 Near the surface, warm moist air that is denser than warmer, dryer air wedges under the drier air in a manner similar to that of a cold front wedging under warmer air.When the warm moist air wedged under the drier mass heats up, it becomes less dense and rises and sometimes forms thunderstorms.At higher altitudes, the warm moist air is less dense than the cooler, drier air and the boundary slope reverses.