Because a substantial part of the Sunís heat is used up in evaporation and rain formation, temperatures in the tropics rarely exceed 35?C; a daytime maximum of 32?C is more common. At night the abundant cloud cover restricts heat loss, and minimum temperatures fall no lower than about 22?C. This high level of temperature is maintained with little variation throughout the year. The seasons, so far as they do exist, are distinguished not as warm and cold periods but by variation of rainfall and cloudiness. Greatest rainfall occurs when the Sun at midday is overhead. On the equator this occurs twice a year in March and September, and consequently there are two wet and two dry seasons. Further away from the equator, the two rainy seasons merge into one, and the climate becomes more monsoonal, with one wet season and one dry season. In the Northern Hemisphere, the wet season occurs from May to July, in the Southern Hemisphere from November to February.