A condition existing when an air mass of lesser density flows or is pushed over by an air mass of greater density. This term is usually applied in the case of warm air ascending the surface of a cold or quasi-stationary front.
A condition where warm moist air rides up an over cooler air at the surface, usually resulting in precipitation.
A condition that exists when an air mass moves up and over a denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation.