Climate is the average weather in a given area over a longer period of time. A description of a climate includes information on, e.g. the average temperature in different seasons, rainfall, and sunshine. Also a description of the (chance of) extremes is often included. Climate change is any systematic change in the long-term statistics of climate variables such as temperature, precipitation, pressure, or wind sustained over several decades or longer. Climate change can be due to natural external forcings (changes in solar emission or changes in the earth’s orbit, natural internal processes of the climate system) or it can be human induced.

The classical period used for describing a climate is 30 years, as defined by the World Meteorological Organization (WMO).