FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesHard
A flight instructor explains that upper-level winds tend to flow parallel to isobars rather than directly from high to low pressure, once the wind has accelerated and stabilized above the friction layer. Which force, acting opposite the pressure gradient force, is primarily responsible for this geostrophic wind balance?
- AConvective force
- BCoriolis force
- CCentrifugal force
- DFrictional force
Show answer & explanationAnswer & explanation
Correct answer: B. Coriolis force
Geostrophic wind results when the Coriolis force, which deflects moving air to the right in the Northern Hemisphere, balances the pressure gradient force. This balance causes the wind to flow parallel to the isobars rather than directly across them from high to low pressure.
Why the other options are wrong
- A. Convective force is not a recognized force in wind balance equations.
- C. Centrifugal force becomes important in curved (gradient) wind flow around tightly curved highs/lows, not the basic geostrophic balance.
- D. Friction is significant near the surface but decreases with altitude, and is not the balancing force for geostrophic wind aloft.
Geostrophic Wind
Wind that flows parallel to isobars above the friction layer, resulting from a balance between the pressure gradient force and the Coriolis force.
- Coriolis force deflects wind to the right in the Northern Hemisphere
- Balance occurs above roughly 2,000-3,000 feet AGL where friction is minimal
- Wind speed is proportional to the pressure gradient (isobar spacing)
Memory trick: Coriolis is the invisible hand curving wind to flow along the isobar lines