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.
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
