Temperature Drop After Shortest Day


Why is it that the shortest day of the year is not the coldest? After December 21, here in Maine it just keeps getting colder and colder until sometime in February.


Here's the story on that. Everything, including the state of Maine is made up of tiny bits of matter, atoms and molecules, that are in constant motion. Even those ledges along the shore that look so solid are actually in constant motion on a microscopic scale. Anything in motion has what is called kinetic energy. When the state of Maine absorbs energy from the sun or a campfire or from somebody hitting it with a hammer, most of that energy ends up as kinetic energy of the molecules of the state.

The total of the kinetic energy of all the molecules in an object is the amount of heat stored in that object. The more heat that is added the faster the molecules dither about their assigned position in a solid object. It turns out that we have a convenient way of measuring the average speed of the molecules in a rock. It is proportional to the rock's temperature. So as heat is added the temperature goes up.

Most things in the universe, with the possible exception of money, tend to flow from where there is lots of it to where there is less of it. It is certainly that way with heat. If two objects are not insulated from one another heat will flow from the higher temperature object to the cooler one.

Now let's zoom in on one square meter of the State of Maine, say a nice level piece of ground somewhere between East Overshoe and Bunganuk. When that ground is tilted toward the Sun, during the summer, the heat energy from the sun pushes its temperature up until the heat it gives off to its surroundings balances the Sun's input. Then sometime in autumn, when the energy from the Sun is not as concentrated, the heat loss to the surroundings exceeds the input from the sun and the temperature begins to drop. As long as the heat input is less than the heat loss, the temperature will keep going down.

bucketIt is true that the least amount of sunshine falls about the December 21, but the heat balance has been favoring losses since October and won't get even again until about March. Then when the gains exceed the losses the temperature will start to rise again.

You might think of it this way. The heat balance is like filling a leaky bucket. Water flows in. Water flows out. As long the inflow exceeds the leakage the level in the bucket will rise. If the inflow is slowly reduced to less than the leakage and then slowly increased to exceed leakage again, the level will decrease and then increase but the minimum level will not necessarily occur at the time of the minimum inflow. The level in the bucket story is analogous to the temperature in the heat balance.