The January thaw is an observed but unexplained weather phenomenon found in mid-latitude North America whereby temperatures tend to rise briefly in mid-winter.

Sinusoidal estimates of expected temperatures, for northern locales, usually place the nadir around January 23 and the crest around July 24, and provide fairly accurate estimates of temperature expectations. Actual average temperatures, in North America, usually exhibit two pronounced deviations from this estimate:

• Mid-autumn temperatures tend to be warmer than as predicted by the sinusoidal model, creating the impression of extended summer warmth known as Indian summer.
• For five days around January 25, empirical expected temperatures are usually significantly warmer than as predicted by the sinusoidal estimate, and also warmer than neighboring temperatures on both sides.

During this "thaw" period, usually lasting for about a week, tempeartures are generally about 10°F (6°C) above normal. This is not invariant from year to year, and temperatures fluctuate enough that such a rise in late-January temperature would be unremarkable; what is remarkable (and unexplained) is the tendency for such rises to occur more commonly in late-January than in mid-January or early February, which sinusoidal estimates have to be slightly warmer.

In some regions (such as northern Canada) this phenomenon will not be manifest as a "thaw" in the technical sense, since temperatures will remain below freezing.

The January thaw is believed to be a weather singularity.

External link

"January Thaw: Winter Cold Relief"