Download Ch 18 Global Climate

Ch 18 Global Climate
Scales of global climate
• Recall that climate is loosely defined as "average
weather"
• However, you need to be careful of what geographic
area you are talking about
• Need to define climate on different spatial scales:
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microclimate - near the ground over your front yard
mesoclimate - over a field or few fields (a few square kilometers)
macroclimate - scale of a state or country
global climate - over entire earth
• We will focus on large-scale climate
• Indeed, climate extremes exist on the planet, so what
controls climate?
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Climate Controls
• Intensity of solar radiation and its variation with
latitude
• Distribution of land and water
• Ocean currents
• Prevailing winds
• Positions of semi-permanent high and low
pressure systems
• Mountain barriers
• Altitude
Distribution of global temperatures
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Temps have been reduced to sea level
Latitudinal change
N.H. - difference between land and ocean
U.S. west coast (upwelling and cold current) v.s. U.S.
east coast (gulf stream)
• Influence of gulf stream in north Atlantic
• highest temps are in the subtropical N.H. desert regions
• West coast of S. America is cool while the east coast is
warm, due to the ocean currents
• Much less variability
in the zonal direction
in the S. H.
• Coldest mean temps
are over Antarctica,
yet the figure to the
right suggests it is
over the arctic.....?
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Extreme climates
• Highest temperature ever recorded in North America -- 57°C (134°F) at
Greenland Ranch in Death Valley, CA
– average max temp in July in Death Valley is 116°F!!
• Hottest urban area is Yuma AZ
– high temps average 108°F in July
• Hottest place in the world - Dallol Ethiopia - located south of the Red Sea
– average annual temperature between 1960-1996 was 94°F !
• One of the coldest spots in U.S. - International Fall, MN
– Average temperature in January is 3°F
• Longest cold streak - Langdon, ND - winter of 1936
– temp remained below 0°F for 41 consecutive days
• Coldest temperature reading in US - Rogers Pass, Montana
– -70°F on 20 Jan, 1954
• Coldest areas of the US:
– Yukon and the Northwest Territories of Canada
• Coldest areas in N.H.:
– Siberia and Greenland
• Coldest temperature ever recorded anywhere in the world:
– -129°F at Vostok, Antarctica
• Global precipitation
distribution is tied to the
general circulation...
• Tropics are wet
– abundant moisture
and ITCZ
• Subtropics are
influenced by subtropical
highs
• Mid latitudes
– wet due to polar front
– dry on lee-side of
mountains
– wet on upslope side
of mountains
• Polar regions tend to be
dry
– little moisture
– polar high
Distribution of
global precipitation
A vertical cross section along a line running
north to south illustrates the main global
regions of rising and sinking air and how each
region influences precipitation.
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Distribution of Global Precipitation
Climate Classification - The Koppen System
• Widely used system that classifies climatic regimes according
to:
– annual and monthly averages of temperature
– annual and monthly averages of precipitation
• Devised by Waldimir Koppen (1846-1940), but modified and
refined.
• identified five primary climate types:
– Tropical moist climates: all months have an average temperature
above 18°C (64°F) - no real winter season
– dry climates: deficient precipitation most of the year. Potential
evaporation and transpiration exceed precipitation.
– moist mid-latitude climates with mild winters: warm-to-hot summers
with mild winters. The average temperature of the coldest month is
below 18°C (64°F) and above -3°C (27°F)
– moist mid-latitude climates with severe winters: warm summers and
cold winters. The average temperature of the warmest month exceeds
10°C (50°F) and the coldest monthly average drops below -3°C (27°F)
– Polar climates: extremely cold winters and summers. The average
temperature of the warmest month is below 10°C (50°F). There is no
real summer season.
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Climate Classification
• The Koppen system has been modified, most notably
by C. Warren Thornthwaite (1899-1963)
• Thornthwaite emphasized the importance of
precipitation (P) and evaporation (E) by devising the
P/E ratio.....
• The P/E ratio is monthly average precipitation divided
by monthly average evaporation.
• Five major humidity provinces and their characteristic
vegetations: rain forest, forest, grassland, steppe,
and desert.
• Potential evapotranspiration (PE)Æ the amount of
moisture that would be lost from the soil and
vegetation if the moisture were available.
Worldwide
distribution of
climatic regions (after
Köppen).
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Worldwide
distribution of
climatic regions
(after Köppen).
Tropical Moist Climates
• General Characteristics - year-round warm temperatures (all months
have mean temperatures above 18°C [64°F]) and abundant rainfall
(typical annual average exceeds 150 cm [59"])
• Extent - 15°-25° north and south of the equator
• Major types:
1. Tropical wet (Af)
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small seasonal temperature variations (less than 3°C)
solar radiation is constant year round
the diurnal temperature variation is larger than the seasonal variation!
moist, convective showers most every afternoon
example at Iquitos, Peru
2. Tropical monsoon (Am)
– similar to Af, with the exception that monthly average precipitation falls
below 6 cm for a month or two - not enough to affect the rain forest,
however.
3. Tropical wet and dry (Aw)
– located just pole ward of the tropical wet regions
– have a distinct dry season jungle areas transforms into savanna
grasslands
– obtain same amount of precip as Af, rain rates fluctuate greatly during
the year and from year to year
– example at Timbo, Guinea (11°N)
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Tropical wet (Af)
Tropical rain forest near Iquitos,
Peru.
Temperature and precipitation
data for Iquitos, Peru, latitude
4°S.
Tropical wet and dry (Aw)
Baobob and acacia trees
illustrate typical trees of
the East African grassland
savanna, a region with a
tropical wet-and-dry
climate (Aw).
Climatic data for Timbo,
Guinea, latitude 11°N.
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Dry Climates
• General Characteristics - deficient precipitation most of the year;
potential evaporation and transpiration exceed precipitation
• Occupies 26% of total land area - more than any other climate type
• evaporation > precipitation
• Extent - the subtropical deserts extend from roughly 20°-30° latitude in
large continental regions of the middle latitudes, often surrounded by
mountains.
• Major types: 1. arid (BW) - the "true desert"
– further subdivided into BWh (hot) and BWk (cold),12% of worlds land mass
– created by subsidence associated with semi-permanent highs or subsidence
on lee side of mountain ranges
– BW has highest temps in the world, winters can be somewhat moderate min temps in the 20s
– example of Phoenix AZ
2. semi-arid (BS)
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further subdivided into BSh (hot) and BSk (cold), also called Steppe climate
short bunch grass, low bushes, sagebrush
similar to BW, but receives more rainfall, found on edges of BW regions
example of Denver, CO
Hot arid BWh
Creosote bushes and
cacti are typical of the
vegetation found in the
arid southwestern
American deserts
(BWh).
Climatic data for Phoenix, Arizona,
latitude 33.5°N. A station with an
arid climate (BWh).
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Semi-arid (BS)
Cumulus clouds forming over the
steppe grasslands of western
North America, a region with a
semi-arid climate (BS).
Climatic data for Denver,
Colorado, latitude 40°N. A station
with a semi-arid climate (BSk).
Moist Subtropical Mid-latitude Climates
• General Characteristics - humid with mild winters (average temperature of
the coldest month below 18°C (64°F) and above -3°C 27°F)); have distinct
summers and winters and ample precipitation
• Extent - on the eastern and western regions of most continents, from about
25°-40° latitude
• Major types:
1. humid subtropical (Cfa)
– largely found along east coasts of continent, hot, muggy summers -- usually on
western side of subtropical highs
– mild winters, some snow, frost, ice, though heavy snows are rare
– afternoon convection is common in summer, tropical storms
– example from mobile, Alabama
2. marine (Cfb)
– extend to 40°-60° latitude on the western sides of continents
– prevailing westerlies moderate the climate near the coast, compared to the same
latitude further inland
– relatively mild winters, cool summers; lots of low clouds, fog, drizzle, especially
during fall, winter, and spring; little snow, except at higher altitudes in the
mountains
– example for Port Hardy, Vancouver, Island
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Humid subtropical
climate (Cfa)
Climatic data for Mobile,
Alabama, latitude 30°N
Marine climate (Cfb)
Climatic data for Port Hardy,
Canada, latitude 51°N.
Moist Subtropical Mid-latitude Climates
3. dry-summer subtropical (also referred to as
Mediterranean) (Cs)
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similar to marine (Cfb), but with a more pronounced dry season
found to the south of marine regions, where high pressure areas
are more dominant (Oregon, California)
mild-hot, dry summer
mild, wet winters- subtropical high moves south allowing midlatitude cyclones to affect the region
further subdivide Mediterranean into:
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coastal Mediterranean (Csb)
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moderated by coastal upwelling
mild, summertime highs in the 70's, lows in the 50's-60's
example at San Francisco, CA
interior Mediterranean (Csa)
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much warmer, summertime highs in the 90's
example at Sacramento, CA
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Comparison of a coastal Mediterranean climate,
Csb (San Francisco, at left), with an interior
Mediterranean climate, Csa (Sacramento, at right)
Moist Continental
• General Characteristics - warm to cool summers and cold winters.
Winters are severe with snow storms, blustery winds, bitter cold climate is controlled by large continent
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controlled by large land masses - found only in northern hemisphere
found between about 40°-70° latitude
warm-cool summers, cold winters
average temperature of coldest month must dip below -3°C (27°F)) Koppen found that this temperature value marks the southern extent of
persistent snow cover during the winter
• Extent - north of moist subtropical mid-latitude climates
• Major types: 1. humid continental with hot (Dfa) summers or cool summers (Dfb)
– observed between 40°-60° latitude
– evenly distributed precip during the year - 20-40 inches per year
– Dfa - hot, humid summers - summertime temps in the 90's - warm,
humid evenings
– Dfa - growing season is about 5-6 months
– example for Des Moines, IA
– Dfb - cooler, located north of Dfa region
– growing season is about 3-5 months
– slightly larger temperature range than Dfa
– example for Winnipeg, Canada
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Comparison of a humid
continental hot summer climate,
Dfa (Des Moines, at left), with a
humid continental cool summer
climate, Dfb (Winnipeg, at right).
The leaves of deciduous trees
burst into brilliant color during
autumn over the countryside
of Adirondack Park, a region
with a humid continental
climate.
Moist Continental
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sub polar (Dfc)
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much of Canada, Alaska, Norway, Siberia
source regions of cP, cA air masses
severe winters, short, cool summers
1-3 months with mean temperature
exceeding 10°C (50°F)
small amounts of precip, though evaporation
rate is also low
hence, there is adequate moisture to support
the boreal forests of conifers and birches
example for Fairbanks, Alaska
Coniferous
forests (taiga)
such as this
occur where
winter
temperatures
are low and
precipitation is
abundant.
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Polar
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General Characteristics - year round low temperature
(average temperature of the warmest month is below
10°C (50°F))
• Extent - northern coast areas of North America and
Eurasia; Greenland and Antarctica
• Major types: 1. polar tundra (ET)
– ground is permanently frozen to depths of hundreds of meters called permafrost
– meager annual precip
– very cool summers - low sun angle
– extremely cold winters
– example for Barrow, Alaska
2. polar ice caps (EF)
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average temperature for each month is below freezing
interior ice sheets of Greenland and Antarctica
very meager precip - no moisture
example for Eismitte, Greenland
Climatic data for Eismitte,
Greenland, latitude 71°N, a polar
ice cap climate (EF).
Tundra vegetation
in Alaska.
Climatic data for Barrow,
Alaska, latitude 71°N. A
station with a polar tundra
climate (ET).
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Highlands
• Polar climates can be observed at lower
latitudes, you just need to increase your
elevation, as one would ascending a mountain
chain
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