What phenomenon causes plants to drop below freezing on calm, clear nights despite the surrounding air being 32 degrees F?

The phenomenon responsible for causing plants to drop below freezing on calm, clear nights, even when the surrounding air temperature is above 32 degrees Fahrenheit, is radiation. On clear nights, the ground and plant surfaces lose heat quickly through radiation. This process occurs because the heat emitted from the earth’s surface radiates into the atmosphere, which can lead to lower temperatures at ground level than the measured air temperature.

The calm conditions associated with clear nights allow for less mixing of the air, which means there's no convection to redistribute warmth. Consequently, as the ground cools, the temperature of the surfaces of the plants can dip below the freezing point, resulting in frost formation on the plants even when the air above remains warmer.

In contrast, frost refers specifically to the icy formation that occurs when the temperature drops to the freezing point or below, and while it can be related, it is the radiation cooling effect that actually causes the temperature differential. Convection involves the transfer of heat through fluid movement, but on calm nights, convection is minimal, and capillary action relates to water movement in plants and does not pertain to temperature drops.