Weather Theory Part II (Group C)

Weather TheoryPart II: Air Movement

Group C: Second Year

PHAK Chapter 11, pages 7-12

Recall• Air always strives to achieve _________ due to

unequal _______ of Earth’s surface• Atmosphere is 78% ________, __% oxygen,

and 1% other gases• Warm air is ____ (____ dense), thus rises while

cool air is _____ (____ dense), thus sinks• ____________ deflects air to the ____ in the

Northern Hemisphere• Air is measured in inches of ________• Pressure and temperature ________ as altitude

________• Lower pressure causes longer takeoff roll• Lower oxygen concentration at higher _______

can/will cause _______

thin lessthick

hypoxia

more

equilibriumheating

nitrogen 21

Coriolis effect right

mercurydecrease

increases

altitudes

Essential Questions• What is the difference between wind and

currents?• What is the difference between cyclonic and anti-

cyclonic circulation?• What effects do surfaces have that retain heat?

Release heat?• What is low-level wind shear and what sorts of

hazards does it present?• How can wind and pressure be depicted on a

surface weather map?

Wind and Currents

• Air always strives equilibrium – air moves from high pressure to low pressure

• Currents – vertical movement of air

• Wind – horizontal movement of air

Wind Patterns• Vertical movements – currents• Horizontal movements – winds• Anti-cyclonic circulation is clockwise movement

of air around an area of high pressure• Cyclonic circulation is counterclockwise

movement of air around an area of low pressure• High pressure:

– Dry, stable, descending– Good weather

• Low pressure:– Unstable, cloudiness and precipitation– Bad weather

• Favorable winds• Large-scale only; doesn’t account for local

conditions, geographical abnormalities, etc.

Favorable Winds

Convective Currents• Releases heat:

– Plowed ground– Rocks– Sand

• Retains heat:– Water– Trees

• Convective currents cause turbulent air when flying low, in warm weather, or over varying surfaces

Releases heat Retains heat

updraft downdraft

Convective Turbulence

Intended Flight Path

Sea and Land Breeze Circulation

Mountain Turbulence

• Air flows smoothly up mountain but follows contour of terrain back down, forcing airplane down the side

Low-Level Wind Shear

• Sudden, drastic change in wind speed and/or direction over a very small area

• Violent updrafts and downdrafts• Abrupt changes to horizontal movement of

aircraft• Hazardous due to close proximity of aircraft

close to ground• Directional wind changes of 180 and speed

changes of 50+ knots associated

Low-Level Wind Shear (cont’d)

• Rapid changes in wind direction and velocity change the wind’s relation to the aircraft, disrupting the normal flight attitude and performance of the aircraft

• Most severe type associated with convective precipitation or rain from thunderstorms

Microburst

• Type of low-level wind shear• Associated with convective precipiration• Normally >1 mile horizontal & ≥1,000 feet

vertically and lasts 15 minutes, during which can produce downdrafts of up to 6,000 fpm

• Microburst behavior:– 1. performance-increasing headwind– 2. performance-decreasing downdraft– 3. rapid tailwind shear– 4. can result in terrain impact or flight

dangerously close to the ground

Microburst (cont’d)

• Difficult to detect

• Low-Level Wind Shear Alert System (LLWAS)

• Can affect any pilot in any airplane

• Undetected, silent danger

Wind on Surface Weather Maps

• Information about fronts, areas of high/low pressure, surface winds

• How to read:– Circle indicates weather station– No line (two circles) represents calm wind– Direction of line indicates wind direction– Half-barb represents 5 knots– Full barb represents 10 knots– Pennant represents 50 knots

Surface Weather Map

Isobars

• Lines drawn on the chart to depict areas of equal pressure

• Reveals pressure gradient or change in pressure over time

• Close isobars represent steep pressure gradient, strong winds

• Further isobars represent shallow pressure gradient, light winds