When the _____ is greater than zero, the air is unsaturated.

The temperature-dew point spread refers to the difference between the air temperature and the dew point temperature. When this spread is greater than zero, it means that the air temperature is higher than the dew point temperature, indicating that the air is unsaturated. Unsaturated air can hold more moisture without condensation occurring, whereas saturated air has reached its moisture capacity and tends to produce condensation.

In contrast, when the temperature-dew point spread is zero or negative, it suggests that the air is either saturated or supersaturated, which can lead to the formation of clouds, fog, or precipitation. Understanding this concept is crucial in meteorology and air traffic control as it affects visibility, weather patterns, and aircraft performance.