In a narrow sense, climate is usually defined as the “average weather” or, more rigorously, as the statistical description in terms of the mean and variability of relevant quantities—most often of surface variables such as temperature, precipitation and wind—over a period of time ranging from months to thousands of years. The classical period is three decades, as defined by the World Meteorological Organization (WMO). In a wider sense, climate is the state, including a statistical description, of the climate system.