So, one would imagine that data for this station is quality proof. But that was not the idea I got when I found out about Arctic heat. We found two sources for temperature data: at Weather Underground and at the National Climate Data and Information Archive.
The first interesting data about this station is it's record high temperature, which according to Wikipedia was reached on July 14, 2009, with 20ºC. On Weather Underground, the page for that day does say that 20ºC was the maximum temperature. But when you check the METAR data, the maximum temperature was 14ºC. Checking the Environment Canada page, the maximum for the date was 14.4ºC. Things were different on the day before, July 13th. Maximum temperature for Weather Underground was also 20ºC, while at Environment Canada was 19.6ºC. But if you check the graphs below, some special heat occurred at 10PM, when temperatures soared some 15ºC!
As Anthony Watts pointed out at Watts Up With That, the Eureka station registered the biggest rise in temperature probably seen on the Earth's surface: 86ºC in one hour, on March 3, 2007! Now this data is available on Weather Underground, but seems not to exist in Environment Canada. The graph differences are clear below:
But that seems not to be the case in other examples. Take January 1st, 2007, for instance. Both Weather Underground and Environment Canada agree: there was a mighty spike at noon. Seems like the "M" problem affects both:
There are times where differences are not so big, but the "M" problem is still there. Check the images from Weather Underground and Environment Canada for September 26, 2006:
Other times, changes are so significant, that something must be wrong. Check out the temperature rise on June 20, 2005. On the left, the weekly graph from Weather Underground shows a great surge in temperatures, confirmed by the Environment Canada graph for the day.