Thermal heat cannot be turned on until what altitude relative to field elevation?

Thermal heat cannot be turned on until what altitude relative to field elevation?

• A. True.
• B. False.
• C. Only at night.
• D. Only on the ground.

Answer: (A)

Thermal lift is produced by solar heating of the surface, which drives rising air in the boundary layer. You can’t rely on or “turn on” a usable thermal from the ground because the buoyant updrafts need vertical development above the surface to become strong and exchange energy efficiently with the surrounding air. In practice, you typically must be some height above the field elevation before you can intercept a reliable thermal, with the exact amount depending on day, sun, and wind conditions. Thermals also require daylight—at night the surface heating stops and such lift is largely absent—so the other options don’t fit.

Thermal lift is produced by solar heating of the surface, which drives rising air in the boundary layer. You can’t rely on or “turn on” a usable thermal from the ground because the buoyant updrafts need vertical development above the surface to become strong and exchange energy efficiently with the surrounding air. In practice, you typically must be some height above the field elevation before you can intercept a reliable thermal, with the exact amount depending on day, sun, and wind conditions. Thermals also require daylight—at night the surface heating stops and such lift is largely absent—so the other options don’t fit.