High School Earth Science/World Climates
A climate zone results from the climate conditions of an area: its temperature, humidity, amount and type of precipitation, and the season. A climate zone is reflected in a region's natural vegetation. Perceptive travelers can figure out which climate zone they are in by looking at the vegetation, even if the weather is unusual for the climate on that day!
- Describe the relationship between the climate zones and the factors that influence climate.
- Discuss the relationship between climate zones and biomes.
- Discuss the different biomes based on a general description.
Climate Zones and Biomes
The major factors that influence climate also determine the different climate zones. The same type of climate zone will be found at similar latitudes and in similar positions on nearly all continents, both in the Northern and Southern Hemispheres. The one exception to this pattern is the climate zones called the continental climates, which are not found at higher latitudes in the Southern Hemisphere. This is because the Southern Hemisphere land masses are not wide enough to produce a continental climate.
Climate zones are classified by the Köppen classification system (Figure 17.7). This system is based on the temperature, the amount of precipitation and the times of year when precipitation occurs. Since climate determines the type of vegetation that grows in an area, vegetation is used as an indicator of climate type. A climate type and its plants and animals make up a biome. The organisms of a particular biome share certain characteristics around the world, because their environment has similar advantages and challenges. The organisms have adapted to that environment in similar ways over time. For example, different species of cactus live on different continents, but they have adapted to the harsh desert in similar ways.
The Köppen classification system recognizes five major climate groups, each with a distinct capital letter A through E. Each lettered group is divided into subcategories. Some of these subcategories are forest (f), monsoon (m), and wet/dry (w) types, based on the amount of precipitation and season when that precipitation occurs.
Tropical Moist Climates (Group A)
Tropical Moist (Group A) climates are found in a band about 15° to 25° N and S of the equator (Figure 17.8). Intense sunshine means that the tropics are warm year-round: each month has an average temperature of at least 18°C (64°F). Rainfall is abundant—at least 150 cm (59 inches) per year. The subcategories of this zone are based on when the rain falls.
Tropical Wet (Af)
The wet tropics lie in a band around the equator, covering about 10% of the Earth's land. The wet tropics are consistently warm, with almost no annual temperature variation. Great amounts of rain fall year-round, between 175 and 250 cm (65 and 100 inches) per year. These conditions support the tropical rainforest biome (Figure 17.9). The forest is dominated by densely packed, broadleaf evergreen trees. Many habitats are found in rainforests, as a result of the high number of plant types and the different environments within the layers of the forest. Rainforests have the highest number of species or biodiversity of any ecosystem.
Tropical Monsoon (Am)
The tropical monsoon climate resembles the tropical wet biome (Af) but has very low precipitation for one to two months each year. During these months, less than 6 cm (2.4 inches) of rain falls. Rainforests can grow here because the dry period is short, and the trees can be supported by moisture trapped in the soil. This climate is found where the monsoon winds blow, primarily in southern Asia, western Africa, and northeastern South America.
Tropical Wet and Dry (Aw)
The tropical wet and dry climate lies north and south of the tropical wet climate, between about 5° and 10° latitude to around 15° to 20° latitude. The average annual temperature is similar to the wet tropics, but the temperature range is greater. This climate zone receives less rain than the wet tropics, about 100 to 150 cm (40 to 60 inches). For more than two months each year, rainfall is less than 6 cm (2.4 inches). Wet and dry seasons are related to the location of the ITCZ. In the summer, when the ITCZ drifts northward, the zone is wet. In the winter, when the ITCZ moves back toward the equator, the region is dry. This climate exists where strong monsoon winds blow from land to sea, such as in India.
Rainforests cannot survive the months of low rainfall, so the typical vegetation is savanna (Figure 17.10). This biome consists mostly of grasses, with widely scattered deciduous trees and rare areas of denser forests. Central Africa is famous for its savanna and the unique animals that live there.
Dry Climates (Group B)
The Dry Climates (Group B) generally have less precipitation than evaporation and are further from the equator than Tropical (Group A) climates. They have cooler winters and longer, harsher dry seasons. Rainfall is irregular; several years of drought are often followed by a single year of abundant rainfall. Summer temperatures are high, and much of the rain evaporates before it reaches the ground. Dry climate zones cover about 26% of the world's land area. Low latitude deserts form as a result of the Ferrell cell high pressure zone. Higher latitude deserts occur within continents or in rain shadows where the air has little humidity.
Arid Desert (Bw)
Low-latitude, arid deserts are found between 15° to 30° N and S latitudes. This is where warm dry air sinks at high pressure zones. True deserts make up around 12% of the world's lands. Deserts are found in southwestern North America, Africa, Australia and central Asia. Humidity is low, and as a result there are few clouds in the sky. The Sahara is the world's largest desert (Figure 17.11).
In the Sonora Desert of the southwestern United States and northern Mexico, most weather stations record sunshine 85% of the time, both in summer and winter (Figure 17.12). Clear skies allow the ground to heat rapidly during the day and cool rapidly at night. The summer sun can be merciless and daytime temperatures are extremely hot. Temperatures often plunge when the Sun goes down and the daily temperature range may be 15° to 25°C (27° to 45°F). Annual rainfall is mostly less than 25 cm (10 inches). The Sonora desert gets much of its rain in the winter, when storms come in from the Pacific. Summer monsoon rains drop a great deal of rain in some areas. The existence of two wet seasons a year allows a unique group of plants and animals to survive in the southwestern deserts.
Vegetation is limited, since water is scarce. Desert plants are adapted to surviving long periods of drought. Cacti and shrubby plants have wide or deep roots to reach water after a rain. Many desert plants are able to store water. Some plants lie dormant as seeds, and bloom after rain falls.
Semi-arid or Steppe (Bs)
Deserts in continental interiors or in rain shadows are found at higher latitudes. Semi-arid deserts often receive more rain than the arid deserts, between 20 to 40 cm (8 to 16 inches) annually. Because land areas change temperature easily, these areas have lower annual average temperatures plus greater annual temperature ranges. In the winter, these climate zones get very cold under cold high pressure cells. In the summer, they heat up, allowing air to rise and rain to fall.
In the United States, the Great Plains, portions of the southern California coast and the Great Basin are semi-arid deserts. The biome is called steppe, which has short bunch grass, and scattered low bushes, or sagebrush (Figure 17.13). A steppe has few or no trees because there is not enough rain for trees to grow.
Moist Subtropical Mid-latitude (Group C)
The Moist Subtropical Mid-latitude (Group C) climates are found along the coastal areas in the United States. Seasons are distinct, although winters are mild. The average temperature of the coldest month ranges from just below freezing to almost balmy, between -3°C and 18°C (27° to 64°F). Summers are often mild with average temperatures above 10°C (50°F). There is plentiful annual rainfall.
Dry Summer Subtropical or Mediterranean Climates (Cs)
The Dry Summer Subtropical climate is found on the western sides of continents between 30° and 45° latitude. Annual rainfall is 30 to 90 cm (14 to 35 inches), most of which comes in the winter. This climate is also called the Mediterranean climate because it is found around the Mediterranean Sea (Figure 17.14). The climate is also typical of coastal California, which sits beneath a summertime high pressure for about five months each year. Land and sea breezes make winters moderate and summers cool. The mild winters and foggy summers of San Francisco represent a coastal Mediterranean climate. Further inland in Sacramento, summer temperatures are much higher, representing an interior Mediterranean climate. Vegetation must survive long summer droughts. This vegetation type is called chaparral. Scrubby, woody plants and trees are common (Figure 17.15).
Humid Subtropical (Cfa)
The Humid Subtropical climate zone is found mostly on the eastern sides of continents (Figure 17.16). Rain falls throughout the year with annual averages between 80 and 165 cm (31 and 65 inches). Summer days are humid and hot, from the lower 30s up to 40°C (mid-80s up to 104°F). Afternoon and evening thunderstorms are common. These conditions are due to warm tropical air passing over the hot continent. Winters are mild, but middle-latitude storms called cyclones may bring snow and rain. The southeastern United States, with its hot humid summers and mild, but frosty winters, is typical of this climate zone.
Forests grow thickly in much of this region, due to the mild temperatures and high humidity. Pine forests are common in the lower latitudes, while oak are more common at higher latitudes (Figure 17.17).
Marine West Coast Climate (Cfb)
This climate lines western North America between 40° and 65° latitude, an area known as the Pacific Northwest (Figure 17.18). Ocean winds bring mild winters and cool summers. The temperature range between seasons and between day and night is fairly small. Rain falls year-round, although summers are drier as the jet stream moves northward. Low clouds, fog, and drizzle are typical. Snowfall is infrequent and short-lived. The mountain ranges that line the western U.S. keep this climate from extending far inland.
Dense forests of Douglas fir thrive in the heavy rain and mild temperatures (Figure 17.19). In Western Europe the climate covers a larger region since no high mountains are near the coast to block wind blowing off the Atlantic.
Continental Climates (Group D)
Continental (Group D) climates are found in most of the North American interior from about 40°N to 70°N. In this climate, summers are cool-to-warm and winters are very cold and stormy. The average temperature of the warmest month is higher than 10°C (50°F) and the coldest month is below -3°C (-27°F). Snowfall is common and the cold temperatures mean that snow stays on the ground for long periods of time. Trees grow in continental climates, even though winters are extremely cold, because the average annual temperature is fairly mild. Continental climates are not found in the Southern Hemisphere due to the absence of a continent large enough to generate this effect.
Humid Continental (Dfa, Dfb)
Humid continental climates are found around the polar front at about 60°N latitude within continental North America and Europe (Figure 17.20). In the winter, middle-latitude cyclones bring chilly temperatures and snow. In the summer, westerly winds bring continental weather and warm temperatures. The average July temperature is often above 20°C (70°F). The region is typified by deciduous trees, which protect themselves in winter by losing their leaves.
The two variations of this climate are based on summer temperatures. In the humid continental climate with long, hot summers (Dfa), the long summers and high humidity foster plant growth (Figure 17.21). Summer days may be over 38°C (100°F), nights are warm and the temperature range is large, perhaps as great as 31°C (56°F)! In the humid continental climate with long, cool summers (Dfb), summertime temperatures and humidity are lower. Winter temperatures are below -18°C (0°F) for long periods. For example, Winnipeg, Canada has a 38°C (68°F) annual temperature range.
The subpolar climate is found between the humid continental and the polar tundra climates (Figure 17.22). This climate is dominated by long, very cold winters with very little precipitation. Continental polar air masses form when air stagnates in this zone. Snowfall is light, but temperatures are so cold that snowfall remains on the ground for months. Most of the approximately 50 cm (20 inches) of annual precipitation falls during summer cyclonic storms. The angle of the Sun's rays is low but the Sun is visible in the sky for most or all of the day, so temperatures may get warm, but are rarely hot. These continental regions have very high annual temperature ranges. The climate of Fairbanks, Alaska is a typical subarctic climate.
The boreal coniferous forests of this climate are called taiga (Figure 17.23). These vast forests stretch across North America from western Alaska to Newfoundland, and across Eurasia from Norway to the Pacific coast.
Polar Climates (Group E)
In the polar regions, winters are entirely dark and bitterly cold. In the summer, days are long, but the sun is low on the horizon. Summers are cool with the average temperature of the warmest month at less than 10°C (50°F). Winters are extremely cold, so the annual temperature range is large. Polar climates receive less than 25 cm (10 inches) precipitation, mostly during the summer. This climate is found across the continents that border the Arctic Ocean, Greenland, and Antarctica.
Polar Tundra (ET)
The polar tundra climate is continental, with severe winters (Figure 17.24). Temperatures are so cold that a layer of permanently frozen ground, called permafrost forms below the surface. This frozen layer can extend hundreds of meters deep. The average temperature of the warmest months is above freezing, so summer temperatures defrost the uppermost portion of the permafrost. In winter, the permafrost prevents water from draining downward. In summer, the ground is swampy. Although the precipitation is low enough in many places to qualify as a desert, evaporation rates are also low, so the landscape receives more usable water than a desert.
The only plants that can survive the harsh winters and soggy summers are small ground-hugging plants like mosses, lichens, small shrubs and scattered small trees that make up the tundra (Figure 17.25). Due to the lack of ice-free land near the South Pole, there is very little tundra in the Southern Hemisphere. The area surrounding the Arctic Ocean is the only part of the globe with much tundra.
Ice Cap (EF)
Ice caps are found mostly on Greenland and Antarctica, about 9% of the Earth’s land area (Figure 17.26). Ice caps may be thousands of meters thick. Ice cap areas have extremely low average annual temperatures, e.g. -29°C (-20°F) at Eismitte, Greenland. Precipitation is low, since the air is too cold to hold much moisture. Snow occasionally falls in the summer.
When climate conditions in a small area are different from those of the surroundings, the climate of the small area is called a microclimate. The microclimate of a valley may be cool relative to its surroundings since cold air sinks. The ground surface may be hotter or colder than the air a few feet above it, since rock and soil gain and lose heat readily. Different sides of a mountain will have different microclimates. In the Northern Hemisphere, a south-facing slope receives more solar energy than a north-facing slope, and so each side supports different amounts and types of vegetation.
Altitude mimics latitude in climate zones. Climates and biomes typical of higher latitudes may be found in other areas of the world at high altitudes. Mt. Kilimanjaro in Africa lies near the equator in the tropics, but the top of the mountain is in the tundra biome and there is also a small glacier at the top.
- A climate zone depends on a region's latitude, continental position, and relationship to prevailing winds, large water bodies, and mountains, among other factors.
- The temperature, rainfall, length of dry season, and other features of the climate zone determine which plants can grow.
- When living organisms develop in similar climates, they must adapt to that same environment. So the organisms in similar climate zones resemble each other, no matter how geographically distant they are. Because the organisms are so similar, a climate zone and its organisms make up a biome.
- What is the Koppen Classification System used to study world climates?
- Why are most climate zones found in similar locations on continents within the Northern Hemisphere?
- What are some reasons that climate zones differ between continents, even though locations are similar?
- Why do organisms in the same biome often look the same even though they are not the same species? Think about desert plants, for example. Why are the plants that live in low latitude deserts on different continents so similar?
- Why is the length of the dry season important in distinguishing different types of climate zones? Give an example.
- Since the equator receives the most solar radiation over the course of a year, why are the hottest temperatures found in the low-latitude deserts? Why are low-latitude deserts often chilly at night, even in the summer?
- What are the differences between arid and semi-arid deserts?
- What conditions bring about the hot and humid summer days found in the American South?
- What is the most important factor in determining the presence of a forest?
- Look at Figure 17.7 and see which major climate types are found in California (ignore the 3rd letter on each symbol). Look at a geographic map at the same time if you need to. Which climate type is the most common and where is it found? Which other two types of climates are found and where? Why does California have so many major climate types?
- Polar regions receive little precipitation. Why are they not considered deserts?
- What is permafrost? Does it stay the same year-round?
- Why are microclimates important to living things?
- The number of species of plants, animals, and other organisms within a particular habitat.
- The living organisms that are found within a climate zone that make that zone distinct, such as rainforests, arid deserts, tundra, and ice caps.
- Scrubby woody plants and widely scattered trees typical of the Mediterranean climate.
- Areas receiving very little precipitation, less than 25 cm (10") per year; found in arid climates; plants are infrequent.
- Douglas fir
- A coniferous, evergreen tree found in enormous forests in western North America.
- ice cap
- Permanent ice that is found mostly around Greenland and Antarctica.
- Permanently frozen ground that is found in the polar regions, where temperatures do not rise above freezing most months.
- The tropical wet biome where temperatures are warm and rain falls nearly every day.
- The tropical wet and dry biome, typified by grasses and widely scattered deciduous trees.
- The biome found in semi-arid deserts, typified by bunch grasses, scattered low bushes and sagebrush.
- Vast, boreal forests of small, more widely spaced trees that are typical of the subpolar climate.
- The treeless area of the arctic with very cold, harsh winters. The only polar climate that contains living organisms.
Points to Consider
- Why aren't biomes always determined by latitude? What geographic features or other factors affect the climate?
- Climate zones and biomes depend on many climate features. If climate changes, which of these features changes too?
- If global warming is increasing average global temperatures, how would you expect biomes to be affected?