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Solstice, shmolstice – Why the coldest days are still to come

Why is the winter's coldest day usually more than a month away?

(Inside Science) -- Residents of the Northern Hemisphere, don't worry about the winter solstice – it's not the middle of winter, and in some places, it's not even the start of wintry weather.

So why exactly is the shortest day of the year so distant from the coldest temperatures? It's usually another month before the bone-aching freezes of winter hit their worst.

That gap is what's known as the seasonal lag, said Anthony Arguez, a physical scientist with the National Oceanic and Atmospheric Administration's National Climatic Data Center in Ashville, North Carolina.  The lag occurs primarily because the earth's land and oceans absorb some of the sun's energy and release it slowly over time.

"There's not a good answer for why people say that December 21 is the beginning of winter," he said. "There's nothing magical that says that winter has to happen after the solstice." While the temperature of soil more than 30 feet below the surface remains basically constant, the soil higher up holds in heat, even while the air temperatures drop off.  Arguez pointed out that summertime temperatures have a similar lag – the hottest day of the year in the Northern Hemisphere is typically in July or August.

In places far north and south, where the difference in the number of daylight hours in summer and winter is huge – like Alaska and Sweden – the solstice is known as mid-winter. It's also necessary to point out that the solstice, traditionally celebrated as the "shortest day of the year" in terms of hours of daylight, actually doesn't have the earliest sunset in some places. The earliest sunset at 40 degrees north latitude (which includes New York, Beijing and Madrid) was on December 7. Closer to the equator, the earliest sunsets of the year come in late November. Closer to the North Pole, they come near the solstice.

Another factor creating the weird lag between the shortest and coldest days of the year becomes apparent by taking a closer peek at the oceans' role – after all, water does cover about 70 percent of the surface of the earth.

If you put a pot of water on a stove, and turn on, it warms up gradually and reaches its highest temperature later, even if you turn the range down from 10 to 8, said LuAnne Thompson, an oceanographer at the University of Washington in Seattle.

"So the maximum temperature is not the same time when you apply the maximum heat."

Water is able to hold about four times more heat than air, so it can carry heat much more efficiently. This is why in the days before electric blankets and space heaters, people would use bottles filled with hot water to keep themselves warm at night, not bottles filled with hot air.

Thompson pointed out that the seasonal differences near the ocean are muted. A place like Rome, Italy will have less extreme cold or hot weather than a place like Des Moines, Iowa – because Rome is surrounded by oceans.

Because of its heat capacity, ocean water is even slower than land to warm up and cool down – the warmest ocean temperatures are in late August, and ocean water is coldest in late January.

To truly define winter on a local scale, Arguez suggests finding the coldest day of the year in your area – and go forward and back 45 days. That could put winter temperatures in January, February and March for many parts of the U.S.. So enjoy your festive autumn season, one and all!

Reprinted with permission from Inside Science, an editorially independent news product of the American Institute of Physics, a nonprofit organization dedicated to advancing, promoting and serving the physical sciences.