The Sun supplies energy to Earth, which absorbs and reradiates some of that energy. Because of the Sun’s high angle in summer, the amount of energy the Northern Hemisphere gains exceeds the energy it loses even after the summer solstice (June 21), when the number of daylight hours begins to diminish. This results in a continued warm-up — reflected in daily average temperatures — for roughly 30 days following the solstice.
Let’s use Trenton, New Jersey, as a reference point. There, the average daily high/low near the summer solstice is 83°/63° F (28°/17° C). However, the maximum summer daily high/low temperatures peak at 86°/68° F (30°/20° C) between July 21 and July 23. The decrease in solar radiation after the summer solstice simply isn’t significant enough to change the summer warming pattern until about 30 days later.
The opposite effect occurs at the winter solstice (December 21). Despite minor increases in solar radiation following the winter solstice — thanks to the Sun’s gradually increasing altitude — the Northern Hemisphere’s energy loss still exceeds the increase in solar energy, and cooler temperatures prevail for the next 30 to 45 days. The average daily minimum high/low on the winter solstice for Trenton is 41°/27° F (5°/-3° C). But the minimum winter daily temperatures — on average, a high/low of 35°/23° F (2°/-5° C) — don’t occur until around January 30. — GRANT BROHM, METEOROLOGIST, MADISON, WISCONSIN