World Climates 5: Climate

Reason for Existence of Dry Climates

Dry climates exist as the result of the global distribution of air pressure and winds. In regions near the tropics of Cancer and Capricorn, the air is subsiding. When air sinks, it is compressed and warmed. Such conditions are the opposite of those needed for clouds to form precipitation. As a result, regions with dry climates experience mostly clear, sunny skies and dry climates. Other dry areas including the Great Basin in North America and the Gobi Desert of Eurasia occur where prevailing winds meet mountain barriers. These arid regions are called rain shadow deserts.

A dry climate is defined by yearly data for _____.

precipitation and evaporation

The Köppen climate classification system uses _____ to classify climates.

temperature and precipitation data

What is the main difference between wet tropical climates and tropical wet and dry climates?

the distribution of annual precipitation

Country X has a humid, tropical climate. Its weather in July is very warm and wet. What is its weather like in January?

warm temperatures like July, and possibly much drier

The Köppen Climate Classification System

Many classification systems have been used to group climates. Perhaps the best-known and most commonly used system is the Köppen climate classification system. The Köppen climate classification system uses mean monthly and annual values of temperature and precipitation to classify climates. This system is often used because it classifies the world into climatic regions in a realistic way

Subartic Climate

North of the humid continental climate and south of the tundra is an extensive subarctic climate region. This climate zone covers a broad expanse. Such climates stretch from western Alaska to Newfoundland in North America, and from Norway to the Pacific coast of Russia in Eurasia. Winters in these regions are long and bitterly cold. By contrast, summers in the subarctic are remarkably warm but very short. The extremely cold winters and relatively warm summers combine to produce the highest annual temperature ranges on Earth.

Humid Mid-Latitude With Mild Winters

As you will see in the figure on the next screen, there are three types of humid mid-latitude climates. Located between about 25° and 40° latitude on the eastern sides of the continents are the humid subtropical climates. The subtropical climate dominates the southeastern United States. In the summer, these areas experience hot, sultry weather since daytime temperatures are generally high. Although winters are mild, frosts are common in the higher-latitude areas. The temperature range is only 16 degrees and that the coldest temperatures occur from October through February.

Coastal Areas

Coastal areas between about 40° and 65° north and south latitude have marine west coast climates. Maritime air masses over these regions result in mild winters and cool summers with an ample amount of rainfall throughout the year. In North America, the marine west coast climate extends as a narrow belt from northernmost California into southern Alaska. The temperature range in this location is only 13 degrees. By comparison to the humid subtropical climate, the overall temperatures are lower.

Highland Climates

Highland climates are localized. This means that they are much different from climates in surrounding areas. Conditions of highland climates often vary abruptly from one place to another. For example, south-facing slopes are warmer than north-facing slopes, and air on the windward sides of mountains is wetter than air on the leeward sides. In general, highland climates are cooler and wetter than nearby areas at lower elevations.

Humid Mid-Latitude Climates

Humid mid-latitude climates include climates with mild winters as well as those with severe winters. Climates with mild winters have an average temperature in the coldest month that is below 18°C but above -3°C. Climates with severe winters have an average temperature in the coldest month that is below -3°C.

Humid Tropical Climates

Humid tropical climates are climates without winters. Every month in such a climate has a mean temperature above 18°C. The amount of precipitation can exceed 200 centimeters (cm) per year. There are two types of humid tropical climates: wet tropical climates and tropical wet and dry climates.

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Polar Climates

Polar climates are those in which the mean temperature of the warmest month is below 10°C. Polar winters are periods of perpetual night, or nearly so, making temperatures extremely cold. During the summer, temperatures remain cool despite the long hours of daylight. Very little precipitation falls in polar regions. There are two types of polar climates—tundra and ice cap. The tundra climate, shown in the figure, is a treeless region found almost exclusively in the Northern Hemisphere. The ice cap climate does not have a single monthly mean above 0°C. The landscape is covered by permanent ice and snow. Icecap climates occur in high mountain areas and in Greenland and Antarctica.

Tropical Wet and Dry

Refer again to the figure on the previous screen. Bordering the wet tropics are climates classified as tropical wet and dry climates. Tropical wet and dry climates have temperatures and total precipitation similar to those in the wet tropics, but experience distinct periods of low precipitation. Savannas, which are tropical grasslands with drought-resistant trees, are typical of tropical wet and dry climates. A savanna in Africa is shown in the figure.

Dry Summer Subtropical Climate

Regions with dry-summer subtropical climates are located between about 30° and 45° latitude. These climatic regions are unique because they are the only humid climate that has a strong winter rainfall maximum. In the United States, dry-summer subtropical climate is found only in California. It is sometimes referred to as a Mediterranean climate.

Reason for Existence of Dry Climates 2

Some of Earth's driest desert climates occur where a cold ocean current affects the west coast of a continent. A cold current cools the air above it. This strengthens the sinking of air in the warm, dry air masses that cause deserts. The Namib desert in Africa and the Atacama desert in Peru are examples of this type of extremely dry coastal desert.

Five Principal Groups

The Köppen system has five principal groups: humid tropical climates, dry climates, humid mid-latitude climates, polar climates, and highland climates. An example of a polar climate is shown in the photo. All of these groups, except climates classified as dry, are defined on the basis of temperature. Dry climates are classified partly according to the amount of precipitation that falls over an area. Each of the five major groups is further subdivided. See the figure on the next screen.

Wet Tropical

The tropical rain forest shown in the figure is typical of a wet tropical climate. Wet tropical climates have high temperatures and a great deal of annual precipitation. Why? Think about how latitude affects climate. The intensity of the sun's rays in the tropics is consistently high. Because the sun is directly overhead much of the time, changes in the length of daylight throughout the year are slight. The winds that blow over the tropics cause the warm, humid, unstable air to rise, cool, condense, and fall as precipitation.

Chicago and the Great Lakes

The winters are severe, while the summers are typically quite warm, precipitation is also generally greater in summer than in winter. The Great Lakes region of the United States, which includes Chicago, falls into the humid continental climate category. The winter temperatures for Chicago are below zero and that the temperature range is 26 degrees.

Arid and Semi-Arid climates

There are two types of dry climates: arid, or desert, and semi-arid, or steppe. Arid and semi-arid climates have many features in common. In fact, the difference between them is slight. The steppe is a marginal and more humid variant of the desert. The steppe represents a transition zone that surrounds the desert and separates it from humid climates.

Humid Mid-Latitude With Severe Winters

There are two types of humid mid-latitude climates with severe winters: the humid continental climates and the subarctic climates. Continental landmasses strongly influence both of these climates. As a result, such climates are absent in the Southern Hemisphere. There, oceans dominate the middle-latitude zone. There are regions that have a humid continental climate, areas with such climates lie between approximately 40° and 50° north latitude.

Compared with lower areas nearby, highland climates are generally _____.

cooler and wetter

Climates with mild winters and climates with severe winters describe the two main groups of _____.

humid mid-latitude climates

Which are three principal groups of climates as classified by the Köppen system?

humid tropical, humid mid-latitude, and polar

The range of annual temperatures is the greatest in which type of humid mid-latitude climate?

subarctic climates

Dry Climates

A dry climate is one in which the yearly precipitation is not as great as the potential loss of water by evaporation. In other words, dryness is not only related to annual rainfall, but is also a function of evaporation. Evaporation, in turn, is closely dependent upon temperature.