Dew formation versus frost formation If the dew point of air is greater than 0°C (i.e., above the freezing point), which form of near-surface condensation will occur on exposed surfaces?

Introduction / Context:
Dew and frost are products of condensation (or deposition) occurring when surfaces cool at night. Whether liquid droplets (dew) or ice crystals (frost) form depends on the dew point relative to 0°C and the surface temperature.

Given Data / Assumptions:

• Dew point is the temperature at which air becomes saturated (100% relative humidity).
• Dew point here is specified as greater than 0°C.
• Exposed surface cools to near the dew point during nighttime radiative cooling.

Concept / Approach:
If the dew point is above 0°C and a surface cools to that temperature, condensation occurs as liquid water (dew). Frost forms only if the surface temperature is at or below 0°C and the frost point (the dew point below freezing) is reached, leading to direct deposition of ice.

Step-by-Step Solution:

Verification / Alternative check:
Meteorology texts state: when T_surface ≤ T_dew and T_dew > 0°C → dew; when T_surface ≤ 0°C and T_dew ≤ 0°C → frost by deposition.

Why Other Options Are Wrong:

• Frost (b) requires subfreezing conditions and a frost point at or below 0°C.
• “Water vapour will be formed” (c) is backwards; vapour condenses instead of forming.
• “Neither” (d) is incorrect when saturation is reached at above-freezing dew point.
• Simultaneous dew and frost (e) needs mixed conditions not indicated here.

Common Pitfalls:
Confusing air temperature with dew point; the dew point alone does not guarantee dew unless the surface cools to it.

Final Answer:
Dew will be formed