Q: What causes cold fronts?
A: The simple explanation is that the leading edge of a cold air mass is moving into an area previously occupied by a warmer air mass, says Brent McRoberts of Texas A&M University. "The air mass behind the cold front is noticeably colder and it pushes out the warmer air," McRoberts explains. "As a cold front moves into an area, the heavier, colder air lifts the warmer, lighter air upward. If the warm air is humid enough, water vapor in it will condense and rain can occur. A greater contrast in temperatures between the two air masses will lead to stronger upward motion, which is why strong cold fronts often trigger thunderstorms."
Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.