GRAND RAPIDS, Mich. (WOOD) — December features the shortest daylight days of the year, but not the coldest. One viewer wants to know why that is the case.
The answer is something called seasonal lag. This is most evident in summer and winter especially: We see the most intense sunlight in June but don’t experience the hottest temperatures until August. Similarly, we see the shortest “days” in December but don’t see the coldest mean temperatures until January.
The whole reason we have seasons is due to the tilt of the Earth.
The Winter Solstice marks the astronomical moment each year that we see the Earth’s axis tilt furthest from the sun. Not only do we see the shortest daylight days, we also see the weakest sun angle.
Still, this doesn’t line up with our statistically coldest days because of seasonal lag. One of the best ways to think of seasonal lag it to imagine a pot of water warming on the stove. It takes a while for that incoming heat to warm the water to boiling, and it takes a while for the water to cool down once it is taken off the stove.
The Earth is about 71% water. When the sun has peak intensity it takes awhile for our planet to absorb and store that energy and then release it back out into the atmosphere. The same is true when the sun intensity is low or limited, the effects are felt about a month or so after.
The length of day is very slow to increase from December to January with much bigger gains occurring closer to the equinoxes.