Q: What causes cold fronts?

A: The simple explanation is that a cold front is where a cold air mass is replacing a warm air mass, says Brent McRoberts of Texas A&M University. "The air mass behind it is noticeably colder and it pushes out the warmer air," McRoberts explains.

"As a cold front moves into an area, the heavier, colder air moves out the warmer, lighter air it's replacing. If the warm air is humid enough, water vapor in it will condense and rain can occur, which is why cold fronts often trigger thunderstorms. The rotation of the earth helps to form the wind that flows between air masses."

Q: How fast can the temperature drop when a cold front arrives?

A: Amazingly fast, McRoberts adds. It is not uncommon for the temperature to drop 15 degrees or more in just one hour after a cold front arrives. "There are some incredible stories about temperature changes," he adds. "On Dec. 20, 1836, in Illinois, the temperature went from 40 degrees to zero in just a few hours. And on Jan. 22, 1943, in South Dakota, the town of Spearfish was at the edge of an Arctic air mass but still under a temperature inversion area. At 7:30 a.m., the temperature in Spearfish was -4 degrees and within two minutes, went to 45 degrees above zero because of warm winds associated with the inversion. By 9 a.m. the temperature was up to 54 degrees, but the conditions changed and the temperature fell to -4 degrees. A 58-degree temperature drop had occurred in only 27 minutes, still believed to be a record."

"Weather Whys" is a service of Texas A&M University's Department of Atmospheric Sciences.