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The climate (ancient Greek: κλίμα) is the weather averaged over a long period of time. A descriptive saying is that "climate is what you expect, weather is what you get." The Intergovernmental Panel on Climate Change (IPCC) glossary definition is:

Climate in a narrow sense is usually defined as the “average weather”, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.

Climate vs weather

The exact boundaries of what is climate and what is weather are not well defined and depend on the application. For example, in some senses an individual El Niño event could be considered climate; in others, as weather.

When the original conception of climate as a long-term average came to be considered, perhaps towards the end of the 19th century, the idea of climate change was not current, and a 30 year average seemed reasonable (but see note 1). Given the current availability of long-term trends in the temperature record, it is harder to give a precise contradiction-free definition of climate: over a 30 year period, averages may shift; over a shorter period, the statistics are less stable.

Climate determinants

In a given geographical region, the climate generally does not vary over time on the scale of a human life span. However, over geological time, climate can vary considerably for a given place on the Earth. For example, Scandinavia has been through a number of ice ages over hundreds of thousands of years (the last one ending about 10,000 years ago). Paleoclimatology is the study of these past climates, their origin, and by extension, the origin of today's climate.

Over historic time spans there are a number of static variables that determine climate including: altitude, proportion of land to water, and proximity to oceans and mountains. Other climate determinants are more dynamic: The Thermohaline circulation of the ocean distributes heat energy between the equatorial and polar regions; other ocean currents do the same between land and water on a more regional scale. Degree of vegetation coverage affects solar heat absorption, water retention, and rainfall on a regional level. Alterations in the quantity of atmospheric greenhouse gases determines the amount of solar energy retained by the planet, leading to global warming (or cooling). The variables which determine climate are numerous and the interactions complex but there is general agreement that the broad outlines are understood, at least in so far as the determinates of historical climate change are concerned.

Climate Indices

Scientists use climate indices in their attempt to characterize and understand the various climate mechanisms that culminate in our daily weather. Much in the way the Dow Jones Industrial Average, which is based on the stock prices of 30 companies, is used to represent the fluctuations in the stock market as a whole, climate indices are used to represent the essential elements of climate. Climate indices are generally identified or devised with the twin objectives of simplicity and completeness, and each typically represents the status and timing of the climate factor they represent. By their very nature, indices are simple, and combine many details into an generalized, overall description of the atmosphere or ocean which can be used to characterize the factors which impact the global climate system. Because the climate indices are generally determined from measurements made in a localized area, they can have impacts in other areas around the globe, through processes sometimes called teleconnections.

References:

• Why and how do scientists study climate change in the Arctic? What are the Arctic climate indices?
• Climate index and mode information

Classifications

In the original sense, climate is a concept used to divide the world into regions sharing similar climatic parameters. Climate regions can be classified on the basis of temperature and precipitation alone. Examples of such climate schemes are the Köppen climate classification or the Thornthwaite climate classification schemes.

For more details about specific climates, please see:

• Tropical climate
• Subtropical climate
• Arid climate
• Semiarid climate
• Mediterranean climate
• Temperate climate
• Oceanic climate
• Continental climate
• Alpine climate
• Subarctic climate
• Polar climate
• Climate of Antarctica

To understand a climate of a specific place or area, please see the article on that place or area.

See also

• Climate change
• Solar variation
• Temperature extreme.

Historical climates

• Climate changes of 535-536
• Medieval climate optimum

National climates

• Climate of the Alps
• Climate of India
• Climate of the United Kingdom

External links

• WorldClimate
• ESPERE Climate Encyclopaedia
• Weatherbase
• Global Climate Data
• The Climate of Peru
• Climate index and mode information
• Why and how do scientists study climate change in the Arctic? What are the Arctic climate indices?
• A near-realtime Arctic Change Indicator Website
• A current view of the Bering Sea Ecosystem and Climate

Notes

1. In "Climatology" by W G Kendrew (OUP; 3rd edition 1949; chapter 38; page 359) we find: "A well-known cycle is one with a mean period of about 35 years... which was worked out by Bruckner... the reality of this cycle seems to be well established, though it is of little use for actual forecasting; it is a basis of the choice of 35 years as the period estimated to give true mean values of climate elements."

• Climate
• Ecology