If we have any kind of break in the cold for a few days this time of year, I guarantee you that the phrase "January Thaw" will be heard over and over again.

When you look at the data, there actually is a tendency for a relatively mild spell in this part of the country sometime during the third or fourth week of January. Here's a look at the average daily high temperatures for Philadelphia from mid-January into early February, using the last 100 years of data. There's clearly a bump in the graph from around the 20th to the 25th of January that peaks on the 22nd and 23rd. If we look at low temperature data, we see basically the same thing - a period from the 20th to around the 25th of slightly higher average lows. Many other big cities in the Northeast show similar trends.

Now of course, thaws can occur anytime during the winter, but there aren't any that happen as consistently as the one in the latter half of January. Quite simply, the January thaw is hard to explain and still somewhat of a mysterious meteorological oddity.