All About Climate | National Geographic Society (2023)

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

micro

climates

, exist in every

climate

region

.

Micro

climates

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

micro

climates

. 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

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

rain

forests

. 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).

Rain

forests

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 semi

arid

. 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

semi

arid

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.

Semi

arid

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

semi

arid

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 sub

arctic

. 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 Na

poleon

Bonaparte’s soldiers, for example, were used to the mild

Mediterranean

climates

of France. Thou

sands

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

.

Sub

arctic

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 thou

sands

of years.

Ice Cap

Few organisms survive in the ice cap climates of the

Arctic

and Ant

arctic

.

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, Ant

arctica

, 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

, semi

arid

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

agri

culture

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 thou

sands

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 Ant

arctic

Peninsula. Warmer ocean

temperatures

and warmer

ambient

air

temperatures

likely contributed to the fracturing of the

ice shelf

and the massive Ant

arctic

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 semi

arid

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.