Wind

Reference ACWM Chapters 5 and 11

Wind is the result of changes in pressure gradient.

• flows from High to Low 
• steeper gradient = stronger wind

Coriolis Effect

• the deflection of wind caused by the Earth's rotation
• varies from zero at equator to maximum strength at poles

Curvature Effect

• when isobars are curved, air moves in an arc >> centrifugal force
• the pressure gradient around lows is generally stronger than around highs

Friction

• As you climb from the surface to 3000 feet, the wind veers (clockwise change in direction) and increases.
• As you descend from 3000 feet to the surface, the wind backs (counterclockwise change in direction) and decreases.
• We must also keep in mind that land and water create different amounts of friction.  There is less friction over water, so wind would blow at less of an angle across isobars and at faster speeds. 

Squalls and Gusts

• characteristics of turbulent flight conditions
• Gusts >> rapid peaks and lulls
• Squalls >> sudden increase lasting for a minute or more, then a decrease

Diurnal Effects

• surface winds are usually stronger and gustier during the day
• Sea Breeze during the day (higher pressure over water than land)
• Land Breeze at night

Topographical Effects

• hilly and mountainous terrain can contribute to mechanical turbulence
• Anabatic winds flow upslope during the day when mountain slops facing the sun are heated
• Katabatic winds flow downslope
• warm = Chinook wind, warms at DALR
• cold = glacier wind, cooling by underlying ice

Wind Shear

• increased performance (headwind) >> airspeed increases
• encountering increased performance on glideslope >> would have to reduce power to recapture glidepath, then increase power to maintain due to stronger headwind
• decreased performance (tailwind) >> airspeed decreases

Lapse Rates and Stability

Adiabatic Processes

• no heat is added or removed from air
• rising air >> lower pressure >> expands >> temperature decreases
• sinking air >> higher pressure >> compresses >> temperature increases

Lapse Rates

• Dry Adiabatic Lapse Rate (DALR): 3 C / 1000 ft
• Saturated Adiabatic Lapse Rate (SALR): 1.5 C / 1000 ft
• Average: 2 C / 1000 ft

• Environmental Lapse Rate (ELR): indicates the temperature of the surrounding air; comparison to lapse rate of air parcel determines if it is stable or unstable
• shallow lapse rate compared to SALR: absolute stability
• lapse rate between DALR and SALR: conditional instability
• steeper lapse rate than DALR: absolute instability

Characteristics of Stable Air

• sustained low visibility (i.e. haze layers, drizzle, fog)
• continuous precipitation
• strato-form clouds (layers)
• steady winds

Characteristics of Unstable Air

• good visibility
• showery precipitation
• cumulo-form clouds (vertical development)
• gusty winds

DO YOU KNOW... why rising saturated air cool less rapidly than rising unsaturated air?

• heat is released during the condensation of water vapour

Critical Point

Also known as... should we return to our point of departure or continue to our destination after an engine failure, if there is no suitable alternate nearby?

Info you need:

• total distance (D)
• reduced (OEI) groundspeed home (RH)
• reduced (OEI) groundspeed out (RO)

For calculating groundspeeds, you need:

• track
• true airspeed
• wind direction and speed

And for calculating the time to the CP, you need

• AEO groundspeed out (GO)

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Formulae:

• Distance to CP = (D x RH) / (RO + RH)
• Time to CP = DCP / GO

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An increase in the tailwind component:

• decreases RH
• increases RO
• moves distance to CP closer to point of departure
• decreases time to CP