FAA Ground School Practice Test

Which conditions result in the formation of frost?

1. The temperature of the collecting surface is above the dewpoint

2. The temperature of the collecting surface is at or below the dewpoint and the dewpoint is below freezing

3. Warm air cooling rapidly at night

4. High humidity conditions at low temperatures

The correct answer is: The temperature of the collecting surface is at or below the dewpoint and the dewpoint is below freezing

The formation of frost occurs when the temperature of a collecting surface is at or below the dewpoint, and the dewpoint itself is below freezing. This scenario allows moisture in the air to deposit directly onto surfaces as ice, bypassing the liquid phase. When air cools to the dewpoint, it becomes saturated, and because the dewpoint is below freezing, the moisture condenses as frost rather than dew. In contrast, if the temperature of the collecting surface is above the dewpoint, frost cannot form because the moisture in the air will not condense; it would remain in vapor form. Similarly, while warm air cooling rapidly at night may lead to cooling effects, it does not necessarily result in frost unless conditions related to surface temperature and dewpoint align correctly. High humidity at low temperatures can lead to conditions where frost is possible, but it is the specific relationship between temperature, dewpoint, and freezing conditions that truly dictates frost formation. Thus, the combination of a collecting surface temperature at or below the dewpoint, with the dewpoint being below freezing, is the key condition for frost to form.