Take Alert Airport, in Nunavut, Canada, for instance. It's part of the Global Historical Climatology Network (GHCN). The temperatures for April 2, 2010, were clearly below -20ºC. But slightly after 6am, surprise! In comes a zero value, and temperature skyrockets, as can be seen in the graphic below, with a special SPECI message. After three hours, cold temperatures resume, but that day should have established a new maximum day temperature, with 0ºC.
Switching to Greenland, where ice is said to be melting, we find more interesting hot temperatures. Take Qaanaaq, and the first week of last February. As can be seen in the following graph, some good warming occurred between late February 3 and early morning next day. From the METAR data, in the table below, it doesn't seem to be a normal "M" missing problem. Even if it was an M2 or M3, it would still be strange...
METAR BGQQ 040050Z AUTO 05001KT //// NCD M11/M11 Q1007
Going back to Nunavut, in Canada, we can look at Cape Hooper temperatures, for February 23, 2010. Now, this was a quite cold day, but something interesting happened slightly after lunch time. Temperatures soared by more than 10ºC, during only one hour. But wait a minute: the graph has two different temperatures for the same hour! How can that be? You have to take a look at the raw data to find out that the METAR and AAXX data don't agree! Looking at the other data at Weather Underground, it seems that it also got quite windy during that period. Might it be an Arctic tornado, or, who wonders, simply a plane taking off?
METAR CWUP 231500Z AUTO 03005KT M19/ A3015 RMK SLP260
While METAR and AAXX seem to agree most of the time, there are many other situations where they disagree by much more than one degree. Don't really know why, but here is another example. From Longstaff Bluff, also in Nunavut, the following is an example of February 23, 2010.
AAXX 23154 71091 45/// /0921 11111 39946 40158 53009
Now, these are examples of nice places in the Arctic, where you can get a little warmth once in a while. But if you are interested in a nice dark place, all day long, but very warm, you should take a look at Phippsoya, in Norway. It's way up in Svalbard, and looks like a hidden treasure on Earth. Polar bears seem to like it, maybe because of Global Warming. March had a mean temperature of 18ºC, and a maximum of 33ºC!!! Almost as good as Portugal in the Summer!