Diurnal temperature patterns in winter

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The high temperature for the day comes in the afternoon and the low in the wee hours of the morning, right? You can probably guess that with such a leading question and the fact that we are talking about Alaska (where “we don’t care how they do it in the Lower 48”) means I am about to try to blow this assumption out of the water.

Fairbanks temperature graph

A look at November 25 on the above graph (click on graph for larger version) seems to support the daytime high pattern mentioned in the opening sentence. However, notice how the temperature rise started before sunrise and started to fall well before sunset. On the 26th the temperature rises from midnight to sunrise, falls during the short daylight hours and rises again after sunset. Assumption blasted.

The physical principles that govern daily temperatures apply anywhere. But in Alaska some of these principles are often more pronounced, and others less. The term diurnal, by the way, is applied in meteorology to things that have some sort of a daily pattern or cycle. The pattern of daily temperature changes mentioned in the lead-in is caused by the sun, and is the true diurnal temperature cycle. Of course at this time of the year in Alaska the sun is pretty much AWOL, which means all the other factors which control the temperature have little trouble overpowering its influence. These include the advection (the moving in on the wind) of warmer or colder air from another area, mixing of warmer (usually) or colder air from layers above the surface, and radiational effects.

Radiation is what the sun is all about, and nothing beats it at this game when it is around. When the sun is down, the earth mostly loses heat through radiation, bringing on the cool half of the classic diurnal cycle. During the Alaska winter we have the radiational heat loss part down well and that is basically why it is so cold in the polar regions. But there is another source of warming radiation in the northern winter…clouds. Yes, clouds. And they don’t simply stop nighttime heat loss as is commonly known (the blanket effect). They do actually radiate heat down to the surface. (Everything radiates infrared heat…and receives radiation from other things. It is when the radiational balance between masses is lopsided that strong warming or cooling takes place.) The clouds simply need to be in a warmer stratum of air than the surface. This is almost always the case in the interior and north and west coast winter, much less so in south coastal areas.

In the Fairbanks example, it is the mixing effect of the wind that is the most pronounced. Even the very slight wind of 3-5 knots (4-6 mph) is enough to stir some warmer air down to the surface in the otherwise calm air, causing a temperature jump.

The cloud cover (column labeled sky conditions in the hourly text reports below) shows a less convincing correlation with the temperature in this case, although the automatic cloud sensing equipment may not be good enough for this kind of scrutiny. In other cases it can be a dominant factor.

Wind can pick up and clouds can move in or out anytime of the day or night, affecting the temperature and causing a fairly large temperature range on the high and low record for the day, but it is not a true diurnal cycle.

Here are the hourly observations from Fairbanks covering roughly the same time as the graph above. The time column is in Alaska Standard Time. Temperature (labeled temp) is shown in Fahrenheit. The wind column gives the direction in the first 3 digits and speed in the last two (in knots).

Site M/A Day Time Sky Conditions           VIS Weather Temp DP Wind(kt)  Alt  RH  Chill Peak
PAFA  AA 25 1153  FEW080 SCT100 BKN200      10           2  -2 00000     936  83%   2
PAFA  AA 25 1253  FEW080 SCT100 BKN200      10           5   1 20003     936  83%   5
PAFA  AA 25 1353  SCT100 SCT200             10           2  -1 00000     935  87%   2
PAFA  AA 25 1453  SCT100 SCT200             10           0  -4 00000     933  83%   0
PAFA  AA 25 1553  SCT100 SCT200             10           0  -4 19005     932  83% -12
PAFA  AA 25 1653  FEW100 SCT200             10          -2  -7 00000     932  79%  -2
PAFA  AA 25 1753  FEW100 SCT200             10          -8 -12 00000     932  82%  -8
PAFA  AA 25 1853  FEW100 SCT200             10          -9 -13 00000     932  82%  -9
PAFA  AA 25 1953  FEW020 SCT100             10          -8 -13 00000     931  78%  -8
PAFA  AA 25 2053  FEW020 SCT100 SCT200      10          -8 -13 00000     929  78%  -8
PAFA  AA 25 2153  FEW020 SCT100 SCT200      10          -9 -13 00000     928  82%  -9
PAFA  AA 25 2253  FEW020 SCT100 SCT200      10         -10 -15 00000     926  78% -10
PAFA  AA 25 2353  FEW020                    10         -10 -15 00000     924  78% -10
PAFA  AA 26 0053  CLR                       10         -10 -15 04003     922  78% -10
PAFA  AA 26 0153  CLR                       10          -9 -14 04003     920  78%  -9
PAFA  AA 26 0253  FEW090 SCT200             10          -5  -9 04003     918  82%  -5
PAFA  AA 26 0353  SCT100 SCT200             10           1  -4 05005     917  79% -10
PAFA  AA 26 0453  BKN100                    10           5   1 00000     917  83%   5
PAFA  AA 26 0553  FEW070 BKN100             10           5   1 21003     919  83%   5
PAFA  AA 26 0653  SCT090 BKN120             10           4   0 00003     920  83%   4
PAFA  AA 26 0753  SCT085 OVC110             10           3   0 00000     923  87%   3
PAFA  AA 26 0853  SCT080 BKN100             10           4   0 00000     925  83%   4
PAFA  AA 26 0953  FEW080 BKN100 BKN200      10           5   1 00000     928  83%   5
PAFA  AA 26 1053  SCT085 BKN120 BKN200      10           6   1 00000     931  79%   6
PAFA  AA 26 1153  FEW085 SCT120 BKN200      10           5   0 00000     932  79%   5
PAFA  AA 26 1253  FEW085 SCT120 BKN200      10           0  -3 00000     933  87%   0
PAFA  AA 26 1353  FEW085 SCT120 BKN200      10           1  -2 00000     934  87%   1
PAFA  AA 26 1453  FEW080 SCT130 BKN200      10          -2  -5 03003     932  87%  -2
PAFA  AA 26 1553  FEW080 SCT130 BKN200      10          -2  -6 00000     931  82%  -2
PAFA  AA 26 1653  FEW080 BKN130 BKN200      10           5  -6 00000     930  59%   5
PAFA  AA 26 1753  FEW080 BKN100 BKN200      10          10   2 00000     930  69%  10
PAFA  AA 26 1853  FEW080 BKN100 BKN200      10          10   0 00000     929  63%  10
PAFA  AA 26 1953  BKN100 BKN200             10          14   0 00000     930  53%  14
PAFA  AA 26 2053  BKN100 BKN200             10          13   2 00000     930  61%  13
PAFA  AA 26 2153  BKN110 OVC200             10          14   6 00000     930  70%  14
PAFA  AA 26 2253  BKN110 OVC200             10          17   3 02003     930  53%  17
PAFA  AA 26 2353  BKN095 OVC200             10          14   4 00000     929  64%  14
PAFA  AA 27 0053  BKN095 OVC200             10          18  10 00000     928  70%  18
PAFA  AA 27 0153  BKN075 OVC090             10          21  12 00000     926  68%  21
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