CLIMATE

1. Climate is commonly defined as the weather averaged over a long period of time. The Intergovernmental Panel on Climate Change (IPCC) glossary definition is: Climate in a narrow sense is usually defined as “average weather”, or more rigorously, as 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.

2. The difference between climate and weather is usefully summarized by the popular phrase “Climate is what you expect and weather is what you get”. Over historical time spans there are a number of nearly constable variables that determine climate, including latitude, altitude, proportion of land to water, and proximity to oceans and mountains. These change only over periods of million of years due to processes such as plate tectonics. Other climate determines are more dynamic: for example, the thermohaline circulation of the ocean leads to a 5^o C warming of the northern Atlantic Ocean compared to other ocean basins. Other ocean currents redistribute heat between land and water on a more regional scale. The density and type of vegetation coverage affects solar heat absorption, water retention, and rainfall on a regional level. Alterations in the quantity of atmospheric greenhouse gases determine the amount of solar energy retained by the planet, leading to global warming or global 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 insofar as the determinants of historical climate change are concerned.

3. There are several ways to classify climates into similar regimes. Originally, climes were defined in Ancient Greece to describe the weather depending upon a location’s latitude. Modern climate classification methods can be broadly divided into genetic methods, which focus on the causes of climate, and empiric methods, which focus on the effects of climate. Examples of genetic classification include methods based on the relative frequency of different air mass types or locations within synoptic weather disturbances. Examples of empiric classifications include zones defined by plant hardiness, evapotranspiration, or more generally the Köppen climate classification which was originally designed to identify the climates associated with certain biomes. A common shortcoming of these classification schemes is that they produce distinct boundaries between the zones they define, rather than the gradual transition of climate properties more common in nature.

4. The most generic classification is that involving the air masses. The Bergeron classification is the most widely accepted form of air mass classification. Air mass classification involves three letters. The first letter describes its moisture properties, the second letter describes the thermal characteristic of its source region and the third one is used to designate the stability of the atmosphere.

5. Based upon the Bergeron classification scheme is the Spatial Synoptic Classification System (SSC). There are six categories within the SSC scheme: Dry Polar (similar to continental polar), Dry Moderate (similar to maritime superior), Dry Tropical (similar to continental tropical), Moist Polar (similar to maritime polar), Moist Moderate (a hybrid between maritime polar and maritime tropical) and Moist Tropical (similar to maritime tropical, maritime monsoon, or maritime equatorial).

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