Why Is the Hottest Month Not the Longest Day?

Step 1 of 5

Temperature depends on a running balance.

The ground gains heat from the Sun and loses it to the sky. It gets warmer whenever the gain is bigger than the loss — not simply when the Sun is highest.

Step 2 of 5

At midsummer the gain is still winning.

The longest day is the peak of arriving sunshine, but the ground is still cool from winter, so it is absorbing far more than it loses.

Step 3 of 5

Warming continues until the two match.

Only when the ground has warmed enough to radiate as much as it receives does the temperature stop rising — and that takes weeks.

Step 4 of 5

Water lags most of all.

The sea takes an enormous amount of heat to warm and gives it up slowly, so coastal places peak later than inland ones and stay milder in winter.

Step 5 of 5

The same lag happens each morning.

The Sun is highest at noon, but the warmest part of the day is mid-afternoon. It is the identical effect on a scale of hours instead of weeks.

The same lag runs the other way in winter: the coldest weeks come well after the shortest day.