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?

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.

Associate Editor