An Overview of Climate

Climate, Climate Classification, and Climate Change

Barometer and Climate
A barometer measures air pressure, an important component to quantifying weather on the short-term and determining climate on the long-term. Peter Dazeley/Getty Images

Climate is defined as the average weather patterns existing throughout several years over a large portion of Earth's surface. Usually, the climate is measured for a specific area or region based on weather patterns over a 30-35 year time period. Climate, therefore, varies from weather because weather is concerned only with short term events. A simple way to remember the distinction between the two is the saying, "Climate is what you expect, but the weather is what you get."

Since climate is composed of long-term average weather patterns, it encompasses the average measurements of various meteorological elements like humidity, atmospheric pressure, wind, precipitation, and temperature. In addition to these components, Earth's climate is also composed of a system consisting of its atmosphere, oceans, land masses and topography, ice and biosphere. Each of these is a part of the climate system for their ability to influence long-range weather patterns. Ice, for example, is significant to climate because it has a high albedo, or is highly reflective, and covers 3% of the Earth's surface, therefore helping to reflect heat back into space.

Climate Record

Although an area's climate is normally a result of a 30-35 year average, scientists have been able to study past climate patterns for a large part of Earth's history through paleoclimatology. In order to study past climates, paleoclimatologists use evidence from ice sheets, tree rings, sediment samples, coral, and rocks to determine how much Earth's climate has changed through time.

With these studies, scientists have found that Earth has experienced various periods of stable climate patterns as well as periods of climate change.

Today, scientists determine the modern climate record through measurements taken via thermometers, barometers (an instrument measuring atmospheric pressure) and anemometers (an instrument measuring wind speed) over the past few centuries.

Climate Classification

Many scientists or climatologists studying Earth's past and modern climate record do so in an attempt to establish useful climate classification schemes. In the past, for example, climates were determined based on travel, regional knowledge, and latitude. An early attempt the classify Earth's climates was Aristotle's Temperate, Torrid and Frigid Zones. Today, climate classifications are based on the causes and effects of climate. A cause, for example, would be the relative frequency over time of a specific type of air mass over an area and the weather patterns it causes. A climate classification based on an effect would be one concerned with vegetation types present an area.

The Köppen System

The most widely used climate classification system in use today is the Köppen System, which was developed over a period from 1918 to 1936 by Vladimir Köppen. The Köppen System (map) classifies the Earth's climates based on natural vegetation types as well as the combination of temperature and precipitation.

In order to classify different regions based on these factors, Köppen used a multi-tiered classification system with letters ranging from A-E (chart). These categories are based on temperature and precipitation but generally line up based on latitude.

For example, a climate with a type A, is tropical and because of its characteristics, a climate type A is almost completely confined to the region between the equator and the Tropics of Cancer and Capricorn. The highest climate type in this scheme is polar and in these climates, all months have a temperature below 50°F (10°C).

In the Köppen System, the A-E climates are then subdivided into smaller zones which are represented by a second letter, which can then be further subdivided to show more detail. For A climates, for example, the second letters of f, m, and w indicate when or if a dry season occurs. Af climates have no dry season (such as in Singapore) while Am climates are monsoonal with a short dry season (as in Miami, Florida) and Aw has a distinctive long dry season (such as that of Mumbai).

The third letter in the Köppen classifications represents the temperature pattern of the area. For example, a climate classified as Cfb in the Köppen System would be mild, located on the marine west coast, and would experience mild weather throughout the year with no dry season and a warm summer. A city with a climate of Cfb is Melbourne, Australia.

Thornthwaite's Climate System

Although Köppen's System is the most widely used climate classification system, there are several others that have been used as well. One of the more popular of these is the climatologist and geographer C.W. Thornthwaite's system. This method monitors the soil water budget for an area based on evapotranspiration and considers that along with total precipitation used to support an area's vegetation over time. It also uses a humidity and aridity index to study an area's moisture based on temperature, rainfall and vegetation type. The moisture classifications in Thornthwaite's system are based on this index and the lower the index is, the drier an area is. Classifications range from hyper humid to arid.

Temperature is also considered in this system with descriptors ranging from microthermal (areas with low temperatures) to mega thermal (areas with high temperatures and high rainfall).

Climate Change

A major topic in climatology today is that of climate change which refers to the variation of Earth's global climate over time. Scientists have discovered that Earth has undergone several climate changes in the past which include various shifts from glacial periods or ice ages to warm, interglacial periods.

Today, climate change is mainly to describe the changes occurring in modern climate such as an increase in sea surface temperatures and global warming.

To learn more about climate and climate change, visit the collection of climate articles and climate change articles here on this site along with the National Oceanic and Atmospheric Administration's Climate website.