Which conditions result in the formation of frost?

• A. The temperature of the collecting surface is at or below the dewpoint of the adjacent air and the dew point is below freezing
• B. Air temperature is well above the dew point
• C. Humidity is zero percent
• D. The sun is shining brightly with warm air aloft

Answer: (A)

Frost forms when a surface cools enough to freeze and there is enough moisture in the air for the vapor to deposit directly as ice rather than become liquid. The best way this happens is when the surface temperature is at or below the dewpoint of the surrounding air, and the dewpoint itself is below freezing. That combination means the air is cold and moist enough that water vapor will deposit as ice on the surface instead of forming liquid dew. If the dewpoint were above freezing, moisture could condense as liquid water (dew) and frost wouldn’t form as readily. Conversely, if the air is not humid (dewpoint very low or humidity zero) or if the surface isn’t cold enough (sunlight or warm air aloft), frost won’t form.