Why is December 21 not the coldest time of the year?

December 21, the winter solstice, marks the shortest day and longest night of the year in the Northern Hemisphere, but it is not typically the coldest time. This phenomenon occurs because of a delay in the Earth's heating and cooling processes, known as thermal inertia. Although the days start getting longer after the solstice, the cumulative effect of shorter daylight hours and the Earth's atmosphere means that temperatures often continue to drop for several weeks before reaching their annual lows, usually in January or February.