Short notes 06: Winds

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Wind – Fundamental Principles

• Wind: Horizontal movement of air.
• Primary cause: Horizontal pressure difference.
• Initiating force: Pressure Gradient Force (PGF) (acts from High → Low, perpendicular to isobars).
• Modifying forces:
  • Coriolis Force (CF): Deflects motion (Right in NH, Left in SH).
  • Friction: Acts near surface, reduces speed and CF.

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Pressure Gradient & Isobars

• Closely spaced isobars → strong PGF → strong winds.
• Widely spaced isobars → weak PGF → light winds.
• Strongest winds: Usually near low pressure systems (cyclones).

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Surface Wind vs Wind Aloft

• Above friction layer (~2000–3000 ft):
  • Wind flows parallel to isobars (Geostrophic / Gradient).
• At surface:
  • Friction reduces speed → CF weakens.
  • Wind crosses isobars toward low pressure.

Typical Surface Deflection (NH)

• Over land (day): ~30°, speed ≈ 50% of 2000 ft wind.
• Over sea: ~15°, speed ≈ 75% of 2000 ft wind.

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Geostrophic, Gradient & Cyclostrophic Winds

• Geostrophic Wind (GW):
  • Balance: PGF = CF.
  • Straight isobars, above friction layer.
  • Valid mainly >15° latitude.
• Gradient Wind (GRW):
  • Balance: PGF + CF + Centrifugal Force.
  • Curved isobars.
  • Low pressure: GRW < GW (sub-geostrophic).
  • High pressure: GRW > GW (super-geostrophic).
• Cyclostrophic Wind:
  • Balance: PGF = Centrifugal Force.
  • CF negligible.
  • Applies to tornadoes, TRS cores, very low latitudes.

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Coriolis Force (CF)

• Acts 90° to direction of motion.
• NH: Deflects to the right.
• SH: Deflects to the left.
• Zero at Equator, maximum at poles.
• Affects direction, not speed.

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Buys-Ballot’s Law

• NH: Back to wind → Low on left.
• SH: Back to wind → Low on right.
• Applies to large-scale winds, not local winds.

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Hemisphere Circulation (Surface)

• NH Low: Counter-clockwise + inward.
• SH Low: Clockwise + inward.
• NH High: Clockwise + outward.
• SH High: Counter-clockwise + outward.

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Diurnal Variation of Surface Wind (Land)

• Day:
  • Stronger winds (mixing from aloft).
  • Wind veers (clockwise).
• Night:
  • Weaker winds (stable layer).
  • Wind backs (anti-clockwise).

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Gusts, Squalls & Gales

• Gust:
  • Peak exceeds mean by ≥10 kt.
  • Duration: seconds (<1 min).
• Squall:
  • Increase ≥16 kt, reaching ≥22 kt.
  • Duration ≥1 minute.
• Gale:
  • Mean wind ≥34 kt or gusts ≥43 kt.

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Thermal Wind (TWC)

• Definition:
  TWC = Upper Wind − Lower (Geostrophic) Wind.
• Represents wind shear due to temperature gradient.
• NH Rule: Thermal wind flows parallel to isotherms, cold air on left.
• Wind increasing with height: Strong thermal wind component.

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Local Winds (Do NOT follow Buys-Ballot)

• Coriolis negligible; flow mainly H → L or gravity driven.

Sea & Land Breeze

• Sea Breeze (Day):
  • Stronger (~10–15 kt).
  • Sea → Land.
  • Causes temp drop + RH rise at coast.
• Land Breeze (Night):
  • Weaker (~5 kt).
  • Land → Sea.

Mountain & Valley Winds

• Anabatic (Day):
  • Upslope.
  • Warm, light (~5 kt).
• Katabatic (Night):
  • Downslope.
  • Cold, stronger (~10 kt).
  • Favors fog/frost in valleys.

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Föhn / Chinook Winds

• Warm, dry, descending leeward winds.
• Cause: Compressional (adiabatic) heating.
• Associated with mountain waves, rotor turbulence.
• Chinook: Rockies (N. America).
• Bora: Cold, strong katabatic wind (NE Adriatic).

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Rotor Clouds

• Form beneath mountain wave crests on lee side.
• Indicate severe to extreme turbulence.
• Avoid flight near rotor zone.

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Charts & Reporting

• Wind on charts: Direction TRUE, speed in knots.
• Isotachs: Lines of equal wind speed.
• Surface wind sensors: ~10 m above ground.

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Aviation One-Line Traps

• Strongest winds → closely spaced isobars.
• Surface wind always slower than wind aloft.
• High to Low – beware below (altimeter over-reads).
• Local winds ≠ Buys-Ballot.
• Gust ≠ Squall (duration matters).
• Sea breeze > land breeze (strength).

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