During the Northern Hemisphere winter season the sun angle is at its lowest elevations. This is due to the earth being tilted on its axis such that the Northern Hemisphere points away from the sun during its winter season. What this means is the far northern latitudes north of Alaska do not experience sunlight during the so-called 'daytime.'. In fact, at the North Pole you have 6-months of sunlight and 6-months of nighttime.
Below is a GOES visible satellite image at 115 pm Friday January 18, 2013, centered on North America. The GOES satellites are at an altitude of 23,000 miles above the earth's surface. Note the dark area across the top of the image - that's nighttime darkness even during the daytime. Of course, the lack of sunshine means air-masses over land can get very cold in the northern latitudes.
Kapela, WFO Milwaukee/Sullivan