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If the sun is most concentrated at the tropic of Capricorn on either the 21st or 22nd of December, shouldn't that also be more or less the coldest date of the year in the northern hemisphere? And December in general? Apparently January 17th and mid-January are the worst, but I'm curious as to why.
"Why isn't the month with the least sunlight also the one with the lowest temperatures?
Because water retains heat. Between 70 percent and 75 percent of the Earth's surface is covered in oceans, rivers, and lakes. (There's even more water vaporized in the air or stored in the ground.) During seasons of longer days and more sunlight, these geographical features are able to store up and retain heat over long periods of time, before emitting it as the days get shorter. A body of water is far more effective as a space heater than, say, a big field of rocks: The water holds on to five times as much heat per gram.
Large bodies of water thus serve as giant heat reservoirs, but they're not the only contributing factor to the seasonal lag in average temperatures. Properties of the Earth's atmosphere also contribute to the delayed onset of winter chills. Daily thermometer readings are determined, in part, by the balance between solar heat that's reflected by the planet surface (and bounced back into space) and solar heat that is absorbed into the climate system. This balance shifts as winter proceeds. At the beginning of the season, the temperature drops, which leads to an increased amount of snow covering the globe. This makes the Earth more reflective, with sunlight bouncing off the bright, white surfaces; more reflection exacerbates the cold weather and maintains the low temperatures. Gradually, as the days get longer, the snow melts, and the Earth's atmosphere begins to absorb more heat."
Totally unscientific, but I would also guess air masses and the gradual disappearance of residual heat from the summer and fall even.
At least in northeastern North America, January and February (especially in late Jan. and early Feb.) I believe have dominant trends of air masses flowing down from the Arctic.
Also, December is closer to the summer and fall, and by the time you get into the new year most of the heat from the warmer months is gone from the air. And the effect of darkness in December with little heat from the sun is also a cold-inducing lag for the two months that follow.
What the above posters said.
Where I live, in west-central Canada, December is the second-coldest month of the year, after January. The statistically coldest period of the year is the 2nd week of January. So we have relatively low seasonal lag in winter compared to some other places.
1981-2010 averages:
December: -13 C
January: -15 C
February: -12 C
If you examine the climate of Fairbanks, AK, or Yakutsk, Russia, it's even more clear that January and December are the coldest months. Oddly enough, if you go south of the Tropic of Cancer, December is often the coldest month, or a close runner up. Examine the climate of Calcutta, India.
But many mid-latitude locations have December as the third-coldest month.
One factor you need to consider is that daylength follows a sinusoidal curve. It doesn't increase or decrease linearly on the calendar. The increase in daylength and solar gain between the winter solstice and late January is small. The biggest increases occur around the equinoxes, and the least around the solstices. This is why the biggest mean temperature changes usually occur in April and October.
December is slightly cooler than February here but only because the days are cooler. February has colder nights and is more prone to very cold weather and snow, so I prefer February to December.
Because the oceans haven't cooled off sufficiently by December yet, the oceans reach their coldest by February. The further in a continent you are the less "seasonal lag" you'll experience. Although, the west coast of North America is usually at it's coldest by late December, despite being right next to the Pacific. I wonder why that is
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