Abstract
An investigation of the January thaw phenomenon, a period of unseasonable warmth, was conducted using daily maximum temperatures recorded at New Brunswick, New Jersey, from 1858–1981. Student's t-tests, comparing long-term means of daily maximum temperature to values from a fitted seasonal trend curve, indicate temperatures higher on 22–23 January and lower on the 29th than seasonally expected.
It was found that the January thaw does not have a fixed time of occurrence but occurs most frequently from the 19th to the 28th. During this time the interannual variability of daily maximum temperature is significantly higher than during the remainder of the month.
Evidence of a tendency for a secondary thaw maximum to occur, centered on the 26th, is evident in several different analyses. Examination of daily temperature curves for 10-, 20- and 40-year periods reveals a shift in the mean thaw date from 22–23 January to the 26th. This change has evolved over the last 30–40 years. It was concluded that the January thaw is more pronounced when the mean circulation is characterized by a contracted polar vortex over North America and abnormally strong midlatitude westerlies.
