Both polar regions of the earth are cold, primarily because they receive far less solar radiation than the tropics and mid-latitudes do. At either pole the sun never rises more than 23.5 degrees above the horizon and both locations experience six months of continuous darkness. Moreover, most of the sunlight that does shine on the polar regions is reflected by the bright white surface.
What makes the South Pole so much colder than the North Pole is that it sits on top of a very thick ice sheet, which itself sits on a continent. The surface of the ice sheet at the South Pole is more than 9,000 feet in elevation--more than a mile and a half above sea level. Antarctica is by far the highest continent on the earth. In comparison, the North Pole rests in the middle of the Arctic Ocean, where the surface of floating ice rides only a foot or so above the surrounding sea.The Arctic Ocean also acts as an effective heat reservoir, warming the cold atmosphere in the winter and drawing heat from the atmosphere in the summer.
Answer originally published May 5, 2003.