Tracking Seasonal Highs: When Is Summer Actually Hottest?

The summer solstice is June 21, the day with the most direct sunlight. But most cities don't hit their hottest averages until late July or even early August.

This is called "seasonal lag." Landmasses take weeks to absorb and release heat. The atmosphere, oceans, and soils all have thermal inertia, so peak temperature trails peak insolation by roughly a month and a half inland, and even longer along coasts.

For Chicago, the warmest month is July (average high 84°F). For Boston, also July, with slightly lower values. Along the Pacific coast, San Francisco is strange — its warmest month is September, because the coastal upwelling cycle keeps summer cool and autumn warm.