Forces

Pressure Gradient Force (PGF)

• The reason we have the wind!

• PGF is always directed from high to low pressure!

• The stronger the pressure gradient, the stronger the wind  (i.e., the closer the isobars are together, the stronger the wind will be)

Note: We don't actually see the wind blow straight across from high-pressure to low-pressure

Coriolis Force

• "Apparent" force due to Earth's rotation

• Wind direction gets deflected either to the...

  • • Right (in the Northern Hemisphere)

    • Left (in the Southern Hemisphere)

• Zero at the equator and relatively strong at the poles

• The faster the motion, the stronger the Coriolis force

• Only applies on large scales (Synoptic & Planetary, not microscale or mesoscale)

  • • Example: Hurricanes, Mid-latitude cyclones, etc.

Friction Force

• Near the surface, the wind is slowed by drag from the ground, trees, buildings, etc.

  • Therefore, the Coriolis force is reduced

Centrifugal Force

Only applies in curved flow:

• Magnitude of centrifugal force is related to the velocity and radius of curvature

  • Faster speeds = greater centrifugal force

  • Tight curves = greater centrifugal force

• When there is curvature, an observer (or an air parcel) in the rotating frame of reference experiences a force directed outward (think of being in a car going around a curve)

• Result:

  • At large scales (i.e., when Coriolis matters): gradient wind

  • At small scales: cyclostrophic wind