The difference in wind direction between the surface and the wind at 5,000 feet AGL is primarily due to which factor?

• A. Coriolis effect
• B. Friction between the wind and the surface
• C. Temperature gradient
• D. Humidity differences

Answer: (B)

Surface friction is the main reason for the directional difference. The wind near the ground is slowed by the rough surface, terrain, and objects, which causes it to cross the line of equal pressure (isobars) and veer toward lower pressure. Up at 5,000 feet AGL, friction is much weaker, so the wind is less deflected and tends to blow more parallel to the isobars (closer to the geostrophic direction set by the pressure gradient and Coriolis effect). That contrast between a heavily friction-affected surface wind and a loft wind with little friction is what creates the noticeable change in direction with height. Temperature gradient or humidity can affect wind speed through density changes but aren’t the primary cause of this directional difference, and Coriolis influence is already at work at both levels but doesn’t by itself explain the surface-to-aloft directional shift as much as friction does.