Climate question for the smart people here

[Quinn]

We've passed the Winter Solstice and the days are getting longer. So how come we're still moving toward the coldest part of the year? Okay, I expect some lag because the ground and water are heat sinks that have to recover, but the Earth is tilting the Northern hemisphere back toward the Sun and we're getting more sunlight, more direct, for longer periods each day. So why are we still heading toward the cold pit that is February?

 

I'd always passed it off by figuring that the Earth's orbit is eliptical and maybe, although the Northern Hemisphere was heading back to more direct sunlight, we were actually further from the magic star. But, if that were true, then the Southern Hemisphere wouldn't have as hot a Winter as we do a Summer.

 

So, smarter people than me, why are we still losing heat for the next month and a half?

 

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[Huzband]

I'm not smart (just ask Moshe ), but I've often wondered the the same thing. How come late January & all of Feb are the coldest, & harshest, of months? I grew up in north Alabama, & those were predictably the worst winter months of the season. Or the best, depending on weather school closings.

[Natche]

I think it's because the earth is on an elliptical orbit and is tilted we are still moving further from the sun in the northern hemisphere. The daylight has less impact on temperature than does the proximity to the sun. In the winter, we are tilted away from the sun and in the summer we are tilted toward it. Someone who really knows, fill in the real truth here but I'm scratching at it somewhat. This is the type of answer I give my wife. She thinks I know everything.

[CoarsegoldKid]

The way I have always understood it is we have an orbit where at one point of the year the earth is close and another far from the sun. Okay you knew that. However the proximity makes no difference in the amount of heat energy absorbed by the earth. When far away some part of the earth is still getting very hot. What does make the difference is the tilt of earth's axis. That allows direct rays of sunlight for summers and glancing rays for winters. The northern hemisphere's glancing rays are slowly turning to direct rays.

[DavidEBSmith]

For the same reason the summer solstice, around June 21, the longest day of the year in the Northern Hemisphere, is not the hottest day of the year.

 

Not a bad answer here: http://www.usatoday.com/weather/resources/askjack/2003-11-07-answers-cold-days_x.htm

 

Think of warmth like money the sun deposits in a bank account, which always has money going out as well as coming in.

 

Sunlight reaching the ocean or land warms the Earth. But the warm ocean or land also give off heat, which escapes out into space as infrared energy. This is why it almost always grows cooler at night.

 

When days are short during the winter, more heat is leaving than coming in. Even as days begin growing longer, more heat continues going out than coming in until maybe late January, depending on the place, when more heat finally begins arriving than leaving, and days start to warm up.

[chrisd]

So why are we still heading toward the cold pit that is February?

 

Actually, it's not quite that bad. Your cold pit is January. February sees the average temps rising again, above December's level.

 

Buffalo and Fairbanks (and all others I looked at) are the same, with February's average temperature higher than January's.

[Jaguar]

The way I have always understood it is we have an orbit where at one point of the year the earth is close and another far from the sun. Okay you knew that. However the proximity makes no difference in the amount of heat energy absorbed by the earth. When far away some part of the earth is still getting very hot. What does make the difference is the tilt of earth's axis. That allows direct rays of sunlight for summers and glancing rays for winters. The northern hemisphere's glancing rays are slowly turning to direct rays.

 

http://www.personal.soton.ac.uk/ejr/DarkMed/6-precession.jpg

[tallman]

This series from Columbia gives you a one page overview.

It covers solar raidation, the physics of radiative heating, electromagnetic radiation, albedo, axial tilt, longitude, effect of the shape of the orbit, greenhouse effect, and provides links to the math if you so desire.

With those one can do the calculations for any planet.

 

[Joe Frickin' Friday]

In technical jargon:

In any system acted upon by a forcing function, the system's first-order response will lag behind the forcing function. If you have a cyclic forcing function, then when the forcing function reaches a minimum (or a maximum), the system's response will reach a minimum (or maximum) at some later time.

 

 

 

Less technical:

Wherever you are, there's always solar heat coming in from the sun (call it "heating power"), and the earth is also radiating heat away into outer space (call it "cooling power"). Cooling power is proportional to temperature: if the earth is hotter, it radiates heat away at a higher rate. Temperature is stable when the heating power is equal to cooling power. If solar power input suddenly jumped by some amount, then the earth would warm up, and cooling power would increase accordingly, until heating power and cooling power were in balance again. These concepts work on a global scale, but the earth can't equalize place-to-place variations, so these concepts also work on a smaller scale; that's why the north and south poles are colder than Kenya.

 

On the winter solstice (in the northern hemisphere), the sun is delivering the lowest heating power to the northern hemisphere. After that, heating power begins increasing as the northern hemisphere begins moving back toward the sun - but the heating power is still very low. At this time cooling power is still very high. The lowest daily temperature happens when solar power delivery has increased to the point where it matches that cooling rate. Depending on where you are, that's some time in late January or early February.

 

FWIW, a similar lagging phenomenon happens with the tides:

 

[quote=The lunitidal interval measures the time lag from the moon passing overhead, to the next high or low tide. It is also called the high water interval.

 

Tides are known to be mainly caused by the moon's gravity. Theoretically, peak tidal forces at a given location occur when the moon is at the meridian, but there is usually a delay before high tide that depends largely on the shape of the coastline, and the sea floor, therefore, the lunitidal interval varies from place to place. The lunitidal interval further varies within about +/- 30 minutes according to the lunar phase.

 

Same fundamental concept: system response lags behind the forcing function. In the OP's case, the forcing function is solar heating; in the case of the tides, the forcing function is lunar gravity.