Primary cause of wind

Wind is the horizontal movement of air caused primarily by atmospheric pressure differences.

Main Forces

1. Pressure Gradient Force (PGF)

It is the initiating force of the wind.

• Acts from High Pressure towards Low Pressure.
• Acts perpendicular (90°) to the isobars.
• The closer the isobars are, the greater the gradient and the stronger the wind.

2. Coriolis Force (CF)

It is an apparent force caused by the Earth's rotation.

• Deflects the wind:
  • To the right in the Northern Hemisphere (NH).
  • To the left in the Southern Hemisphere (SH).
• Magnitude:
  • Proportional to wind speed (more speed = more Coriolis).
  • Proportional to latitude ($sin(\phi)$):
    • Zero at the Equator ($sin(0°) = 0$).
    • Maximum at the Poles ($sin(90°) = 1$).
• Acts perpendicular (90°) to the wind direction.

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Theoretical Wind Types

Geostrophic Wind

Occurs when the PGF and the Coriolis Force are in perfect balance.

• Flows parallel to straight isobars.
• Occurs above the friction layer (> 2000-3000 ft).
• Does not exist at the Equator (needs Coriolis, latitude > 15°).

Relationship with Latitude: For the same pressure gradient (same isobar spacing):

• Lower latitude $\rightarrow$ Less Coriolis $\rightarrow$ Wind must be faster to balance the PGF.
• Higher latitude $\rightarrow$ More Coriolis $\rightarrow$ Wind is slower.
• Formula: $V \propto \frac{1}{sin(\phi)}$

Gradient Wind

Occurs when isobars are curved. Involves PGF, Coriolis, and Centrifugal Force (inertia).

1. Around a LOW (Cyclone):

   • Wind rotates counter-clockwise (NH).
   • PGF pulls inward, Coriolis and Centrifugal pull outward.
   • Coriolis has "help" from centrifugal, so it doesn't need to be as strong.
   • Sub-geostrophic Wind: It is slower than the geostrophic wind for the same gradient.

2. Around a HIGH (Anticyclone):

   • Wind rotates clockwise (NH).
   • PGF and Centrifugal pull outward (or oppose in a way that Coriolis must be larger).
   • Coriolis must balance PGF + Centrifugal.
   • Super-geostrophic Wind: It is faster than the geostrophic wind for the same gradient.

Rule: Around a Low the wind is slower; around a High it is faster (for the same isobar spacing).

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Effect of Friction (Surface Wind)

Friction with the terrain reduces wind speed in the lower layer (up to 2000-3000 ft).

1. Friction reduces speed.
2. Lower speed $\rightarrow$ Less Coriolis Force.
3. The PGF (which does not change) becomes dominant.
4. The wind crosses the isobars towards Low Pressure.

Change in Direction and Speed (Northern Hemisphere)

Surface	Speed (% of wind aloft)	Direction (Change)
Sea	~70%	Backs (Turns left) ~10°
Land	~50% (Day) / ~30% (Night)	Backs (Turns left) ~30° (Day) / ~40° (Night)

• Descent (Aloft $\rightarrow$ Surface): Wind decreases and Backs (turns counter-clockwise, e.g., 270° $\rightarrow$ 240°).
• Ascent (Surface $\rightarrow$ Aloft): Wind increases and Veers (turns clockwise, e.g., 240° $\rightarrow$ 270°).

In the Southern Hemisphere: Surface wind Veers (turns clockwise) relative to wind aloft, crossing isobars towards low pressure.

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General Circulation and Weather

Buys Ballot's Law

In the Northern Hemisphere, if you stand with your back to the wind, the Low Pressure is on your left. (In the SH it is on the right).

Convergence and Divergence

System	Surface	Aloft	Vertical Movement	Typical Weather
Low Pressure (Cyclone)	Convergence (inward)	Divergence	Ascent (air rises)	Clouds, precipitation, bad weather.
High Pressure (Anticyclone)	Divergence (outward)	Convergence	Subsidence (air sinks)	Good weather, clear skies, inversions.

Altimetry and Wind Rules

• If you fly with wind from the left (NH), you are flying towards a Low Pressure.
  • "High to Low, Look out below" $\rightarrow$ Your true altitude is decreasing.
• If you fly with wind from the right (NH), you are flying towards a High Pressure.
  • Your true altitude is increasing.