Earth

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Chapter 15 Earths Climates
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Climate refers to the statistical properties of
the atmosphere and is concerned with the
long-term behavior, or expected (typical)
conditions. The most widely used climatic
classification scheme is the Koeppen system based
on world distribution of natural vegetation
boundaries and the combinations of monthly mean
temperature and precipitation associated with
those boundaries.
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Climate types according to Koeppen.
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World map of Koeppen climates.
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The Tropical Climates consist of three
groups, each of which is warm year-round. The
tropical wet climate has significant
rainfall every month of the year, the tropical
wet and dry climate has a pronounced dry
season, and the monsoonal climate
undergoes relative dryness for 1 to 3 months but
receives sufficient moisture that vegetation need
not be adapted to seasonal drought.
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Tropical Wet (Af) climates have no dry period and
precipitation is almost always convectional with
strong solar heating of the surface
triggering brief but heavy thundershowers in the
mid to late afternoon.
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Monsoonal (Am) climates usually occur along
tropical, coastal areas subjected to predominant
onshore winds that supply warm, moist air to the
region throughout most of the year.
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Tropical Wet and Dry (Aw) often occur along the
poleward margins of the Tropics and border dry
climates on one side and tropical wet climates on
the other. They undergo much greater seasonality
in precipitation and temperature than do the
tropical wet and the monsoonal climates.
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Tropical wet and dry climates are associated with
the savanna. This vegetation consists mainly of
grasses interspersed with widely separated trees
or clumps of trees. The lack of forest results
from numerous factors, including recurrent fire,
waterlogged soils, and the development of hard
layers within the soil.
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The Dry Climates of the world can be divided by
the level of aridity and by their latitudinal
position. Semi-deserts are transitional zones
that separate the true deserts from adjacent
climates. They are also called steppe climates,
with reference to the associated vegetation type
consisting of short grasses. True deserts are so
dry that only sparse vegetation consisting
entirely of xerophytic species can take hold.
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Subtropical Deserts (BWh) tend to exist in the
subtropical regions, particularly within the
western portions of the continents as a result
of subsidence associated with the Hadley
circulation.
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Subtropical Steppe (BSh) are marked by aridity,
high year-to-year variations in precipitation,
extreme summer temperatures, large annual
and daily temperature ranges, and commonly border
the subtropical deserts.
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Mid-latitude Deserts (BWk) result from extreme
continentality in regions deep within continental
interiors or downwind of orographic barriers that
cut off the supply of moisture from the ocean.
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Mid-latitude Steppe (BSk) accounts for most of
the arid regions of western North America and has
the same temperature characteristics as the
mid-latitude deserts. The primary difference
between the two is the greater amount of
precipitation in the steppes.
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The Mild Mid-latitude climates are located in
parts of the latitude range between 30 and 60
in either hemisphere. The term mild refers to
the winter temperatures and not necessarily
those of the summer. Mediterranean climates can
be found along the west coasts between about 25
and 40 latitude. At about the same range of
latitude on the eastern side of continents are
the humid subtropical climates. The marine west
coast climates lie adjacent to and poleward of
the mediterranean climates.
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Mediterranean (Csa, Csb) climates are the only
ones that have a distinct summer dry season and a
concentration of precipitation in the winter.
Annual precipitation increases with latitude and
with elevation along windward slopes in
mediterranean climates.
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Humid subtropical (Cfa, Cwa) climates lie within
the lower middle latitudes of eastern North
America, South America, and Asia. They have a
distinct tropical feel during their long summers
and receive abundant precipitation.
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Marine West Coast (Cfb, Cfc) climates normally
occur poleward of mediterranean climates. Both
summers and winters are typically mild with low
annual temperature ranges and a wide range of
precipitation.
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The Severe Mid-latitude climate group includes
two climates, humid continental and subarctic,
both of which are marked by very cold winters.
These climates require large continental
areas within the high-middle latitudes between
about 40 and 70. Both of the severe
mid-latitude climates receive precipitation throug
hout the year and have no true dry season.
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Humid Continental (Dfa, Dfb, Dwa, Dwb) climates
are found between 40 N and 55 N in the eastern
parts of continents with warm and often hot
summers followed by cold winters. The coniferous
forest found here is referred to as the boreal
forest in North America and the taiga in Asia.
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Subarctic (Dfc, Dfd, Dwc, Dwd) climates occupy
the northernmost extent of the severe
mid-latitude regions. Summer temperatures are
lower than humid continental regions and winter
mean monthly temperatures can be extremely low.
Precipitation is greater in the summer than
winter.
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Polar Climates exist in the highest latitudes
poleward of about 70 at the high latitude
boundaries of the subarctic climates. The most
equatorward and milder of the two is the tundra.
At the most poleward regions of the globe lie the
true ice cap climates.
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Tundra (ET) climates are named for the associated
vegetation type that consists primarily of
low-growing mosses, lichens, and flowering
plants, with a few woody shrubs and trees.
Winters are severe with a perennially frozen
layer below the surface called permafrost.
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Polar Ice Cap (EF) areas exist where ice covers
the ground the entire year. The mean temperature
of the warmest month does not rise above 0
C. Most areas of ice cap receive little
precipitation because of the intense cold.
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Highland Climates (H) are governed by topography
and found in large mountain or plateau
areas. This group contains an extremely rich
collection of climates. In high mountains there
can be large changes in mean temperature over
short distances while mountain slopes can enhance
precipitation on their windward sides and
simultaneously create a rainshadow downwind.
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The next chapter examines climate changes past
and future.