The days have been getting shorter since June. So why does the weather keep getting warmer?

Aug 3, 2019 at 3:00pm

The Washington Post, by By Jeremy Deaton

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.

“The hottest day of the year is related to how long it takes any given place to heat up in response to the heat incoming from the sun,” said Karen McKinnon, a climate scientist at UCLA who studies variability in the Earth’s climate. “The Earth is interesting because we have land and we have ocean, and they respond to the heat that comes from the sun in different ways.” It all comes down to something called heat capacity.

“What that means is, how much heat do you have to put into a material in order to heat it up,” she said. “If you think about trying to boil a pot of water, you can turn on the flame under the water, but water has a pretty high heat capacity, and so it takes a good amount of time after you put the heat into the water for the water temperature to actually increase and then come to a boil.”

In places where there is more water, temperatures rise more slowly. In San Francisco, which is bounded by water on three sides, the hottest day of the year is typically in late September. In El Paso, which sits in the middle of the desert hundreds of miles from the nearest ocean, the hottest day of the year is usually right around the summer solstice.

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