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Weather Whys

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.

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

A: Sometimes 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, with one in particular that stands out" he adds. "On Jan. 22, 1943, the town of Spearfish, SD was sitting on the edge of an arctic air mass and recorded a temperature of -4 degrees at 7:30 in the morning. Within two minutes, the thermometer shot up to 45 degrees because of strong ‘Chinook’ winds descending down a nearby mountain peak. By 9 a.m. the temperature was up to 54 degrees, but as the winds died down and the cold air mass settled in, the temperature fell to -4 degrees within 27 minutes, a 58-degree temperature drop that is still believed to be a record."

Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.

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