The Average Date of the Hottest Day in the US

The picture below shows the average date where the hottest temperature is reached in many cities across the US. It probably doesn’t surprise you that most of those dates are late-July into early August. But check out San Francisco. The average date of the hottest temperature is actually in late September. Why? The coastal fog gets blown in all summer long by a strong and persistent sea breeze. That sea breeze lightens up considerably in late September.