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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:
– 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• 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
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.
• 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
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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) 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)
– 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)
Temperature and precipitation data for Iquitos, Peru, latitude 4°S.
Tropical rain forest near Iquitos, Peru.
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.
Tropical wet and dry (Aw)
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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) – 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
Climatic data for Phoenix, Arizona, latitude 33.5°N. A station with an arid climate (BWh).
Creosote bushes and cacti are typical of the vegetation found in the arid southwestern American deserts (BWh).
Hot arid BWh
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Climatic data for Denver, Colorado, latitude 40°N. A station with a semi-arid climate (BSk).
Cumulus clouds forming over the steppe grasslands of western North America, a region with a semi-arid climate (BS).
Semi-arid (BS)
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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Climatic data for Port Hardy, Canada, latitude 51°N.
Climatic data for Mobile, Alabama, latitude 30°N
Humid subtropical climate (Cfa)
Marine climate (Cfb)
Moist Subtropical Mid-latitude Climates3. dry-summer subtropical (also referred to as
Mediterranean) (Cs) – 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 mid-
latitude cyclones to affect the region – further subdivide Mediterranean into:
• coastal Mediterranean (Csb) – 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) – 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– 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 Continental2. sub polar (Dfc)
– 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• 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) – 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 Barrow, Alaska, latitude 71°N. A station with a polar tundra climate (ET).
Climatic data for Eismitte, Greenland, latitude 71°N, a polar ice cap climate (EF).
Tundra vegetation in Alaska.
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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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