The weather systems undergo incredibly rich variety and never repeat. If left alone in the atmosphere, mid-latitude storms would continually march eastwards, modulated by very large-scale atmospheric waves – planetary waves – set up by the land–sea differences and continental mountain ranges. The latter are mainly in evidence in the northern hemisphere and vary from summer to winter because the mid- to high-latitude land is colder than the adjacent seas in winter, but warmer in summer. Nevertheless, this idealized state seldom exists and frequently the weather systems get stuck into a certain repeating pattern of sorts, called weather regimes.