It all adds up

December 21 is the shortest day of the year, but it doesn’t seem to be the coldest. Why isn’t the day with the least sunlight the coldest?

Winter solstice has the fewest hours of sunlight and the most diffuse sunlight (because the sun remains low in the sky), so it is the day the Northern Hemisphere receives the least amount of energy from the sun. It is not the coldest time of year because the Earth and its atmosphere store thermal energy; therefore, a net loss of energy accumulates over time.