CLIMATIC REGIONS Climate = "Long-term average weather, including an indication of temperature levels, rainfall totals and their seasonality and extremes".

4 major elements of climate: temperature, precipitation, winds, humidity.

Temperature and moisture are the two most prominent elements; not surprisingly, the controls on climate are all factors which determine the input and output of heat and moisture for a specific location.

7 controlling factors: latitude, altitude, distribution of land and sea, ocean currents, distribution of mountains, wind patterns, location of major centers of high and low pressure.

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Koppen Classification This is the most widely used classification scheme - it is based simply on mean annual and monthly temperatures and precipitation levels. This data has the advantage of being simple to use and easily graphed as a climograph.

There are 6 major classifications, A, C, D, E (all based only on temperature criteria), H (= Highland Climate - highly variable; controlled mainly by altitude and aspect - not discussed), B (Dry climates – based on temperature and precipitation criteria). These are subdivided by 2nd and 3rd letters (based on temperature or precipitation).

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Fig 10.4

The climate zones clearly correspond to LATITUDE, but there are some obvious exceptions.

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“A” Climates Harry Williams, Earth Science 4

A: Tropical Climates Average temperature of each month > 18 0 C. Characteristics: Low latitudes (ca. 20N-20S); warm year-round (no winter); Abundant rainfall. 2nd Letters: f: rain forest - average rainfall each month at least 6 cm m: monsoon - 1-3 months with < 6 cm rainfall w: savanna; 3-6 months with < 6 cm rainfall

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Fig. 10.7

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Controls On “A” Climates Mainly equatorial; warm and wet every day -> seasonless; very small annual temperature range (2-3 0 C). Average temperatures usually about 80 0 F. Controlled by latitude -> constant high insolation. Usually 60-100 inches rainfall per year. Located close to ITCZ -> convergence of Trade Winds and convergent uplift. Heating -> convection. Onshore winds on east coasts -> orographic rainfall e.g. world record average annual rainfall = 460 inches in Hawaii (Denton gets about 33 inches). Major areas = Amazon Basin; Zaire Basin; East Indies.

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“B” Climates Harry Williams, Earth Science 8

B: Dry Climates Potential evapotranspiration > precipitation Characteristics: cover 30% of land surface (largest of any group); no rain due to lack of uplift associated with subtropical highs - descending air). 2nd Letters: W: arid; precipitation less than half potential evapotranspiration. S: semi-arid; precipitation more than half potential evapotranspiration. 3rd Letters: h: hot - average annual temperature > 18 0 k: cold - average annual temperature < 18 0 C.

C.

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Bwhclimateeg.jpg

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Fig. 10.25a

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Controls On “B” Climates.

- descending air, little precipitation. STH's are most pronounced on eastern side of ocean basins -> strongest effect on western edge of adjacent continents (deserts do not extend to eastern side of continents). The presence of Asia makes North Africa like a continent without an east coast (no source of moisture), therefore desert conditions extend further inland.

- Atacama desert is created by high pressure; cold Humboldt current offshore -> stable air; Andes Mountains to east -> rainshadow for air coming from east.

- the clear skies and tropical latitudes result in intense insolation in summer -> world record highest temperature (136 0 F) world record daily temperature range (100 0 F); world record highest average annual temperature (94 0 F); world record least annual average precipitation (0.03 inches).

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“C” Climates Harry Williams, Earth Science 14

C: Mild Midlatitude Climates Average temperature warmest month > 10 0 C; average temperature of coldest month > 0 0 C. Pronounced seasons and variability (meeting of tropical and polar air).

2nd Letters: s: summer dry - driest summer month has < 1/3 average precipitation of wettest winter month.

w: winter dry - driest winter month has < 1/10 average precipitation of wettest summer month.

f: moist - no dry season (doesn't fit either above) 3rd Letters: a: hot summers - average temperature of warmest month >22 0 C.

b: warm summers - average temperature of warmest month < 22 0 C. At least 4 months with average temperatures > 10 0 C.

c: cool summers – 1 to 3 months with average temperatures > 10 0 C.

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Controls On “C” Climates Humid Subtropical (Cfa) - on eastern side of continents in midlatitudes away from influence of STH's. Hot humid summers, mild winters with occasional severe cold outbursts.

e.g. winter temperatures minimums can be 10-20 0 F lower than in west coasts climates (Mediterranean) at similar latitudes. Summer maximum rainfall due to convection (thunderstorms in North Texas) and cyclonic storms; onshore air flows.

Mediterranean (Csb) Influenced by STH’s in summer - west coast of large continents. Pronounced summer drought. Warm year round.

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“D” Climates Harry Williams, Earth Science 19

D: Severe Midlatitude Climates Warmest month average > 10 0 C; coldest month average < 0 0 C. Continentality -> large annual temperature range. Pronounced seasons. Summer maximum rainfall due to convection; winter rainfall due to cyclonic activity.

2nd and 3rd Letters same as for C climates, plus d = very cold winters - average temperature of coldest month < -38 0 C.

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Controls on “D” Climates: Central-East side of continents in the higher midlatitudes; Day to day weather, especially in winter, characterized by variability associated with passage of pressure systems, cyclonic storms. Much winter precipitation is snow - especially further north. Inland locations exhibit continentality (NOT moderated by oceans - more “extreme” climates).

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“E” Climates Harry Williams, Earth Science 23

E: Polar Climates No month with average temperature > 10 0 C; no true summer. Precipitation is very low, but so is evaporation, so the whole group is classified as humid. Low insolation -> coldest summers, lowest annual and absolute temperatures.

2nd Letter: T: tundra - at least 1 month with average temperature > 0 0 C < 10 0 C. F: frost - no month with average temperature > 0 0 C.

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The tundra climate zone is defined mainly on the basis of vegetation: its equator-ward boundary is the 10 0 C average temperature of the warmest month isotherm this is the poleward limit of tree growth (the tree line); its poleward limit is the 0 0 C average temperature of the warmest month isotherm - this is the poleward limit of vegetation growth. Found mostly in the northern hemisphere - little land at suitable latitudes in southern hemisphere. Harry Williams, Earth Science 26