel to isobars• Tight isobars = high wind

Ellsworth AFB reported 87MPH Gusts!

If a person does not know to which port he is steering, no wind is favorable to him.— Seneca, "ignoranti quem portum petat, nullus suus ventus est."

Temperature

• In the Tropopause (0-10 miles up) the atmosphere normally gets cooler with altitude (Question: is there a common exception?)

• Warm air can carry more water vapor• The dew point is the temperature

at which water vapor will condense• Temp = dew point = clouds• At 0 AGL temp = dew point = fog

Stability– When temperature decreases uniformly and rapidly as you

climb (approaching 3° C per 1,000 feet), you have an indication of unstable air.

– If temperature remains unchanged or decreases only slightly with altitude, the air tends to be stable.

– If the temperature increases with altitude through a layer - an inversion - the layer is stable and convection is suppressed. Air may be unstable beneath the inversion.

– When air near the surface is warm and moist, suspect instability. Surface heating, cooling aloft, converging or upslope winds, or an invading mass of colder air may lead to instability and cumuliform clouds.

Clouds

I will ascend above the heights of the clouds; I will be like the most High.

— Isaiah 14:14

VisibilityOr “what does one thousand and three really buy me …”

• Fair weather cumulus clouds often indicate bumpy turbulence beneath and in the clouds. The cloud tops indicate the approximate upper limit of convection; flight above is usually smooth and the air clear

• Stratiform clouds indicate stable air. Flight generally will be smooth, but low ceiling and visibility might make VFR flight impractical.

• Restricted visibility at or near the surface over large areas usually indicates stable air. Expect a smooth ride, but poor visibility might make VFR flight impractical.

"I just whip out my blue card with a hole in it and read what it says: 'When color of card matches color of sky, FLY!'“

-Gordon Baxter

Surface Depiction Chart

Synoptic Weather Picture• Synoptic means seen with one eye• Get the Big Picture:

– Where are the Highs/Lows– Where are the Fronts?– Visibility/Precipitation/Adverse Conditions– Winds

• Flight Planning– Current conditions at departure?– Current and Forecast conditions en-route?– Current and Forecast conditions at the Destination?

ADM/D.E.C.I.D.E.• Am I flying towards better or worse?• Which direction is the good weather?• How confident am I in the forecast?

– How old is the forecast?– How have the previous forecasts held up?– Do winds aloft match what I was expecting?– Red flags: temperature and dew-point are close or converging

Everybody talks about the weather, but nobody does anything about it. Mark Twain

Take-Home QuizPick a favorite destination within 300 miles . Using any of: printed WX maps from the

newspaper, internet WX sources, official aviation WX sources – Plan a VFR flight for the following Saturday AM to return Sunday PM– Prognostic, 3-2 and 1 day prior

• Describe potential hazards, forecast ceilings, visibility and winds for departure, enroute and destination

• If VFR flight will not likely be practical, what changes could you make?– Saturday Morning

• Brief for flight• Describe current conditions at Departure and current and forecast

conditions Enroute and Destination (hazards, ceilings, visibility and winds)– Sunday Night

• Brief for return flight• Describe current conditions at Departure and current and forecast

conditions Enroute and Destination (hazards, ceilings, visibility and winds)

Take Home Quiz

3 Day Prognostic ____________2 Day Prognostic ____________24 Hour Prognostic __________Departure Briefing __________Return Briefing _____________If you were able to make the outbound trip, but not

the scheduled return – watch the subsequent weather and determine the likely length of delay