Soil heat flux is upward during

Correct option is D

The correct answer is (D) Midnight

Explanation:
• Soil heat flux moves according to temperature gradients. Heat always moves from a warmer area to a cooler area.
• During the day: The sun heats the soil surface. The surface becomes warmer than the subsoil, so heat flux is downward (into the soil). This happens during forenoon, noon, and afternoon.
• During the night: The soil surface loses heat rapidly through terrestrial long-wave radiation to the atmosphere. The surface becomes cooler than the deeper soil layers. Consequently, heat stored in the subsoil during the day moves upward toward the surface. This is most pronounced at night and midnight.

Information Booster:
• The point where the heat flux changes direction is called the "turnover."
• Soil serves as a heat sink during the day and a heat source during the night.
• Diurnal temperature fluctuations are highest at the soil surface and decrease exponentially with depth.

Additional Knowledge:
• At Noon (Option A): Heat flux is strongly downward because solar radiation is at its peak.
• After sunset, the surface cooling begins, and the upward flux starts.
• This upward flux at night helps in preventing the air immediately above the soil from cooling too drastically, which is important in frost management.