12.1: Regional Climates

Controls on Climate

As mentioned earlier, climate is the average weather at a particular location over a long period, typically 30 years or more. Climate can change, but only over long periods of time. The climate of a region depends on its position relative to many things. These factors are described in the following sections. A useful mnemonic device for remembering all the major influences is LACEMOPS. These are some factors influencing the weather, but over a long period:

• Latitude: The primary factor influencing a region's climate is its latitude, as different latitudes receive varying amounts of solar radiation. As discussed in Chapter 2, the equator receives the most solar radiation. Days are equally long year-round, and the sun is just about directly overhead at midday. The polar regions, on the other hand, receive the least solar radiation. The night may last nearly six months during the winter, at the highest latitudes. Even in summer, the sun never rises very high in the sky. Sunlight filters through a thick wedge of the atmosphere, making it much less intense. The high albedo, because of ice and snow, reflects a good portion of the sun’s light. Thus, the equatorial regions of Earth will experience a warm climate year-round, whereas the higher latitudes will be much colder, with significant seasonal temperature variations.
• Air Masses: Recall from Chapter 8 that large bodies of air over some areas of the Earth take on the temperature and moisture characteristics of the surface over which they form. They often end up prescribing the region's climate.
• Continentality: When a particular location is near an ocean or large lake, the body of water significantly influences the region’s climate. In a maritime climate, temperatures vary relatively little from season to season and from day to day. For a location to have a true maritime climate, the winds must most frequently come off the sea. On the other hand, a continental climate is more extreme, with greater temperature differences between day and night and between summer and winter.
• Elevation: As altitude increases, temperature generally decreases because the air is thinner and the land is farther from the Earth's surface, which is a source of heat. Locations at higher elevations usually experience lower annual temperatures and drier conditions compared to their sea-level counterparts.
• Mountain barriers (orographic effect): Mountain ranges have two effects on the surrounding region's climate. The first is the rain shadow effect, which brings a warm, dry climate to the leeward side of a mountain range. The second effect mountains have on climate systems is the ability to separate coastal regions from the rest of the continent. Since a maritime air mass may have trouble rising over a mountain range, the coastal area will have a maritime climate, but the inland area on the leeward side will have a continental climate.
• Ocean Currents: The temperature of the offshore waters influences the temperature at a coastal location, particularly when winds blow off the sea. The cooler waters along the western United States are caused by a clockwise-rotating ocean current that brings cold water from the Arctic toward the equator. The climatic effect is that coastal regions of California, Oregon, and Washington are cool. Coastal upwelling also brings cold, deep water to the ocean surface off California, contributing to the cool coastal temperatures. In the Atlantic Ocean, the northern ocean current, called the Gulf Stream, brings warm water from the tropics to the southern states. This is a significant reason the southern states experience humid conditions in the summer and tornadoes: all this warm, moist air. The Gulf Stream also impacts Europe by bringing warm water northward, making this region warmer than expected.
• Pressure Systems: High- and low-pressure systems influence wind direction and atmospheric circulation. Recall from Chapter 7 that semi-permanent highs and lows persist in roughly the same locations worldwide. They affect the climate in those locations.
• Storm Tracks: The paths that weather systems, such as mid-latitude cyclones and tropical storms, tend to follow are called storm tracks. The location of these tracks is often influenced by prevailing winds, the jet stream, and high- and low-pressure systems.

Climate Zones and Biomes

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 determine which climate zone they are in by looking at the vegetation, even if the weather is unusual for that climate on that day.

The major factors that influence climate determine the different climate zones. In general, the same climate zone is found at similar latitudes and in similar positions across nearly all continents, in both the Northern and Southern Hemispheres. The one exception to this pattern is the continental climate, which is not found at higher latitudes in the Southern Hemisphere. This is because the landmasses of the Southern Hemisphere are not wide enough to support a continental climate.

The most common system for classifying climatic zones is the Köppen classification, as illustrated in Figure \(\PageIndex{1}\). This system is based on temperature, precipitation amounts, and the times of year when precipitation occurs. Since climate determines the types of vegetation that grow in an area, vegetation is used as an indicator of climate.

Figure \(\PageIndex{1}\): 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, such as forest (f), monsoon (m), and wet/dry (w), based on precipitation amount and the season in which it occurs. (CC BY 4.0; Beck et al., 2023 via Wikimedia Commons). Alternative description of the image.

A climate type, together with its plants and animals, makes up a biome. Organisms across biomes share specific characteristics worldwide because their environments offer similar advantages and challenges. The organisms have adapted to that environment in similar ways over time. For example, different cactus species live on different continents, but they have adapted to harsh desert conditions in similar ways.

Tropical Moist Climates (Group A)

These climates are found in a band about 15° to 25° N and S of the equator. What climate characteristics are the tropical moist climate group likely to have?

• Temperature: Intense sunshine; the average monthly temperature is at least 18°C (64°F).
• Rainfall: Abundant, at least 150 cm (59 inches) per year.

Figure \(\PageIndex{2}\): Mt. Kinabalu, Borneo, with the typical vegetation of tropical forests. (CC BY-SA 2.5; NepGrower via Wikimedia Commons)

The wet tropics have almost no annual temperature variation and year-round rainfall of 175-250 cm (65-100 inches). These conditions support the tropical rainforest biome. Densely packed, broadleaf evergreen trees dominate Tropical Rainforests (Af), as depicted in Figure \(\PageIndex{2}\). These rainforests have the highest biodiversity of any ecosystem.

The Tropical Monsoon (Am) climate has very low precipitation for 1 to 2 months each year. Rainforests grow here because the dry period is short, and the trees survive on soil moisture. This climate is found where the monsoon winds blow, primarily in southern Asia, western Africa, and northeastern South America.

The Tropical Wet and Dry (Aw) climate lies between about 5° and 20° latitude, around the Intertropical Convergence Zone (ITCZ). In the summer, when the ITCZ drifts northward, the zone is wet. In the winter, when the ITCZ moves 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. This biome consists mostly of grasses, with widely scattered deciduous trees and rare areas of denser forests.

Dry Climates (Group B)

These climates receive less precipitation than they lose through evaporation. Dry climate zones cover about 26% of the world’s land area. What climate characteristics are the dry climate group likely to have?

• Temperature: Abundant sunshine. Summer temperatures are high; winters are cooler and longer than in Tropical Moist climates
• Rainfall: Irregular; several years of drought are often followed by a single year of abundant rainfall

Figure \(\PageIndex{3}\): The Namib Desert in Southern Africa. (CC BY-SA 3.0; Martin Cígler via Wikimedia Commons)

Low-latitude, Arid Deserts (Bw) are found between 15° and 30° N and S latitudes, as depicted in Figure \(\PageIndex{3}\). This is where warm, dry air sinks, creating high-pressure zones, as a part of the Hadley Cycle. True deserts make up around 12% of the world’s land. In the Sonoran Desert of the southwestern United States and northern Mexico, skies are clear. The typical weather is extremely hot summer days and cold winter nights. Although annual rainfall is less than 25 cm (10 inches), rain falls during two seasons. Pacific storms bring winter rains, and monsoons bring summer rains. Since organisms do not have to go too many months without some rain, a unique group of plants and animals can survive in the Sonoran Desert.

Higher-latitude Semi-arid Deserts (Bs), also called steppe, are found in continental interiors or in rain shadows. Semi-arid deserts receive between 20 and 40 cm (8 to 16 inches) of rain annually. The annual temperature range is large. In the United States, the Great Plains, portions of the southern California coast, and the Great Basin are semi-arid deserts.

Moist Subtropical Mid-latitude (Group C)

A moist subtropical climate is a type of climate found within the temperate or subtropical regions of the Earth. What climate characteristics are the moist subtropical group likely to have?

• Temperature: The coldest month ranges from just below freezing to almost balmy, between -3°C and 18°C (27° to 64°F). Summers are mild with average temperatures above 10°C (50°F). Seasons are distinct.
• Rainfall: There is plentiful annual rainfall.

Figure \(\PageIndex{4}\): The sage and chaparral vegetation of coastal California. (CC BY-SA 2.0; Jeff Turner via Wikimedia Commons)

The Dry Summer Subtropical (Cs) 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. The climate is typical of coastal California, which sits beneath a summertime high-pressure system for about five months each year. Land and sea breezes make winters moderate and summers cool. Vegetation must survive long summer droughts. The scrubby, woody vegetation that thrives in this climate is called chaparral, as depicted in Figure \(\PageIndex{4}\).

The Humid Subtropical (Cfa) climate zone is found mostly on the eastern sides of continents. Rain falls throughout the year, with annual averages of 80-165 cm (31-65 inches). Summer days are humid and hot, ranging from the lower 30s to 40°C (mid-80s to 104°F). Afternoon and evening thunderstorms are common. These conditions are caused by 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.

The Marine West Coast (Cfb) climate zone extends across western North America between 40° and 65° latitude, an area known as the Pacific Northwest. Ocean winds bring mild winters and cool summers. The temperature range, both daily and annually, is fairly small. Rain falls year-round, although summers are drier as the jet stream moves northward. Low clouds, fog, and drizzle are typical. In Western Europe, the climate covers a larger region because there are no high mountains near the coast to block wind blowing off the Atlantic.

Humid Continental (Group D)

Continental climates are found in most of the North American interior from about 40°N to 70°N. What climate characteristics is the continental group most likely to have?

• Temperature: The average temperature of the warmest month is above 10°C (50°F), and the coldest month is below -3°C (-27°F).
• Precipitation: Winters are cold and stormy (look at the latitude of this zone and see if you can figure out why). Snowfall is common, and snow can stay on the ground for long periods.

Figure \(\PageIndex{5}\): Fall foliage near Hogback Mountain in Vermont. (CC BY-SA 4.0; chensiyuan via Wikimedia Commons)

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 because of the absence of a continent large enough to generate this effect.

The Humid Continental (Dfa, Dfb) climate is found along the polar front in North America and Europe. 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 in the fall, leading to vibrant fall colors as depicted in Figure \(\PageIndex{5}\).

The two variations of this climate are based on summer temperatures:

• Dfa, long, hot summers: 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). The long summers and high humidity foster plant growth.
• Dfb, long, cool summers: summer temperatures and humidity are lower. Winter temperatures are below -18°C (0°F) for long periods.

The Subpolar (Dfc) climate is dominated by continental polar air masses that cover the frigid continent. Snowfall is light, but cold temperatures keep snow 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 during the summer, so temperatures may get warm, but are rarely hot. These continental regions have extreme annual temperature ranges. The boreal conifer forests found in subpolar climates are called taiga and are characterized by small, hardy, widely spaced trees. Taiga vast forests stretch across Eurasia and North America.

Polar Climates (Group E)

Polar climates are found across the continents that border the Arctic Ocean, Greenland, and Antarctica. What climate characteristics is the polar climate group most likely to have?

• Temperature: Winters are entirely dark and bitterly cold. Summer days are long, but the sun is low on the horizon, so summers are cool. The average temperature of the warmest month is less than 10°C (50°F). The annual temperature range is extensive.
• Precipitation: The region is dry, with less than 25 cm (10 inches) of rainfall annually; most falls during the summer.

Figure \(\PageIndex{6}\): The shrubs from Denali National Park in Alaska. (Public Domain; Spruce Peak via National Park Service)

The Polar Tundra (ET) climate is continental, with severe winters. 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 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 would. Because of the lack of ice-free land near the South Pole, there is very little tundra in the Southern Hemisphere. The only plants that can survive the harsh winters and soggy summers are small, ground-hugging plants like mosses and lichens, small shrubs, and scattered small trees that make up the tundra, as depicted in Figure \(\PageIndex{6}\).

Ice caps (EC) are found mainly on Greenland and Antarctica, covering about 9% of the Earth’s land area. Ice caps may be thousands of meters thick. Ice cap areas have extremely low average annual temperatures, e.g., -29°C (-20°F) at Eismite, Greenland. Precipitation is low because the air is too cold to hold much moisture. Snow occasionally falls in the summer.