Which condition is explicitly listed as a frost-formation prerequisite regarding dew point?

• A. Dew point below freezing and surface temperature at or below dew point
• B. Dew point above freezing only
• C. Dew point irrelevant if temperatures are below freezing
• D. Dew point must be exactly equal to air temperature

Answer: (A)

Frost forms when the air around a surface is saturated and the surface itself is cold enough for moisture to deposit as ice rather than condense as liquid. The explicit condition is that the dew point is below freezing and the surface temperature is at or below the dew point. When the dew point is below 0°C, the only way to get moisture onto a surface is for it to deposit as ice, which happens if the surface is as cold as or colder than the dew point. That combination ensures air near the surface is saturated and the surface temperature is low enough for deposition to occur.

If the dew point were above freezing, you’d typically get liquid condensation (dew) rather than frost unless the surface temperature drops below freezing. Ignoring the dew point misses the moisture condition that drives frost formation, and requiring the dew point to be exactly equal to the air temperature isn’t necessary for frost to occur.