From the June 2019 issue

In the Northern Hemisphere, the summer solstice happens on or around June 21. Yet the summer is usually hottest the first week of August. The opposite is true for the winter solstice. Why the delay?

Ronald Cohen Brooklyn, New York 
By | Published: June 24, 2019 | Last updated on May 18, 2023
The summer solstice is when the Sun reaches its farthest point north in the sky. This date is the official beginning of summer in the Northern Hemisphere, but the season’s hottest temperatures lag behind by weeks or months because the planet doesn’t heat up instantaneously. It takes time to warm the land, sea, and air, so the hottest portion of the summer is delayed.
The reason for the delay between the longest (or shortest) day and the hottest (or coldest) temperatures of the year is similar to the reason the hottest or coldest parts of the day occur after noon and in the earliest hours of the morning, respectively. Heating and cooling aren’t instantaneous — it takes time for land, water, and air to either absorb heat and warm up or release stored heat and cool down. 

In the case of summer, the energy received by the hemisphere in question increases as the days grow longer in the time leading up to the solstice. Although the planet releases heat overnight, it is receiving energy faster than it can cool off, so it warms up. On the solstice, the Sun’s energy input is at a maximum, then slowly begins to decrease in the following days and weeks. But the portion of the planet experiencing summer is still receiving high energy input from the Sun after the solstice. This energy input remains higher than the rate at which heat is released, which causes the hemisphere to continue warming, for several weeks — thus, the hottest portion of the season is delayed. The effect is the same in winter, only the processes are reversed as land, air, and water continue to release heat faster than they absorb it for several weeks after the winter solstice.

Alison Klesman

Associate Editor