Photograph courtesy of John B. Anderson, Rice University. The Antarctic atmosphere is cleaner. It contains far fewer particles to reflect and scatter incoming radiation. There is much more industry in the Northern Hemisphere, so the Arctic atmosphere contains large quantities of such particles. However, Antarctica is more than a degree colder than the Arctic on average.
Water holds heat longer than land.
This heat passes into the atmosphere through the thin sea ice, or from the sea surface directly, so the temperature of the air stays warmer. The Antarctic is a large continent covered by a thick ice sheet. The sea ice of the Arctic is not quite as reflective, and the ocean actually absorbs most of the incoming solar radiation. The amount of solar radiation reflected from a surface is called albedo. The land mass distribution is different in the Northern Hemisphere and the Southern Hemisphere.