It doesn't take long for a temperature to drop.
Don't use the mild weather in the middle of January to justify leaving the house without a jacket. The weather records from January 10, 1911, in Rapid City, South Dakota, show how quickly temperatures can drop. The temperature began at a pleasant 55°F, then dropped to 8°F in 15 minutes due to a powerful cold front. That day still holds the record for the world's fastest cold snap.