The climate varies widely due to its large geographical size, but by far the largest part of Australia is desert or semi-arid. Only the south-east and south-west corners have a temperate climate and moderately fertile soil. The northern part of the country has a tropical climate, varied between tropical rainforests, grasslands and desert. Because Australia is a small continent, separated from polar regions by the Southern Ocean, it is not subject to the movements of frigid polar air that sweep over the continents in the northern hemisphere during winter. Consequently, its winter is relatively mild, so that there aren’t any great contrast between summer and winter temperatures that is present in the northern continents. Yet in many parts of the country, seasonal highs and lows can be considerable: temperatures have ranged from above 50 °C (122 °F) to well below zero. Nonetheless, minimum temperatures are moderated.