The Sun gives, the Earth takes it in
At the summer solstice, around 21 June, the Sun climbs to its highest and the day stretches to its longest. It is the time of year when your half of the planet takes in its biggest dose of solar energy. You would expect that day to be the hottest too. It almost never is.
It comes down to something we tend to forget: the Earth does not only receive the Sun’s heat, it gives heat back. Like any warm body (your skin, a stone left out in the sun), its surface is constantly radiating its own heat to the sky. And the warmer it grows, the more it radiates.
So the temperature only climbs as long as the Sun delivers more than the Earth sends back: that surplus is exactly what warms the ground, the air and the oceans. Fresh out of spring, the surface is still cool, so it radiates little, and the gap stays wide: the heat piles up fast.
Make no mistake: past the solstice, the Sun does not put out any more heat. It gives a little less each day, in fact, as the days shorten. Yet what it delivers stays greater than what the Earth gives back for weeks more. As long as the input outweighs the loss, the total keeps climbing; the peak only comes when the two finally even out.
The peak comes when the books balance
Think of a bank account. The solstice is the month your income is at its highest. But your balance keeps growing as long as you earn more than you spend, even once the income starts to drop. Temperature plays the part of the balance: it peaks not when the Sun gives the most, but when what it brings in and what the Earth lets back out finally come level.
That balance point falls three to six weeks later, in late July or early August at our latitudes. The oceans stretch the delay further: water stores enormous amounts of heat and releases it with the slowness of inertia. Coastal regions reach their peak later than the interior, and the sea itself can go on warming into September.
The same delay, every day
You feel this lag on the scale of a single day, too. The Sun is highest at noon, yet it is not hottest at noon: the peak holds off until three or four in the afternoon. Same mechanism. As long as the ground takes in more than it loses, the afternoon keeps warming, long after the Sun has started back down.
Nature never answers on the spot. Between turning the heating up and the room finally being warm, there is always that delay. Summer’s heat is simply the most patient of them all.
And since it all plays out in the Sun’s light and its journey through the air: Why are sunsets red?