It’s been a little over five weeks since the summer solstice, the longest day of the year. Since June 21, the days have grown progressively shorter. The sun has been in retreat, casting off a little less light every day, beaming down a little less heat. And yet, in many parts of the United States, the weather has only gotten warmer. Scientists have a name for this phenomenon: seasonal lag. In many places, the hottest day of the year comes weeks, or even months, after the longest day of the year. The reason for that has a lot to do with how water soaks up heat.