People often confuse weather and climate; the difference is really a matter of time. Climate is the long-term pattern of weather in a particular area. In contrast, 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.

More than 100 years ago, British geographer Andrew John Herbertson described the difference between climate and weather like this: "Climate is what we expect, weather is what we get."

Climate System

Different parts of the world have different climates. Between the icy poles and the steamy tropics are many climates that contribute to Earth’'s biodiversity and geologic past.

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

The hydrosphere corresponds to Earth's water. Its flow is affected by variations in temperature">temperature and salt levels. Changes in the hydrosphere occur at much slower rates than changes in the atmosphere.

The cryosphere is made up of ice sheets and glaciers, which reflect sunlight rather than absorb it. The thermal conductivity of ice and permafrost—which is to say the ease with which they transmit heat—profoundly influences temperature. The cryosphere also helps regulate thermohaline circulation, which is the movement of ocean currents.

Vegetation and topography">topography, the physical features of land, influence climate. They affect what happens to the sun's energy once it reaches Earth's surface. Types of land cover—such as soil, sand, or asphalt—impact evaporation and temperature, as does plant cover.

The biosphere—the sum total of living things on Earth—also influences climate. Through photosynthesis, plants help regulate the flow of gases in the atmosphere. Living organisms alter the landscape through both natural growth and buildings, such as mounds and dams. These altered landscapes can influence patterns of wind and temperature.

Climate Features

The most familiar features of a region's climate are probably average temperature and precipitation. Seasonal variations, or changes, also help determine climate.

Other climate features include windiness, humidity, cloud cover, atmospheric pressure and fogginess. Landscape, a region's elevation or height, and its distance from water can all impact climate.

All climates are the product of many factors, including elevation and latitude. For example, higher elevations are typically cooler than lower elevations. The location on a continent and distance from the ocean also affect climate.

The rainy, tropical climate of West Africa, for example, is influenced by the region's location near the equator, which is the line at 0 degrees latitude. Its position on the western side of the continent is also a significant factor. The area receives direct sunlight year-round, and is located where moist trade winds meet. As a result, the region's climate is warm and rainy.

Microclimates

No climate is the same across a whole region. Small variations, called microclimates, exist in every climate region. Microclimates are largely influenced by the presence of lakes, vegetation, and cities. In large urban areas, streets and buildings absorb heat from the sun. The absorbed heat energy raises the average temperature of the city higher than average temperatures of more open areas nearby. This is known as the “urban heat island effect.”

Köppen Classification System

The most popular system of classifying climates was proposed in 1900 by Wladimir Köppen, a Russian-German scientist. 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 classifying climate regions. The five climate groups they came up with are: tropical, dry, mild, continental, and polar.

Tropical: Wet

Places with a tropical wet climate are also known as rainforests. These regions are near the equator and have the most predictable weather on Earth. They have warm temperatures and heavy, regular rainfall, which exceeds 150 centimeters (59 inches) in a year. The coolest temperature is between 20° to 23° Celsius (68° and 73° Fahrenheit). Afternoon temperatures usually reach 30° to 33° Celsius (86° and 91° Fahrenheit). Rainforests experience very little seasonal change, meaning average monthly temperatures remain fairly constant throughout the year.

Tropical wet climates are in the area between 10° of latitude above and 10° below the equator. The U.S. state of Hawai'i and the city of Kuala Lumpur in Malaysia are areas with tropical wet climates.

Tropical: Monsoon

Tropical monsoon climates are mostly found in southern Asia and West Africa. A monsoon is a wind system that flips 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 rain to tropical monsoon regions. India and Bangladesh are famous for their monsoon climate patterns.

Tropical: Wet and Dry

Tropical wet and dry climates sit just near the equator. They create a special grassland ecosystem called savanna and have three seasons. One season is cool and dry—while another is hot and dry. The last season is hot and wet as the region experiences months of tropical wet climate.

During years when rains are light, people and animals suffer through drought">drought. Africa's vast Serengeti Plain is in the wet and dry tropics.

Dry: Arid and Semiarid

In the dry climate regions, precipitation is low. There are two dry climate types: arid and semiarid. Most arid climates receive 4 to 12 inches of rain each year, and semiarid climates receive enough to support wide 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 Death Valley National Park, California, reached 56.7° Celsius (134° Fahrenheit) on July 10, 1913. That was the highest weather temperature ever recorded.

Rainfall is limited in all dry climates, but there are some parts of the world that never see rain at all. One of the driest places on Earth is the Atacama Desert of Chile, on the west coast of South America.

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

Mild: Mediterranean

Regions with mild and continental climates are also called temperate regions. In these parts of the world, climate is influenced mostly by latitude and the region's position on the continent.

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 have clear skies, cool nights, and little rain.

Mild: Humid Subtropical

Humid subtropical climates are usually found on the eastern sides of continents. In cities like Savannah, Georgia, in the U.S., or Shanghai, China, summers are hot and humid. Winter can be very cold. Precipitation is about 76 to 165 centimeters (30-65 inches) in a year. Hurricanes and other violent storms are common in these regions.

Mild: Marine

Weather on both sides of a continent generally becomes cooler as it gets farther from the equator. The marine west coast climate, a type of mild climate typical of cities, such as Seattle, Washington, 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: Warm Summer

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

In autumn, 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.

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.

Continental: Cool Summer

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

Continental: Subarctic

North of regions with cool summer climates are regions with subarctic climates. These regions, including Siberia, a large part of Russia, experience very long, cold winters with little precipitation. Subarctic climates are also called boreal climates or taiga.

Polar: Tundra

The polar climates lie within the Arctic and Antarctic Circles near the North and South Poles. In tundra climates, summers are short, but there are many plants and animals. 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.

Polar: Ice Cap

Few organisms survive in the ice cap climates of the Arctic and Antarctic. Temperatures rarely rise above freezing, even in summer. The 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.

High Elevation Climates

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

There are two high elevation climate types: upland and highland. Upland areas are flat areas high above the sea. Highland areas are mountainous. They are both based on the fact that climbing a mountain or reaching a plateau can be like moving toward the poles. On some mountains, such as Mount Kilimanjaro in Tanzania, the climate is tropical at the base and polar at the top.

Influence of Climate

Earth has an enormous variety of life. This diversity is largely due to the variety of climates that exist and the climate changes that have occurred in the past.

Climate has also greatly influenced the development of cultures and civilizations. People have adapted in various ways to the many climates where they have settled. Climate affects the foods they eat, the homes they build and clothing they wear.

Climate Change

Climate does not change from day to day like weather, but it does vary and change over time. Climate changes happen slowly over hundreds or even thousands of years. For example, glacial periods have covered large portions of Earth with ice caps.

Climate change can happen for many reasons. 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. Climates around the world were changed for years.

Since the Industrial Revolution of the 19th century, human activity has been impacting climate. Humans began putting massive amounts of waste gases into the atmosphere.

Global warming is often associated with a runaway “greenhouse effect.” The greenhouse effect describes the process in which certain gases trap solar radiation in a planet's lower atmosphere. These gases include carbon dioxide (CO2), methane, and ozone. Greenhouse gases absorb some of the energy from sunlight and trap it as heat inside the atmosphere. In this way, these gases act like the glass walls of a greenhouse.

The greenhouse effect is a natural phenomenon. However, human activities that include burning fossil fuels and cutting down forests release greater amounts of greenhouse gases into the atmosphere. As a result, our planet's temperature has risen about 2° F since the late 19th century. Sixteen of the last 17 warmest years on record have occurred in the 21st century.

Most climate scientists say that we should reduce our greenhouse-gas emissions. Changing humanity's impact on the world's climate, though, requires long-range planning and commitment.