Questions: Geostrophic Wind and Pressure-Coriolis Balance

Question 1

In the Northern Hemisphere, air around a low-pressure system initially accelerates toward the center (down the pressure gradient). What causes the resulting wind to flow counterclockwise around the low instead of into it?

• A. Surface friction deflects the air to the left
• B. The Coriolis force deflects moving air to the right until it is flowing parallel to the isobars
• C. The low-pressure center repels air like a magnetic pole
• D. Air rises so quickly that horizontal inflow stops

Answer: (B)

As air accelerates toward low pressure, the Coriolis force deflects it to the right (in the Northern Hemisphere). The deflection continues until the Coriolis force exactly balances the pressure gradient force — at that point the air is moving parallel to the isobars, not toward the low. This steady state is geostrophic balance, producing the counterclockwise circulation observed around Northern Hemisphere lows.

Question 2

Geostrophic wind speed increases as isobars are spaced farther apart on a weather map.

• T. True
• F. False

Answer: False

Geostrophic wind speed is proportional to the pressure gradient — the rate of pressure change with distance. Isobars spaced farther apart indicate a weaker pressure gradient (pressure changes slowly with distance), which produces lighter winds. Tightly packed isobars indicate a strong gradient and faster geostrophic winds. This is why closely spaced contours on a weather map signal strong winds.

Question 3

Why is the geostrophic approximation more accurate at mid-latitudes than near the equator?

Model answer: The Coriolis force is proportional to the sine of latitude, so it approaches zero at the equator. Near the equator, the Coriolis force is too weak to balance the pressure gradient force, so air flows toward low pressure rather than parallel to isobars. Geostrophic balance requires a significant Coriolis force, which only exists at mid-to-high latitudes.

The Coriolis parameter f = 2Ω sin(φ), where φ is latitude. At the equator sin(0°) = 0, so f ≈ 0 and the Coriolis force vanishes. Tropical meteorology therefore requires different dynamical frameworks. The geostrophic approximation is most accurate at mid-latitudes (30°–60°) where f is large enough to dominate the force balance in synoptic-scale systems.