air is rising to a colder region
winter is colder_than_summer. tops_of_mountains is colder_than_surrounding_countryside. air is region. air is thinner_at_higher_altitudes. air is thinner_higher_go. summer is hotter_than_winter. atmosphere is thinner_as_altitude_increases. summer has more_sunshine_than_winter thermals is rising_currents_of_warm_air. hot_air is lighter_than_cold_air.