Monday, August 26, 2013

Loss of Solar Insolation

Rick's post on Sunday highlighted the hard freeze that occurred Sunday morning at certain locations near Fairbanks and in the Upper Yukon valley (notably, 16 °F at the Beaver Creek RAWS).  For those who are less familiar with Alaska climate, it is often surprising how rapidly temperatures begin to drop off in August, even while lower latitude locations are still baking in the heat of high summer.

The chart below partially explains why northern continental locations cool off so quickly.  The chart shows the percent drop in daily incoming solar radiation ("insolation") compared to the annual maximum at the summer solstice, for latitudes between 30°N and the North Pole.  Note that these are theoretical numbers assuming a clear sky (no clouds), no refraction, and no reduction in solar radiation from other factors like dust or haze.  I've plotted the data for two dates in the autumn: August 25 (blue line) and September 22 (the autumn equinox, red line).

By August 25, the maximum possible daily solar radiation has dropped by more than 35 percent in Fairbanks compared to late June, largely because of the shortened day.  In Barrow, at over 71 °N, more than 45 percent of the sun's daily energy is lost (although of course it's rarely sunny in Barrow at this time of year).  By contrast, in southern regions of the lower 48, the daily radiation has dropped only 10-15 percent by late August, which is barely perceptible to most people: the sun maintains nearly all of its summer strength.

Looking ahead to the autumn equinox, when day and night are of equal length across the globe, Fairbanks will have lost over 60 percent of its peak daily insolation, and the high Arctic will approach the long night of winter.


8 comments:

  1. Rick and I briefly discussed this aspect earlier with a slightly different perspective. Seems like the Sun's angle above the horizon (~35 deg 8/26/13, losing ~1 degree/three days ) and the distance of atmospheric penetration before reaching the earth's surface were the culprits.

    We may have more hours of daylight, but we have less solar insolation than say a more southerly location with a lower daily photoperiod.

    Gary

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  2. Gary,

    Thanks for the comment. I agree that the decreasing solar elevation angle (and greater atmospheric reflection/scattering/absorption as a result) is responsible for a large part of the drop-off in energy received as the season advances; I could have emphasized that more. Length of day plays an important though perhaps secondary role. At lower latitudes (e.g. 30-40 N) nearly all the radiation arrives between 6 am and 6 pm, both in June and August, whereas in Fairbanks the radiation outside those hours is significant in June, then becomes small by late August. So the contraction of the northern day in late summer puts another hit on the daily available energy.

    It's interesting to observe that on June 21 the theoretical daily insolation is just about equal in Fairbanks and at 30 N (e.g. Houston!) - although not quite in practice because of the greater path length in the north. And it's even about 10 percent higher at the Pole - the highest of any latitude - again in theory.

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    1. Seems like the effect of more absorptive aerosols and contaminants from volcanoes would effect the insolation drastically at our latitude.

      Like this deal: http://en.wikipedia.org/wiki/Volcanic_winter

      Worse for us than Houston?

      Gary

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    2. Yes, I think so. Doubling the absorption would make a bigger absolute difference where the absorption is already high. Very interesting.

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  3. I was going to chime in here and mention that sun angle has may have more to do with this than day length. As an example for Fairbanks from August 1st to August 31st the sun angle is 25% lower by the end of the month. More surprising from October 1st- October 31st the change is almost 50%!
    (Supports my Greenland argument from a few posts ago)
    (Anyone want to do a research project? ❄)

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    1. I'll bet some of the solar energy research facilities that keep track of daily panel output could shed some light on the matter. Bad pun but that's where I'd start if trying to tie the effect of sun angle and duration into local climate.

      Gary

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    2. Mike,

      Thanks - I think you're right. I put up a new post with some thoughts/results about this. Your and Gary's comments were very helpful.

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