Chapter 13
Global Climate
Summary
This chapter examines the different climatic regions found on the earth. The primary
emphasis is on global scale climates, but micro- and macroscale climates are also mentioned
briefly. The chapter reviews the factors that affect and determine the climate of a particular
region. These climatic controls include seasonal and latitudinal variations in incident sunlight,
proximity to land or sea, ocean and wind currents, and topographical effects. Global distributions
of mean temperature and annual precipitation amounts are presented and form the basis for
climate classification using the Köppen system.
Each of the five major climatic types in the Köppen classification system are discussed in
detail. Examples of yearly temperature variations and monthly precipitation amounts are given for
a representative location in each group and for the major sub-categories. A tropical climate, for
example, is characterized by abundant rainfall and very little seasonal variation in mean
temperature. Seasonal changes are much larger at middle latitudes, and several different climate
zones can be identified depending on whether summers are warm or cool, dry or moist, and by
the severity of the winter. Arid zones are found on the earth in areas dominated by subtropical high
pressure systems or in the rain shadow of large mountain ranges.
Teaching Suggestions, Demonstrations and Visual Aids
1.
Initially, the variety of labels used for the major groups and subcategories in the Köppen
classification system may be confusing to students. Keep a map, such as Figure 13.5, displayed
throughout the discussion and use a specific city or region to illustrate each climatic zone.
Encourage students to learn the classification groups by understanding the differences
between them and the cause of those differences. Show how climate depends on latitude by
observing the changes that occur as one moves in a line from the equator toward the North Pole at
constant longitude. Then examine how climate is modified as one moves from west to east across
the United States at a single middle latitude. Show where features in the global circulation such as
the subtropical highs and the ITCZ are located at various times during the year and explain how
these affect climate.
2.
Present and discuss representative examples (plots of average temperature and monthly
precipitation totals) of each of the important climate types. List one or two of the key
characteristics that can be used to distinguish between the different climate classifications. Then
present some new data, but do not reveal the location where the data was obtained. Ask the
students how these data would be classified. Then ask the students where, within a particular
region such as the United States, these data might have been obtained. After a period of
discussion, reveal the actual location. This is also a good point to be sure that students are familiar
with United States and world geography.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 13: Global Climate
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Student Projects
1.
Have students collect and prepare a plot of yearly average temperature data and
average monthly precipitation totals for their city. How would their town be classified using the
Köppen system?
2.
Have students attempt to locate different macroscale climate regions in the city
where they live. The students will have to devise a system to be used to identify and classify
different climate zones. Students could, for example, compare average conditions in high- and
low-lying areas in their town, average conditions near and far from a body of water, or conditions
inside a city with conditions in a rural area nearby. Are there significant differences in macroscale
climates in their city? Are any differences reflected in the types of vegetation found at different
locations within the city.
3.
In some locations, students could identify and classify the different climatic zones found at
different altitudes on a nearby mountain range. Students could, for example, determine the
predominant vegetation types at different levels and attempt to relate this seasonal temperature
and precipitation variations.
4.
Students might examine how or whether parameters such as the primary agricultural
product in a region, energy consumption, population density or life expectancy depend on climate.
5.
Use the Temperature Trends section of the ThomsonNow web site to examine
estimates of future global temperatures. Using the slider bar, set the animation to 120
years into the future. Based on the climate model estimates of future conditions, do you
think the global pattern of climate will change substantially? In what way(s)?
6.
Choose three cities in three different continents, each in a different Köppen
climate classification. Using the Forecasting section of the ThomsonNow web site,
record the temperature in each of these cities for five consecutive days. Do the
temperatures conform to your expectations? Why or why not?
Answers to Questions for Review
1.
The general circulation of the atmosphere; distribution of mountain ranges and high
plateaus; temperature; passage of high and low pressure systems; proximity to semi-permanent
highs and lows; proximity to large open water sources.
2.
Winter: Pacific high moves south, giving storms access to the west coast of the U.S.,
Bermuda high moves south, carrying less moisture to the eastern half of the U.S. Summer: Pacific
high inhibits precipitation along the west coast, Bermuda high brings moisture to eastern two-thirds
of the U.S.
3.
Vegetation type.
4.
Tropical moist climate: All months have an average temperature above 18°C (64°F). Polar
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Instructor’s Manual
Chapter 13: Global Climate
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climate: Extremely cold winters and summers. The average temperature of the warmest month is
below 10°C (50°F).
5.
a. D – Moist climate with severe winter. b. A – Moist tropical climate. c. B – Dry climate.
6.
Potential evapotranspiration exceeds precipitation.
7.
Tropical rain forest: Tropical Moist Climate. Xerophytes: B – Dry climate. Steppe: B –
Dry climate. Taiga: D – Moist climates with severe winters. Tundra: E – Polar climates. Savanna:
A – Moist tropical climate.
8.
a. tropical wet and dry: In summer, the ITCZ moves poleward, bringing with it heavy
precipitation, usually in the form of showers. Rainfall is enhanced by slow moving shallow lows
that move through the region. Tropical wet-and-dry climates not only receive less total rainfall
than the tropical wet climates, but the rain that does occur is much less reliable, as the total rainfall
often fluctuates widely from one year to the next. In the course of a single year, for example,
destructive floods may be followed by serious droughts. As with tropical wet regions, the daily
range of temperature usually exceeds the annual range, but the climate here is much less
monotonous. There is a cool season in winter when the maximum temperature averages 30°C to
32°C (86°F to 90°F). At night, the low humidity and clear skies allow for rapid radiational cooling
and, by early morning, minimum temperatures drop to 20°C (68°F) or below. b. Mediterranean:
The extreme summer aridity of the Mediterranean climate, which in California may exist for five
months, is caused by the sinking air of the subtropical highs. In addition, these anticyclones divert
summer storm systems poleward. During the winter, when the subtropical highs move
equatorward, mid-latitude storms from the ocean frequent the region, bringing with them much
needed rainfall. Consequently, Mediterranean climates are characterized by mild, wet winters, and
mild-to-hot, dry summers. c. marine: During much of the year, marine climates are characterized
by low clouds, fog, and drizzle. The ocean’s influence produces adequate precipitation in all
months, with much of it falling as light or moderate rain associated with maritime polar air masses.
Snow does fall, but frequently it turns to slush after only a day or so. In some locations, topography
greatly enhances precipitation totals. d. humid subtropical: A trademark of the humid subtropical
climate is its hot, muggy summers. This sultry summer weather occurs because Cfa climates are
located on the western side of subtropical highs, where maritime tropical air from lower latitudes is
swept poleward into these regions. Generally, summer dew-point temperatures are high (often
exceeding 23°C, or 73°F) and so is the relative humidity, even during the middle of the day. The
high humidity combines with the high air temperature (usually above 32°C, or 90°F) to produce
more oppressive conditions than are found in equatorial regions. Summer morning low
temperatures often range between 21°C and 27°C (70°F and 81°F).Occasionally, a weak summer
cool front will bring temporary relief from the sweltering conditions. e. subpolar: The exceedingly
low temperatures of winter account for these areas being the primary source regions for continental
polar and arctic air masses. Extremely cold winters coupled with cool summers produce large
annual temperature ranges. Precipitation is comparatively light in the subpolar climates, especially
in the interior regions, with most places receiving less than 50 cm (20 in.) annually. A good
percentage of the precipitation falls when weak cyclonic storms move through the region in
summer. The total snowfall is usually not large but the cold air prevents melting, so snow stays on
the ground for months at a time. Because of the low temperatures, there is a low annual rate of
evaporation that ensures adequate moisture to support the boreal forests of conifers and birches
known as taiga. f. polar ice cap: It occupies the interior ice sheets of Greenland and Antarctica,
where the depth of ice in some places measures thousands of meters. In this region, temperatures
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are never much above freezing, even during the middle of “summer.” The coldest places in the
world are located here. Precipitation is extremely meager with many places receiving less than 10
cm (4 in.) annually. Most precipitation falls as snow during the “warmer” summer. Strong
downslope katabatic winds frequently whip the snow about, adding to the climate’s harshness.
9.
Because winds (outside the tropics) are typically westerly, giving the west coasts a marine
influence.
10.
D climates are controlled by a large continent, and continents typically experience a larger
annual temperature range as compared to oceans.
11.
There are fewer large continents in the southern hemisphere, and the largest southern
hemisphere land masses are in the tropics.
12.
The tropical wet-and-dry climate has a distinct dry season, whereas the tropical rain forest
(tropical wet) climate does not.
13.
Winters are colder in Dfa climates.
14.
Arid deserts are dominated by the sinking air of a subtropical high pressure system. This
factor prevents significant precipitation even if oceans are nearby.
15.
Köppen found that the average monthly temperature of 10°C tended to represent the
minimum temperature required for tree growth.
16.
The southern desert region of North America is dry because it is dominated by the
subtropical high most of the year, and winter storm systems tend to weaken before they move into
the area. The northern region is in the rain shadow of the Sierra Nevada. These regions are
deficient in precipitation all year long, with many stations receiving less than 13 cm (5 in.)
annually.
17.
In the polar tundra (ET), the average temperature of the warmest month is below 10°C
(50°F), but above freezing. The temperature, rather than the precipitation, is the defining factor of
this climate type.
18.
Because of the low temperatures, there is a low annual rate of evaporation that ensures
adequate moisture to support the boreal forests of conifers and birches known as taiga. Hence, the
subpolar climate is known also as a boreal climate and as a taiga climate.
Answers to Questions for Thought and Exploration
1.
Cities located east of the Rockies receive moisture from the Gulf of Mexico. The Rockies
effectively block Gulf moisture from reaching cities located to the east of the Sierra Nevada
Mountains. In addition, the Sierras shield the region east of them from Pacific moisture.
3.
The prevailing westerly winds at this latitude give Boston a continental-type climate.
Ahrens Essentials of Meteorology, 5th
Instructor’s Manual
Chapter 13: Global Climate
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4.
In polar regions, the ground is frozen except in summer when the upper part thaws. Hence,
the summer landscape often turns swampy. During the winter, warmth from a heated building
could melt the frozen ground beneath it. This could cause the structure to settle into the ground
unevenly. To prevent this, some structures are built upon pilings.
5. The ground in Cfa climate areas is often drier than the ground in areas with Af climates. Dry
ground warms up faster than wet ground, allowing surface temperatures to increase.
7. In arid desert climates. Here the temperatures are warm enough to support liquid precipitation,
but relative humidity is low enough to evaporate the rain before it reaches the surface.
8. San Francisco is closer to the Pacific Ocean and experiences a maritime climate. Sacramento is
inland, and air approaching Sacramento from the west must first cross a mountain range where it
may contain less moisture due to orographic lifting and precipitation.
9. The Rocky Mountains are very tall and are oriented perpendicular to the prevailing wind flow
(westerly). The Appalachians are shorter and are roughly parallel to the prevailing wind directions
(southerly and northerly).
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Instructor’s Manual
Chapter 13: Global Climate
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