Climate is the long-term pattern of weather in a particular area. Weather can change from hour-to-hour, day-to-day, month-to-month or even year-to-year. A region’s weather patterns, usually tracked for at least 30 years, are considered its climate.

Climate System

Different parts of the world have different climates. Some parts of the world are hot and rainy nearly every day. They have a tropical wet climate. Others are cold and snow-covered most of the year. They have a polar climate. Between the icy poles and the steamy tropics are many other climates that contribute to Earth’s biodiversity and geologic heritage.

Climate is determined by a region’s climate system. A climate system has five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface, and the biosphere.

The atmosphere is the most variable part of the climate system. The composition and movement of gases surrounding the Earth can change radically, influenced by natural and human-made factors.

Changes to the hydrosphere, which include variations in temperature and salinity, occur at much slower rates than changes to the atmosphere.

The cryosphere is another generally consistent part of the climate system. Ice sheets and glaciers reflect sunlight, and the thermal conductivity of ice and permafrost profoundly influences temperature. The cryosphere also helps regulate thermohaline circulation. This “ocean conveyor belt” has an enormous influence on marine ecosystems and biodiversity.

Topography

Topography and vegetation influence climate by helping determine how the Sun’s energy is used on Earth. The abundance of plants and the type of land cover (such as soil, sand, or asphalt) impacts evaporation and ambient temperature.

The biosphere, the sum total of living things on Earth, profoundly influences climate. Through photosynthesis, plants help regulate the flow of greenhouse gases in the atmosphere. Forests and oceans serve as “carbon sinks” that have a cooling impact on climate. Living organisms alter the landscape, through both natural growth and created structures such as burrows, dams, and mounds. These altered landscapes can influence weather patterns such as wind, erosion, and even temperature.

Climate Features

The most familiar features of a region’s climate are probably average temperature and precipitation. Changes in day-to-day, day-to-night, and seasonal variations also help determine specific climates. For example, San Francisco, California, and Beijing, China, have similar yearly temperatures and precipitation. However, the daily and seasonal changes make San Francisco and Beijing very different. San Francisco’s winters are not much cooler than its summers, while Beijing is hot in summer and cold in winter. San Francisco’s summers are dry and its winters are wet. Wet and dry seasons are reversed in Beijing—it has rainy summers and dry winters.

Climate features also include windiness, humidity, cloud cover, atmospheric pressure, and fogginess. Latitude plays a huge factor in determining climate. Landscape can also help define regional climate. A region’s elevation, proximity to the ocean or freshwater, and land-use patterns can all impact climate.

All climates are the product of many factors, including latitude, elevation, topography, distance from the ocean, and location on a continent. The rainy, tropical climate of West Africa, for example, is influenced by the region’s location near the Equator (latitude) and its position on the western side of the continent. The area receives direct sunlight year-round, and sits at an area called the intertropical convergence zone (ITCZ, pronounced “itch”), where moist trade winds meet. As a result, the region’s climate is warm and rainy.

Microclimates

Of course, no climate is uniform. Small variations, called microclimates, exist in every climate region. Microclimates are largely influenced by topographic features such as lakes, vegetation, and cities. In large urban areas, for example, streets and buildings absorb heat from the Sun, raising the average temperature of the city higher than average temperatures of more open areas nearby. This is known as the “urban heat island effect.”

Large bodies of water, such as the Great Lakes in the United States and Canada, can also have microclimates. Cities on the southern side of Lake Ontario, for example, are cloudier and receive much more snow than cities on the northern shore. This “lake effect” is a result of cold winds blowing across warmer lake water.

Climate Classification

In 1948, American climatologist Charles Thornthwaite developed a climate classification system that scientists still use today. Thornthwaite’s system relies on a region’s water budget and potential evapotranspiration. Potential evapotranspiration describes the amount of water evaporated from a vegetated piece of land. Indices such as humidity and precipitation help determine a region’s moisture index. The lower its moisture index value, the more arid a region’s climate.

The major classifications in Thornthwaite’s climate classification are microthermal, mesothermal, and megathermal.

Microthermal climates are characterized by cold winters and low potential evapotranspiration. Most geographers apply the term exclusively to the northern latitudes of North America, Europe, and Asia. A microthermal climate may include the temperate climate of Boston, Massachusetts; the coniferous forests of southern Scandinavia; and the boreal ecosystem of northern Siberia.

Mesothermal regions have moderate climates. They are not cold enough to sustain a layer of winter snow, but are also not remain warm enough to support flowering plants (and, thus, evapotranspiration) all year. Mesothermal climates include the Mediterranean Basin, most of coastal Australia, and the Pampas region of South America.

Megathermal climates are hot and humid. These regions have a high moisture index and support rich vegetation all year. Megathermal climates include the Amazon Basin; many islands in Southeast Asia, such as New Guinea and the Philippines; and the Congo Basin in Africa.

Köppen Classification System

Although many climatologists think the Thornthwaite system is an efficient, rigorous way of classifying climate, it is complex and mapping it is difficult. The system is rarely used outside scientific publishing.

The most popular system of classifying climates was proposed in 1900 by Russian-German scientist Wladimir Köppen. Köppen observed that the type of vegetation in a region depended largely on climate. Studying vegetation, temperature, and precipitation data, he and other scientists developed a system for naming climate regions.

According to the Köppen climate classification system, there are five climate groups: tropical, dry, mild, continental, and polar. These climate groups are further divided into climate types. The following list shows the climate groups and their types:

Tropical

• Wet (rainforest)
• Monsoon
• Wet and dry (savanna)

Dry

• Arid
• Semiarid

Mild

• Mediterranean
• Humid subtropical
• Marine

Continental

• Warm summer
• Cool summer
• Subarctic (boreal)

Polar

• Tundra
• Ice cap

Tropical Climates

There are three climate types in the tropical group: tropical wet; tropical monsoon; and tropical wet and dry.

Tropical Wet: Rainforests

Places with a tropical wet climate are also known as rainforests. These equatorial regions have the most predictable weather on Earth, with warm temperatures and regular rainfall. Annual rainfall exceeds 150 centimeters (59 inches), and the temperature varies more during a day than it does over a year. The coolest temperatures, about 20° to 23° Celsius (68°-73° Fahrenheit), occurs just before dawn. Afternoon temperatures usually reach 30° to 33° Celsius (86°-91° Fahrenheit). Rainforests experience very little seasonal change, meaning average monthly temperatures remain fairly constant throughout the year.

Tropical wet climates exist in a band extending about 10° of latitude on either side of the Equator. This part of the globe is always under the influence of the intertropical convergence zone. The ITCZ follows a pendulum-like path during the course of a year, moving back and forth across the Equator with the seasons. It moves north during summer in the Northern Hemisphere, and south during the northern winter.

Some tropical wet climates are wet throughout the year. Others experience more rainfall during the summer or winter, but they never have especially dry seasons. The U.S. state of Hawaii; Kuala Lumpur, Malaysia; and Belém, Brazil, are examples of areas with tropical wet climates.

Tropical Monsoon

Tropical monsoon climates are most found in southern Asia and West Africa. A monsoon is a wind system that reverses its direction every six months. Monsoons usually flow from sea to land in the summer, and from land to sea in the winter.

Summer monsoons bring large amounts of rainfall to tropical monsoon regions. People living in these regions depend on the seasonal rains to bring water to their crops. India and Bangladesh are famous for their monsoon climate patterns.

Tropical Wet and Dry: Savanna

Tropical wet and dry climates are sometimes called “savanna” climates after the grassland ecosystem defined by wet and dry periods.

Tropical wet and dry climates sit just outside the ITCZ, near the Equator. They have three seasons. One season is cool and dry—when the warm, moist ITCZ is in the opposite hemisphere. Another season is hot and dry as the ITCZ approaches. The last season is hot and wet as the ITCZ arrives and the region experiences months as a tropical wet climate.

Life in these tropical wet and dry regions depends on the wet season’s rains. During years when rains are light, people and animals suffer through drought. During especially rainy years, regions may experience flooding. Havana, Cuba; Kolkata, India; and Africa’s vast Serengeti Plain are in the wet and dry tropics.

Dry Climates

Regions lying within the dry climate group occur where precipitation is low. There are two dry climate types: arid and semiarid. Most arid climates receive 10 to 30 centimeters (four to 12 inches) of rain each year, and semiarid climates receive enough to support extensive grasslands.

Temperatures in both arid and semiarid climates show large daily and seasonal variations. The hottest spots in the world are in arid climates. The temperature in the arid Death Valley National Park, California, U.S., reached 56.7° Celsius (134° Fahrenheit) on July 10, 1913—the highest temperature ever recorded.

Although rainfall is limited in all dry climates, there are a few parts of the world where it never rains. One of the driest places on Earth is the Atacama Desert of Chile, on the west coast of South America. Stretches of the Atacama may have never received rain in recorded history.

Semiarid regions, such as the Australian outback, usually receive between 25 and 50 centimeters (10-20 inches) of rainfall every year. They are often located between arid and tropical climate regions.

Arid and semiarid climates can occur where the movement of warm, moist air is blocked by mountains. Denver, Colorado, just east of the U.S. section of the Rocky Mountains, has this type of dry climate, known as a “rain shadow.”

Mild Climates

Regions with mild and continental climates are also called temperate regions. Both climate types have distinct cold seasons. In these parts of the world, climate is influenced mostly by latitude and a region’s position on the continent.

Mediterranean

Mediterranean climates have warm summers and short, mild, rainy winters. Mediterranean climates are found on the west coasts of continents between 30° and 40° latitude, and along the shores of the Mediterranean Sea.

Mediterranean summers feature clear skies, cool nights, and little rain.

Humid Subtropical

Humid subtropical climates are usually found on the eastern sides of continents. In cities such as Savannah, Georgia, in the U.S.; Shanghai, China; and Sydney, Australia, summers are hot and humid. Winter can be severely cold. Precipitation is spread evenly through the year and totals 76 to 165 centimeters (30-65 inches). Hurricanes and other violent storms are common in these regions.

Marine West Coast

Weather on both sides of a continent generally becomes cooler as latitude increases.

The marine west coast climate, a type of mild climate typical of cities such as Seattle, Washington, in the U.S. and Wellington, New Zealand, has a longer, cooler winter than the Mediterranean climate. Drizzle falls about two-thirds of winter days, and temperatures average about 5° Celsius (41° Fahrenheit).

Continental Climates

Areas with continental climates have colder winters, longer-lasting snow, and shorter growing seasons. They are the transition zones between mild and polar climates. Continental climates experience extreme seasonal changes.

The range of weather in continental climate regions makes them among the most spectacular sites for weather phenomena. In autumn, for instance, vast forests put on their annual show of brilliant color before shedding their leaves as winter approaches. Thunderstorms and tornadoes, among the most powerful forces in nature, form mostly in continental climates.

There are three types of continental climate—warm summer, cool summer, and subarctic. All these climates exist only in the Northern Hemisphere. Usually, continental climates are found in the interior of continents.

Warm Summer

Warm summer climate regions often have wet summer seasons, similar to monsoon climates. For this reason, this climate type is also called humid continental. Most of Eastern Europe, including Romania and Georgia, has warm summer climates.

Cool Summer

Cool summer climates have winters with low temperatures and snow. Cold winds, sweeping in from the Arctic, dominate winter weather.

People living in these climates have grown accustomed to the harsh weather, but those unprepared for such cold may suffer. Many of French Emperor Napoleon Bonaparte’s soldiers, for example, were used to the mild Mediterranean climates of France. Thousands died in bitter cold as they retreated from Russia’s cool summer climate in the winter of 1812.

Subarctic

North of regions with cool summer climates are regions with subarctic climates. These regions, including northern Scandinavia and Siberia, experience very long, cold winters with little precipitation. Subarctic climates are also called boreal climates or taiga.

Polar Climates

The two polar climate types, tundra and ice cap, lie within the Arctic and Antarctic Circles near the North and South Poles.

Tundra

In tundra climates, summers are short, but plants and animals are plentiful. Temperatures can average as high as 10° Celsius (50° Fahrenheit) in July. Wildflowers dot the landscape, and flocks of migratory birds feed on insects and fish. Whales feed on microscopic creatures in the region’s cold, nutrient-rich waters. People have adapted to life on the tundra for thousands of years.

Ice Cap

Few organisms survive in the ice cap climates of the Arctic and Antarctic. Temperatures rarely rise above freezing, even in summer. The ever-present ice helps keep the weather cold by reflecting most of the Sun’s energy back into the atmosphere. Skies are mostly clear and precipitation is low. In fact, Antarctica, covered by an ice cap 1.6-kilometers (one-mile) thick, is one of the largest, driest deserts on Earth.

High Elevation Climates

Many geographers and climatologists have modified the Köppen classification system over the years, including geographer Glen Trewartha, who added a category for high-elevation climates.

There are two high elevation climate types: upland and highland. Both highland and upland climates are marked by very different temperatures and levels of precipitation. Climbing a lofty mountain or reaching a plateau can be like moving toward the poles. On some mountains, such as Mount Kilimanjaro, Tanzania, the climate is tropical at the base and polar at the summit. Often, high-elevation climate differs from one side of the mountain to the other.

Influence of Climate

The enormous variety of life on Earth is largely due to the variety of climates that exist and the climate changes that have occurred in the past.

Climate has influenced the development of cultures and civilizations. People everywhere have adapted in various ways to the climates in which they live.

Clothing

Clothing, for example, is influenced by climate. Indigenous Arctic cultures of Europe, Asia, and North America, for example, developed warm, durable, fur and animal-skin clothing. This clothing was necessary for survival in the icy climate near the North Pole. Many parkas worn by Arctic peoples are not only insulated, but waterproof. This combats both the frigid temperatures and precipitation found in polar climates.

Lightweight, papery tapa cloth, on the other hand, is part of many cultures in the warm, humid climates of Polynesia, in the South Pacific. Tapa cloth was traditionally made from dried leaves, coconut fibers, and breadfruit bark. Tapa cloth is delicate and loses strength when wet, which would be deadly near the poles but only inconvenient near the Equator.

Shelter

Climate also influences how civilizations construct housing. For instance, the ancient Anasazi people of southern North America built apartments into tall cliffs. The sheltered, shady area kept residents cool in the hot, dry desert climate.

The yurt is a part of the identity of many cultures across the windy, semiarid steppe of Central Asia. Yurts are a type of original “mobile home,” a portable, circular dwelling made of a lattice of flexible poles and covered in felt or other fabric. Yurts protect residents from fierce winds, and their portability makes them an ideal structure for nomadic and seminomadic herding cultures on the grassland.

Agriculture

The development of agriculture was very dependent on climate. Ancient agricultural civilizations, such as those in Mesopotamia and India, flourished where the climate was mild. Communities could grow crops every season, and experiment with different types of crops, livestock, and farming techniques.

The mild, Mediterranean climate in which the Roman Empire developed, for instance, allowed farmers to cultivate crops, such as wheat, olives, grapes, barley, and figs. Livestock included cattle, sheep, goats, pigs, and even honeybees.

Like the ancient Romans, ancient cultures of the Amazon Basin in South America were also able to develop agricultural practices. The chief domesticated trees in the Amazon were mostly harvested for food and medicine: Brazil nuts, Inga ynga fruit (commonly known as “ice-cream beans”), Amazon tree grapes, abiu (another tropical fruit), and cacao fruits (whose seeds are known as cocoa beans).

Today, farmers are still in tune with the climate. They plant certain crops according to the expected amount of rainfall and the length of the growing season. When the weather does not follow the typical climate pattern, it can mean hard times for farmers and higher food costs for consumers.

Climate Change

Climate does not change from day to day like weather, but it does change over time. The study of historic climate change is called paleoclimatology.

Climate changes happen slowly over hundreds or even thousands of years. For example, periodic glacial periods have covered large portions of Earth with ice caps. Some paleoclimatology evidence shows that the Sahara Desert was once covered by plants and lakes during a warm “wet age.”

Climate change can happen for many reasons. The movement of tectonic plates, volcanic activity, and the tilt of Earth’s axis all have effects on climate. For example, after the eruption of the island volcano of Krakatoa, Indonesia, in 1883, winters and even summers in Asia and Europe were colder and darker. Volcanic ash blocked the sun. Farmers had to adjust to shorter, weaker growing seasons. Climates around the world were changed for years.

The so-called “Little Ice Age” was a period of climate change extending from the 12th through the 19th centuries. The Little Ice Age was not a true glacial period, but describes colder climates around the world. In Europe, canals in Great Britain and the Netherlands were often frozen solid, allowing for ice skating. In North America, European colonists reported especially harsh winters.

Global Warming

Since the Industrial Revolution of the 19th century, human activity has begun to impact climate. The current period of climate change is sometimes called “global warming.”

Global warming is often associated with a runaway “greenhouse effect.” The greenhouse effect describes the process of certain gases (including carbon dioxide (CO2), methane, nitrous oxide (N2O), fluorinated gases, and ozone) trapping solar radiation in a planet's lower atmosphere. Greenhouse gases let the sun’s light shine onto Earth’s surface, but they trap the heat that reflects back up into the atmosphere. In this way, they act like the glass walls of a greenhouse.

The greenhouse effect is a natural phenomenon and keeps Earth warm enough to sustain life. However, human activities that include burning fossil fuels and cutting down forests release greenhouse gases into the atmosphere at an unprecedented rate.

The current period of climate change has been documented by rising temperatures, melting glaciers, and more intense weather phenomena.

Our planet’s temperature has risen about 1.1° C (2° F) since the late 19th century. Sixteen of the last 17 warmest years on record have occurred in the 21st century. According to NASA, not only was 2016 the warmest year on record, but eight of the 12 months that make up the year were the warmest on record for those respective months.

The current period of climate change is also associated with the massive retreat of glaciers, ice sheets, and sea ice. Warmer temperatures have reduced the number of glaciers of Montana’s Glacier National Park from 150 in 1850 to just 26 today. In 2017, one of the largest icebergs ever recorded entered the ocean as a huge chunk of the Larsen C ice shelf broke off the Antarctic Peninsula. Warmer ocean temperatures and warmer ambient air temperatures likely contributed to the fracturing of the ice shelf and the massive Antarctic ice sheet associated with it. Finally, both the extent and thickness of Arctic sea ice has declined rapidly during the past several decades. The famed Northwest Passage, the treacherous route connecting the North Atlantic and North Pacific ocean basins, is now habitually free of ice and safe enough for cruise ships to navigate.

Melting glaciers and ice sheets, as well as expansion of seawater as it warms, have contributed to unprecedented sea level rise. Sea level rises at about 2.3 millimeters (0.2 inch) every year, contributing to up to 900% more frequent flooding in coastal areas.

Increasing temperatures can change the climate impacts and even the classification of a region. For instance, low-lying islands may be flooded as seawater rises. The populations of island nations, such as Maldives or Comoros, have been forced to contemplate becoming “climate refugees”—people forced to leave their homes and migrate to a different region.

Heat in the atmosphere may increase the interaction of diverse weather systems. Unusually arid climates in a semiarid region may prolong droughts, for instance. In regions with mild climates, the increased atmospheric moisture associated with humid climates may increase the likelihood of hurricanes and typhoons.

Climate change is also impacting organisms and species range. Organisms that have adapted to one climate may have to migrate or adapt to warmer temperatures. Manatees, for instance, are marine mammals native to tropical waters. As temperatures increase, manatees have been migrating as far north as New York City, New York. Polar bear populations, on the other hand, are venturing farther south as Arctic sea ice becomes more scarce.

Climate change can be mitigated through reducing greenhouse-gas emissions. This can mean investing in new technologies, relying more on renewable energy sources, making older equipment more energy-efficient, or changing consumer behavior.