Days Are Getting Shorter, But Typically The Hottest Weather Is Yet To ...

The day length has peaked, but usually the summer heat hasn’t.

The longest day of the year was June 21 this year. It’s the first day of astronomical summer and the summer solstice. We have already begun to have less daylight hours. Actually, the sunrise started getting later on June 26, and the sunset just started getting earlier on July 3. So far we’ve lost about eight minutes of daylight since the longest day on June 21.

While it’s not noticeable yet, the sun is also inching lower in the sky, gradually cutting its heating power.

So the hottest weather of the summer must be over, right? No. In most years, the hottest temperatures come sometime in July or the first week of August. There are two ways to see this lag of hottest temperatures occurring after the longest daylight hours of the year. We can look at the long-term average high temperatures and the record high temperatures.

In the graph above I highlight the day before the long-term average high temperature starts to decline for Grand Rapids. Most other Michigan cities are very similar, maybe differing by only a day or two.

When we scan down the record high temperatures at Grand Rapids in the table below, we see the 100-degree records mostly in July. In fact, the last week of July is the only week with all 100-degree or higher record highs. The hottest temperature ever at Grand Rapids occurred on July 13.

Why are the hottest temperatures of the year typically a few weeks after the longest day of the year? It’s because of the heating of the ground and water. The ground and water continue to gain heat for the few weeks after the longest day. It takes a while for the strongest sunlight, which is on June 21, to heat the ground.

We see this same temperature lag on a daily basis, too. The coldest temperatures in the morning are usually one hour after sunrise. The hottest temperatures during the day occur around 3 p.m. or 4 p.m., not when the sun is straight overhead at noon.

So many Michiganders are lovers of the hot weather of summer. Well, don’t let your mood get down because you notice the sun setting a little earlier, or the sun rising a little later. The hottest temperatures of the summer are probably still yet to come.

The hottest temperatures probably won’t come in the next week, but could as we get into the last 10 days of July.

By Aug. 7, you can probably assume the hottest days are behind us.