At what two points of the day is thermal crossover common?

• A. Dawn and dusk
• B. Noon and midnight
• C. Sunrise and sunset
• D. Morning and Evening

Answer: (A)

Thermal crossover happens during the two times when the surface rapidly changes its heating or cooling status, flipping how the near-surface air behaves. At dawn, the sun starts warming the ground, and the lowest part of the atmosphere shifts from the cool, stable night state to a warmer, more mixed daytime state. At dusk, the sun sets and the surface begins to cool, reversing to a cooler, more stable night regime. These sign changes in the surface energy input cause the boundary layer to transition, which is why dawn and dusk are the typical moments of thermal crossover.