Autumn Cooling

There's a slight chill in the air today across interior Alaska, with afternoon temperatures around Fairbanks only in the mid-40s at valley-level thanks to clouds and some light rain.  Temperatures are in the mid-upper 30s in the higher hills; and a glance at the calendar reveals the reason.  Here are some of the observations as of about 6pm (click to enlarge).

Yesterday was the first day with a high temperature below 50°F in Fairbanks, and this is about a week late compared to normal.  Fairbanks usually sees its first sub-50°F day more than a week earlier than Anchorage, which illustrates the contrast between the rapid cooling of the interior and the slower cooling that occurs closer to the waters of the North Pacific.  Anchorage has never seen a sub-50°F day in August during the modern historical record, but it's not too uncommon in Fairbanks and happened just two years ago.

Here are a couple of webcam views of Fairbanks; autumn colors look to be a little past peak.

At this time of year it's always interesting to be reminded of the rapid drop-off in solar radiation at northern latitudes during autumn.  Thanks to 15 years of high-quality data from the Fairbanks CRN site (actually 11 miles northeast of town), we know what the normal incoming solar radiation curve looks like - see below.  I've added the normal cloud cover from Fairbanks airport to give an idea of relative changes in cloudiness through the year, although cloudiness is presumably a bit greater at the CRN site.

Notice how quickly the solar radiation diminishes in mid to late September; the average amount of incoming solar energy drops by 50% in the last 3 weeks of the month.  Of course, at the beginning of September it's already down by about 50% from the peak in June.

Here's the equivalent chart for Barrow, also from 15 years of CRN data; there's little available solar energy by this date in Barrow, and the very high cloud cover in late summer and autumn only exacerbates the rate of decline.

Just for fun, here's the equivalent solar data from the most southerly CRN site in the continental U.S., at Everglades City in Florida, along with the normal cloud cover from nearby Fort Myers.  By September 20 the normal solar radiation has decreased by only about 20% from the solstice, although the annual peak in south Florida occurs in May prior to the wet season.  The slow decline of the solar input, along with the maritime environment, explains why September is basically still high summer in Florida.

But the most interesting aspect of the comparison may be that Barrow receives more solar energy on average in June than south Florida - even though Barrow is very much more cloudy!  This is the result of 24-hour daylight; the noon sun in Barrow is nothing like the noon sun in Florida, but the hours of sunshine really add up in the far north.