 A similar phenomenon to a weather front is the dry line , which is the boundary between air masses with significant moisture differences . When the westerlies increase on the north side of surface highs , areas of lowered pressure will form downwind of north – south oriented mountain chains , leading to the formation of a lee trough . Near the surface during daylight hours , warm moist air is denser than dry air of greater temperature , and thus the warm moist air wedges under the drier air like a cold front . At higher altitudes , the warm moist air is less dense than the dry air and the boundary slope reverses . In the vicinity of the reversal aloft , severe weather is possible , especially when a triple point is formed with a cold front . A weaker form of the dry line seen more commonly is the lee trough , which displays weaker differences in moisture . When moisture pools along the boundary during the warm season , it can be the focus of diurnal thunderstorms . 
