The difference between air temperature and its dew point. For unsaturated air it is positive. Near terrestrial surface conditions, a closed ascending parcel's lifting condensation level is approximately times this difference, because its temperature decreases about faster than its dew point.
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Dew point depression refers to the difference between the air temperature and the dew point temperature. The dew point is the temperature at which air becomes saturated with moisture, meaning that it can no longer hold all the water vapor in the form of gas, and condensation begins to occur.