1. In the equation for the time constant of a thermometer, a quantity equal to the product of wind speed and air density. The time constant varies inversely with ventilation. The concentration of an air pollutant is inversely proportional to the ventilation, the mass flux of “clean air” moving past the observer. Stagnation, a condition caused by the lack of ventilation, is historically associated with major air pollution episodes. It occurs due to the lack of horizontal wind speed and the lack of vertical wind speed, for example, caused by an inversion. 2. The exchange of properties with the surface layer such that property concentrations are brought closer to equilibrium values with the atmosphere. Such exchange may occur without water mass formation. 3. In weather-observing terminology, the process of causing “representative” air to be in contact with the sensing elements of observing instruments; especially applied to producing a flow of air past the bulb of a wet-bulb thermometer.