In the atmosphere, convection is the dominant vertical transport process in convective boundary layers, which are common over tropical oceans and, during sunny days, over continents. Moist convection in the atmosphere is characterized by deep, saturated updrafts and downdrafts, and unsaturated downdrafts driven by the evaporation and melting of precipitation. This form of convection is made visible by cumulus clouds and cumulonimbus clouds in the case of precipitation convection.