It's been more than a month since the first day of winter, when the low sun angle and small number of daylight hours resulted in the weakest sunshine of any day of the year. But it's not until now, in late January, that we've reached what is, on average, the coldest time of the year.

To understand why the coldest time of the year doesn't coincide with the first day of winter, think about making pudding on your stove, and then refrigerating it. If you only wait 5 minutes to take it out, you'll still have warm, runny pudding. It just takes time for hot pudding to cool, even in a refrigerator.

In the same way, even though the sun's heating power hits rock bottom in late December, it takes time for the continents, the oceans, and the air above them to chill down. As a result, average temperatures don't bottom out until a month or so after winter officially begins.