The “Lag of Season” – Why it Stays Hotter Longer into the Summer

"Lag of Season" – Why it Stays Hotter Longer into the Summer

As anyone who has ever stepped outside on a hot summer day knows, the air can take some time to heat up. The same is true for the atmosphere as a whole. Though the summer solstice marks the longest day of the year, it is not necessarily the hottest. That’s because it takes time for the atmosphere to absorb the Sun’s energy and convert it into heat. Similarly, midday is not necessarily the hottest time of day. That’s because it takes some time for the Sun’s rays to travel through the atmosphere and reach the ground. The same principle applies to the summer solstice: it can take some time for the atmosphere to heat up, even though the Sun is at its highest point in the sky.  Today we are going to talk about why it stays hotter longer into the summer or the “Lag of Season”.

From an Expert

It always sticks in my mind as an HVAC technician, but I’m reminded of it when KCRA weatherman Mark Finan talks about it every Summer. In an article on the channel’s weather blog, he said, “June is the month with the highest sun angle and the longest hours of daylight. By the time we get to the solstice (June 20 this year), we have 14 hours and 51 minutes of daylight. By August, we are seeing shorter days and a lower sun angle. By mid-August, we have lost about an hour and a half of daylight.

Despite the shorter days, August is a hotter month than June. The average high on June 1 is 85. On August 1, it is 94. June averages four days in the 100s, while August has 6. So why?

It’s called the lag of seasons…As we transition from Spring to Summer, the Sun gradually warms the surface of the Earth. The gradual warming lags behind the length of the day. So the hottest part of the year is roughly a month after the solstice. The opposite is true in the winter when you’ll find the coldest part of the season is often in January, a month or so after the Winter solstice.”

Lag of Season Explained

What I gather from this is that even though the Sun is at the same angle in the sky in June as it is in August, or May as it is in September, the Earth’s surface is cooler in the earlier summer months tater in the Summer. The winter cools the Earth’s surface, so less radiant heat is absorbed and retained in the atmosphere.

We notice this same phenomenon during the day as noon is not the hottest part of the day; it always falls around 4 pm to 5 pm. That’s the atmosphere heating up and trying to put on the brakes as night comes. It just takes a little longer to reject the day’s heat, which is why it can still be 90 degrees outside at bedtime.

This was a topic I’ve wanted to mention this topic in the Summer before, but I found it hard to explain on paper. Seasonal Lag, or as Mark Finan calls it, “Lag of Seasons,” makes it hotter in the later months of Summer than it is in the earlier months. It’s directly linked to the surface temperature of the Earth and its ability to retain heat as the warmer days upon the surface earth as well as its rivers, lakes, and strea