The summer solstice (June 21) is passed, but temperatures continue rising for about another month because of the cumulative effect of the summer season. Why does the same effect continue after the winter solstice (Dec. 21), when temperatures don’t start their upswing for another month?
Chicago’s temperatures continue rising for about five weeks after the summer solstice because, on average, incoming heat from sunlight is still greater than heat lost into space. Incoming heat (in the Northern Hemisphere) reaches its maximum on the summer solstice, then begins decreasing, but it takes about five weeks for it to be overtaken by (outgoing) heat loss. The opposite occurs on the winter solstice (about Dec. 21). Incoming heat begins to increase after the winter solstice, but heat lost into space is still greater than incoming heat for another month.