What is the day of the latest first frost in the Raleigh area? first freeze? What is normal?
Posted October 13, 2007
Updated October 14, 2007
MIKE MOSS SAYS: Steve, The "normal" first freeze date for the Raleigh-Durham airport is October 28th, but the variability of that date from year to year is fairly high, with a standard deviation of about 9 days either way (meaning about two thirds of the time, the first reading at or below 32 degrees occurs within +/- 9 days of the 28th, and about 95% of the time within +/- 18 days of the 28th and so on).
Note that the same statistics for a couple of other Wake County weather stations located farther southeast show normal first freeze dates around Nov 1st and Nov 5th, with standard deviations of almost two weeks.
Note that the terms "first frost" and "first freeze" are sometimes used interchangeably in terms of statistics like this. In reality, it is fairly simple to develop a climatology of first freeze by tracking air temperature observations. However, frost may form on surface objects, in some cases, while air temperatures remain above freezing. However, there usually are not reliable and complete records of such frost formation available with which to develop a separate climatology of first frost occurrences. If there was, we would probably see first frost dates a few days earlier than first freeze dates in the fall, and vice versa for the spring.
Finally, for the RDU airport, the latest first freeze on record is Nov 17th.