why is the hottest time of the year in the Northern Hemisphere around August or September not Summer Solstice?

Answer 1

Air temperature lags about 4 weeks behind the sun's strongest and weakest rays. The hottest time of year in the Northern Hemisphere is from mid-July till early August. The coldest time is from mid-January till early February.

The air takes time to warm up so it lags behind the period of maximum solar radiation. Similarly in winter it takes longer to cool down. The sea has an even greater lag.

Answer 2

The way I learned it -- it's called the "lag of seasons". The sun may be highest in the (northern hemisphere) sky at the summer solstice, which does contribute to warming, but it still is not an instantaeous heating effect. It takes a while (months) for the atmosphere to gain all that heat and effectively heat the rest of us up.

That's a pretty simplistic explanation, and for a better answer there may be some atmospheric expert / meterologist who can answer more extensively.

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Answer 3

As stated above, thermal lag. But the biggest thermal lag effect is from the oceans, not the atmosphere.

Answer 4

Hi. Thermal inertia.