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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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