a warm air mass slowly passes over a cold air mass above a continent
thermals is rising_currents_of_warm_air. air_mass is atmosphere. high is air_mass. air_mass is not atmosphere. hot_air is air. africa is hot_continent. africa is not hot_continent. hot_air is lighter_than_cold_air. thermals is not rising_currents_of_warm_air. high is not air_mass.