What is the best visual indication of a thermal?

• A. Smooth cumulus clouds with concave bases
• B. Thunderstorm anvil clouds
• C. Cirrus wisps high aloft
• D. Cumulus clouds with convex bases

Answer: (A)

The key idea is that a thermal is a localized updraft lifting warm air. The best visual cue is a discrete cumulus cloud with a smooth, concave base, which forms where rising air cools and moisture condenses at roughly the same altitude along the updraft. That smooth, bowl-like underside indicates a steady lift beneath the cloud, signaling a reliable source of rising air for gaining altitude.

Thunderstorm anvils show far more developed storm activity and outflow, not the gentle lift of a thermal. Cirrus wisps high aloft indicate upper-level winds and aren’t associated with near-ground lifting. Cumulus clouds with convex bases can occur in different air patterns and don’t consistently signal the steady updraft a glider pilot relies on.