This course has introduced the polar climate by showing the annual cycle of mean atmospheric surface temperature at two locations. In their respective winters, both hemispheres are extremely cold and there are very large differences in temperature between winter and summer. The low temperatures are caused by reduced energy from the Sun coupled with a high albedo. It is possible to define the Arctic using both temperature and the northern limit of tree growth. But defining the Antarctic it is more problematic, and here an ocean front that separates warm and cold waters is a sensible definition.