Ask Ellen: Why don’t our coldest temps line up with our shortest days?

GRAND RAPIDS, Mich. (WOOD) — Most of the way through January, the cold has settled in across the Mitten.

You may have already started to notice the days are getting a little longer. In general, we are gaining just over two minutes of daylight each day. But why doesn’t our “shortest” day match up with our coldest time of year? The answer is something called a seasonal delay.

Let’s start at the beginning. We all know the shortest day of the year is the winter solstice, which hits in December around the 21st and the 22nd. In West Michigan, we only see nine hours and one minute of daylight on that day (our longest day in summer kicks out a whopping 15 hours, 21 minutes of daylight).

However, our average high temperatures bottom out a whole month after our days with the least daylight.

So why does this happen? It has to do with how the atmosphere is warmed and we can look at a typical summer day as a perfect example.

Think of a perfectly quiet summer day. The sun is highest in the sky around noon, giving us our most direct angle of incoming sunshine. The sun’s rays enter the atmosphere and a good portion of that incoming energy is absorbed into the ground.

Courtesy: Arizona Atmospheric Program

That energy doesn’t just stay in the ground, though. Instead, the ground begins radiating it back out into the air near the surface. It takes a little bit of time for the rays to be absorbed and the energy to be re-emitted, which is why our typical high temperature for the day is usually in the mid-to-late afternoon, even though the sun is highest in the sky around lunchtime.

The sun heats the ground. The ground heats the air.

This happens on a seasonal cycle, too. Our days are shortest in late December, but it takes time for the ground to relinquish some of the seasonal heat it has built from the last several months. It’s the same with water: Lake Michigan takes much, much longer to let go of the heat it has stored all summer and fall.

It isn’t until we get to late January that we see our coldest days because the ground has finally caught up to the shortage of sunlight.

Watch for the reverse in summer. The longest day of the year is always around June 20 to June 22, but our hottest days tend to arrive in July.

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