Weather Words: Flash Freeze

A flash freeze happens when temperatures drop so quickly that any moisture on roads, sidewalks, or other surfaces freezes almost instantly. This often occurs when a strong cold front sweeps in behind rain or melting snow, sending temperatures plunging from above freezing to well below 32 degrees Fahrenheit in just a few hours, sometimes even minutes.

What makes a flash freeze especially dangerous is how little warning it gives. Pavement can look merely wet when, in reality, it has turned into a sheet of ice. Drivers may not realize conditions have changed until they lose traction, and pedestrians can be caught off guard by suddenly slick sidewalks and steps.

Flash freezes are most common in late fall and winter, particularly during the evening or overnight hours when temperatures are already falling. Gusty winds behind a cold front can speed up the freezing process by rapidly replacing milder air with much colder air.

Because of the rapid onset and high risk to travel, the National Weather Service often issues Flash Freeze Warnings when conditions are favorable. These events may not bring snow or ice from the sky, but they can be just as hazardous as a winter storm because the danger develops so fast.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.