Geography 370: CLIMATOLOGY
Fall, 2003
CLIMATE REGIONS
Major fires are burning from north of Los Angeles (centre) to the Mexican border (Image: Jacques Descloitres, MODIS Rapid Response Team at NASA GSFC)
Southern California Fires -- Images -- October 25-27, 2003
From lecture notes:
TROPICAL CLIMATES -- WET, SEASONALLY WET, AND DRY
Major measurable characteristics of climate:
Precip and Temperature
Tropical Climates controlled by seasonal migration of HADLEY CELL CIRCULATION and ITCZ
TRADE WIND INVERSION
Associated with the Hadley Cell Circulation --
Where major area of cloud formation in tropics is around the ITCZ, area of instability, Convergence, rising air
Areas poleward of ITCZ are subject to a general inversion -- subsiding air descending from Hadley Cell
Air heats by adiabatic warming
Traps somewhat cooler air at surface
This Inversion and Subsidence generally discourages precipitation from occurring between about 30o N and S and the seasonal position of the ITCZ
Since subsidence is not caused by thermal conditions, but rather by the general circulation, most heating at the surface is not sufficient to cause instability
Warm surface trade winds travels across the ocean, picking up much moisture, but cannot release it, cannot release latent heat energy, until they reach the ITCZ
Small amount of cooling at surface can cause fog, since air is close to saturation
Compare CLIMOGRAPHS for Tropical Wet (rain forest), Tropical Wet and Dry (Savanna), Tropical Steppe (Warm Semiarid), and Warm Desert
Characterized by Temperature Variations, and Precipitation Variations
TEMPERATURE
Three main characteristics of Temperature
1) Temperatures are generally warm -- high surplus radiation
2) Temperature (and day length) vary only a small amount throughout the year -- 4-5o F, 2-3oC annually
3) Temperatures vary more diurnally -- between night and day than seasonally - more diurnal variations in dry than moist climatesEnergy balances vary in the tropics, depending on Precipitation, cloudiness
PRECIPITATION
Precipitation is the characteristic that varies most, bringing seasonal changes to much of the Tropics
Three main types of Tropical Climates:
1) Rainy all year (Af)
2) One or two dry seasons (Aw)
3) Dry all year (BS [grading from Aw], BW)
Rainy All Year (Af)
Relatively Narrow area, patches, which is perennially under the influence of the ITCZ
As ITCZ migrates with the sun -- there is a lag between time of high sun and following ITCZ, so there may be periods that are somewhat drier, somewhat moister
Found only where January and July positions of ITCZ are quite close together in terms of latitude:
Central Africa, Amazon Basin, East Indies, Malaysia, Indonesia
These areas are labeled Af on Köppen classification
TEMPERATURES
Tropical wet, temperature varies only a 2-3oC year round, a few degrees between day and night
Always humid, always warm
Days of nearly equal length
Max. High temperatures (afternoons) around 30-35oC (89-95oF)
Lows, before sunrise, around 20-24oC (68-75oF)
Relative Humidity always high, year round, throughout the day and night
PRECIPITATION
Affected year round by ITCZ rainfall
What is this precip like? --
Usually almost Daily, in Afternoons, Convective Thunderstorms, Some Wave patterns in Easterlies (sometimes several days of steady rain)
Usually short, Intense precip -- Highest Record Precip Intensities measured in Tropics
Occasionally longer, gentle rains, associated with tropical wave
Very humid -- Dew Point may be close to 70o F
When night cools, heavy dew, fog
Mostly Cloudy - nearly every day for several hours, afternoons and evenings, so less insulation at surface
VEGETATION:
Rain Forest vegetation -
Thick, multi-level, very tall trees, lianas (vines), epiphytes, high diversity in insects, animals, vegetation species
Very specialized --sensitive to small changes in temp, precip, humidity
Tropical Evergreen -- loses some leaves year-round, always making new leaves
No limits on moisture or energy --
Growth will slow down if it gets too hot, but rarely
High biomass, tall trees top of forest canopy, many levels below tree tops, broadleaf evergreen,
Highest species Diversity in all forms of life, very rapid cycling of biomass, high rates of growth and decay
Tropical Rain Forests provide a significant sink for CO2 -- when vegetation is burned, CO2 that was stored is released to atmosphere
Tropical Rain Forests also provide significant amount of oxygen to the atmosphere
Significant disturbance from clearcutting -- deforestation, especially in Amazon basin, Indonesia
DISEASES
Endemic diseases, parasites --Malaria, Dengue Fever, Sleeping Sickness, River blindness
Warm temperatures year round allow diseases to flourish, never dampened by frost
Diseases carried in soil, water, insects, cattle
Many West African people feel sick all their lives
SOILS:
Deep weathering, poor in nutrients because vegetation cycles so rapidly, little humus is left in soils,
High rates of leaching and oxidation because of lots of precipitation and warm temps -- iron and aluminum oxidized and leach down, accumulating lower levels in soils and sometimes form LATERITE -- red brick clayey soils, harden to stone if exposed in sun (more likely in Aw climate, winter dry season)
Soils that have been stripped of vegetation, biomass burned, become sterile very quickly, in a few years -- and rainforest vegetation may return only slowly or not at all, if soil has dried out and turned to stone
Much of the stripped top soils are washed away, down streams and out to the oceans
(SOILS TAKE THOUSANDS OF YEARS TO REFORM, very slowly renewable)
AGRICULTURE
May have several harvests each year, no fallow season
But climate conditions can vary from year to year, so crops can fail
Mostly shifting agriculture - Slash and burn
Cut Rainforest to grow crops -- soils get used up in a few years -- plots abandoned
Crops: Cocoa beans, rice, bananas, also tropical hardwoods, rubber
Tropical Wet and Dry Climates -- Am, Aw
including both Monsoon and Tropical Savanna
Poleward of Wet Tropical, Af, Between about 5o and
20o N and S latitude
TEMPERATURES
Vary annually a few degrees, Still warm all year, more diurnal than annual range - 2oC to 6oC annually
Mean Annual Temperature around 25-27o C
With Short Dry Season,
More annual temperature variation - heavy clouds in wet season intercept insulation - cooler temperatures
Clear skies in dry season - higher temperatures
On a Continuum, from nearly Af to nearly BS
greater annual temp range than the Tropical Wet, because of distinct dry seasons -- but may be humid all year, depending on location
Temperature peak may come at end of dry season, just before beginning of wet season
Therefore "winter" is warmer than "summer"
PRECIPITATION
Dry Hadley circulation brings one or two dry seasons:
Summer Precipitation -- When ITCZ passes overhead
Dry Winters -- When ITCZ migrates into the other Hemisphere
This may be Accentuated by Monsoon Circulation
Here, Seasonal Precipitation is much more extreme than any other climate type
Monsoon rains can vary from 150 to > 400 centimeters (60-150 inches)
Especially enhanced if orographic effect is present, e.g. Himalayas
Monsoon climates, often briefer dry season with some rain, keeps soils from drying out - vegetation doesn't have dormant season
VEGETATION:
Grading from tropical forest -jungle
To thorn forest, to savanna grasslands with dry-adapted trees, e.g. acacia
Vegetation adapts, may go dormant during dry season,
Shorter trees
Forests less thick - More Shrubs, grasses, open canopy
Lower diversity
Also animals adapt -- fish that burrow in mud during dry season
SOILS:
Nearer tropical wet: Deeply weathered, enriched in oxidized iron and aluminum (other minerals leached out) Laterite -- if stripped of vegetation and allowed to dry out - BrickGrassland soils, deeply weathered (less than wet tropics), usually high in humus, nutrients, but subject to erosion if overgrazed or over cultivated, especially during dry season
AGRICULTURE
Yams, manioc, maize, beans, squash (Milpas, slash & burn)
For export: Coffee, tea, sugar, rice, tropical fruits, melons, tubers, peanuts, sesame, palm oil, cattle, tobacco
Savanna - Grasslands - E.G. Kenya , land of Masai cattle herders, Llanos of Venezuela, campos in Brasil
Tropical Semiarid: (BSh -- Steppe) E.G. Sahel, Sub-Sahara, Namib
Has short rainy season, warm in rainy season, hot in dry season
-- transitional to Tropical Desert, low humidity
Found at ~20-25 o N & S Lat, especially on west
sides of continents
TEMPERATURE
Higher temperatures, less latent heat store, more insolation goes to sensible heat
Greater diurnal temperature range, due to radiational cooling at night
Annual temp range around 10-12oC, mean temp around 25-27oC
PRECIPITATION
Highly variable from year to year, short rainy season from edges of summer ITCZ
VEGETATION
Xerophytic, adapted to drought much of the year, grasses, brush, few low thorny trees, small leathery leaves
SOILS
May be quite rich in nutrients, not as thick as grassland soils, not deeply weathered, may have calcic horizons (calcium carbonate, calcrete) or duripans (silica cemented), hardpans
Susceptible to Desertification -- very sensitive to disturbance -- overgrazing, over-cultivation can remove vegetation -- soil dries out, does not hold moisture, blows away (deflation), so becomes hotter, more wind, more deflation -- positive feedback loop
Tropical Deserts -- Perennially Dry, BWh, Hot
CONTROLS: Descending arm of Hadley Cell - Subtropical
Inversion associated with Subtropical Highs - especially along
East sides (west coasts)
Clear skies most of the time
Location - Some Restricted by Mountains -- Atacama, Namibian, Kalahari deserts, Baja Calif. also
TEMPERATURES
Very warm year round -- High sun, so no real cold season
High mean annual and mean monthly temperatures -- little moisture to be evaporated, so most energy goes into sensible heat
High diurnal temperature ranges -- because no clouds, dry air, much longwave energy lost at night, so cold nights, hot days
Considerable Seasonality -- Mean annual Temperature Range may be as much as 20 deg C or more
Daytime highs -- may be very high -- 45 Deg C. (113 deg F) or more
Nighttime lows -- may be quite low, 10 deg C (50 deg F) or less
Low relative humidity -- may be much water vapor in air, but warm air can hold much moisture
Potential Evapotranspiration significantly greater than Actual Evapotranspiration
PRECIPITATION
Low precipitation -- infrequent, and usually small amounts - can be extremely variable -- no rain for several years, then deluge in a day
High wind velocities -- strong differential heating at surface,
plus low surface friction -- no trees
So may be Sand Storms, Dust Storms, can last for Days
VEGETATION:
Much less diversity than tropical forests, may be sparse, much bare ground -- plants compete for moisture (often in form of dew at night) -- may put out toxins to keep other plants away, adapted to drought, small, leathery leaves or needles (cactus), few small trees, mostly shrubs with bare ground between,
However, desert can have much more life than it appears -- many insects, snakes, small animals -- I have seen civet cats, badger, coyote, antelope, mice, many jack rabbits, owls, eagles, snakes, spiders, scorpions
SOILS:
Thinly weathered, unless quite old, calcrete, hardpan, clay horizons, formed in loess (dust), often stony desert, desert pavement covers soil, protects from erosion, Aridosols, Mollisols
MIDLATITUDE DESERTS: BWk, BSk (Steppe)
Interior Midlatitude Deserts --
k = Temperature -- average annual temperature below 18 ºC
(64.4 ºF)
k' = avg warmest month below 18 ºC
Either in Rainshadow of Mountains -- Midlatitudes of North
America, South America (Argentina)
Or Far to Interior -- far from source of water -- Continental
climate, E.G. Gobi Desert
No particular latitudinal constraints within the Midlatitudes
However, many of these drier colder climates have less moisture
deficit because of colder temps -- raises humidity, cools temps to
dew point -- e.g. Central Siberia
May merge with Tropical Deserts (BWh) at low latitudes
BWk: Located in southwestern Great Basin (e.g. Death Valley), between the Sierra Nevada and the Rockies; Northern Chile, Southern Argentina, large areas of Central Asia east of Caspian Sea, Gobi Desert
BSk: the Great Basin, Western U.S., up into South Dakota, N. Dakota, up into Saskatchewan; huge areas of Central Asia
Air masses: Continental Tropical (cT) and Continental Polar (cP)
TEMPERATURE
Can be very hot in summer -- well over 38oC (100oF) in daytime, due to dominant High Pressure, clear skies; Winters can be cold, below 0oC.
PRECIPITATION
Generally arid, low and highly variable
Summer -- convective showers, flash floods, some summer monsoon in SW US
Winter -- occasional westerly cyclonic storms bringing rain or snow -- precipitation is not seasonal
VEGETATION
Sparse, desert scrub, saltbrush, sagebrush, mesquite (No. America), adapted to conserving moisture
SOILS:
Aridisols, Mollisols --
may be highly alkaline in basins, weakly weathered, calcium carbonate, gypsum, silica cementation, fragile soils, take long time to develop\
AGRICULTURE
Low and variable precipitation requires irrigation for most crops. In many cases, ground water is mined (fossil water, not recharged in present climate). Cattle and sheep grazing.
MIDLATITUDES (C AND D CLIMATES)
Both Temperature and Precipitation Variations throughout
year
Characterized by greater seasonality, compared with diurnal
variations
While Tropical Climates are dominated by Hadley Cell
Circulation
Midlatitudes are Dominated by Westerly Circulation, seasonal
migration of the Rossby Waves, the Polar Front, the Jet Stream
Midlatitude climates influenced by
Air Masses, both Polar and Tropical
And by Cyclonic Circulation -- passage of High and Low Pressure
systems embedded in Westerly Circulation
SEASONS are caused by annual migration of the sun, changes in
radiation received, due to angle of sun, day length
Summer, sun is higher in the sky, days are longer, equator-pole
temperature gradient is weaker,
So Westerly Circulation weakens and shrinks poleward , so Hadley
Cell Circulation also expands poleward
In Winter, sun is lower in sky, days are shorter, equator-pole
temperature gradient is stronger,
So Westerly Circulation strengthens and moves equatorward, Hadley
Cell Circulation weakens and shrinks equatorward
Thus, Midlatitude climate is influenced by both Hadley Cell
Circulation, mainly in summer, and Westerly Circulation, mainly in
winter
Westerly Circulation is influenced by seasonal strengthening and
weakening of semipermanent High Pressure Cells and Low Pressure
Cells over Oceans
In Summer, Midlatitudes more influenced by Tropical Air Masses
moving poleward
In Winter, more influenced by Polar and Arctic Air Masses, moving
equatorward
SEVERAL TYPES OF MIDLATITUDE CLIMATES:
(from West to East, as air flows)
C-types, mild temperatures, usually good for agriculture
Mediterranean (Cs) wet winters, dry summers
On West Coasts between about 30-40o N and S
Latitude
Lies between Subtropical Highs in Summer (dry) and Westerly
Circulation in Winter (wet)
E.G. California, Central Chile, Mediterranean (Spain, Italy,
Greece) Northernmost Africa (Tunisia, Algeria), Capetown, South
Africa, southern & western Australia
TEMPERATURE:
Summers -- mild to hot, depending on continentality and latitude
e.g. Central Valley highs can be over 100 for week or two, coasts rarely over 80s.
High summer diurnal temp range inland, lower at coasts, because of drought
Winters, mild to below freezing
Thus Annual temp range can be quite large, inland, but very small at coast
PRECIPITATION
Wet winters, from Westerlies,
Dry summers, Strong descending air from Subtropical Highs, plus cold water upwelling along coasts
Cooler, foggy coasts due to cold water upwelling
VEGETATION:
Chaparral, scrub woodlands, bunch grasses, plants adapted to fire, live oaks, summer deciduous buckeye, coastal cypress
North coast Redwoods -- feed on fog in summer
SOILS:
Great variety, depending on other factors, because both wet and dry -- Mollisols (grassland), Alifisols (woodland)
AGRICULTURE:
Excellent if Irrigation provided --precipitation comes at wrong time of year
Good Soils and lots of sunshine = great abundance and variety
E.g. Olives, grapes, cotton, wheat, citrus and stone fruits, nuts, rice, vegetables, tomatos, artichokes avocados
Humid Subtropical (Cfa, Cwa) -- warm, humid, mild winters
On east coasts, between about 20-40o and S Lat -- broader area than Mediterranean, including Midwest, East coast U.S., southeastern South America, Coastal South Africa, eastern Australia, eastern Asia from Northern India through south China to Southern Japan
Controlled by humid, onshore air movement in summer (Marine Tropical air masses), midlatitude cyclonic storms in winter, including Continental Polar air at times
TEMPERATURES
Warm Summer temperatures, High Sun, Long Days, High humidity;
summers like humid tropics, low diurnal range
Winters cold, may be subfreezing at times
PRECIPITATION:
Abundant precipitation (75 to 250 cm per year)
Summer convective storms, Winter cyclonic storms, some snow
May be wetter in summer than winter (have summer precip maximum)
May also be summer moisture deficit because of higher temps, but
always humid
In China, Northern India, monsoon rains in summer
VEGETATION:
Mixed deciduous forests, some grasslands, (tall grasses) broadleaf deciduous, some pines, adapted to cooler winters, warm moist summers
SOILS:
Well leached, moderate in nutrients, may be moderately weathered, depending on age, some plinthite (like brick) in Carolinas, from leaching of iron, clay horizons well developed
AGRICULTURE
Excellent for all sorts of crops, mixed livestock and grains, in general crops can be watered with natural precipitation
Marine West Coast (Cfb)
Poleward of Cs, around 40-55 o N & S lat:
Under year-round influence of Westerlies, humid, mild, may have
cold winters
e.g. Oregon (somewhat), Washington, British Columbia, southern
Alaska, Northwest Europe (British Isles, France, Netherlands,
Belgium, Germany, Denmark, etc.), southeast Australia, New
Zealand,
TEMPERATURE:
Mild winters and summers, relatively low annual temperature range for higher latitude, e.g. ~15oC
Coastal, so temp moderated by marine influence
Mean annual temp relatively mild, e.g. ~10-12oC
often foggy, often cloudy, high humidity
PRECIPITATION:
Frequent cyclonic storms in westerlies, year round, may be frosty winters
VEGETATION:
Tall conifer forests, Douglas fir, hemlock, fir, green all year -- some parts midlatitude rain forests -- E.G. Olympic Peninsula
Maple, alder, willow in riparian zones
SOILS:
Deep, well leached soils, if under coniferous forest, may not be high in nutrients, acidic soils, Spodosols, Alfisols
AGRICULTURE
Many types of crops thrive in this climate, field crops, grains, also specialties such as apples, stone fruit, berries, nuts, watered by natural precipitation
D-Climates
Very cold winters, higher latitudes, generally more
continental
Between about 40 to 70 o North lat -- not found in
Southern Hemisphere
Humid Continental, with warm Summers (Dfa, Dfb, Dwa, Dwb)
Dfa, Dwa: Lower Midlatitudes, 35-45oN,
Eastern and midwestern U.S., from east coast to about 100th
meridian, Eastern Europe, Asia
Continental, but with westerly cyclonic storms in summer, usually
drier in winter due to Continental Polar High -- hot, humid
summers, cold winters, large annual temperature ranges, moderate
precipitation
Actual Evapotranspiration close to Potential Evapotranspiration,
so little soil moisture deficit
Dfb, Dwb: mild summers, fairly short growing season, long
winters, year-round precipitation from westerly cyclonic storms,
less precipitation than farther south, may be winter drought from
continental Polar air mass, high pressure, large annual
temperature ranges
Mid-latitudes, 45-55o N and S, New England, Great
Lakes, south-central Canada, Scandinavia, Eastern Europe, western
Russia, northern Japan, northern China
TEMPERATURES
Mild to hot, humid summers (e.g. 20 to 25oC), cold winters (e.g. -5 to -12oC mean), so large annual temperature ranges (e.g. 30-35oC); mean annual temperatures vary greatly within this classification, may be ~4-10oC
PRECIPITATION
Westerly or convective storms in summer, snowy or blizzard conditions in winter
Commonly summer precipitation maximum
VEGETATION
Conifer forest grading into mixed broadleaf deciduous forests
SOILS
Well leached, can be quite fertile and deep, depending on vegetation
AGRICULTURE
Shorter growing season, but many types of crops successfully grown with natural precipitation, including grains, corn, mixed livestock and crops, dairies, etc. depending on soils
Subarctic (Dfc, Dwc, Dfd, Dwd)
Higher Latitudes, 50-70o, Northern Canada, Newfoundland to Alaska, Northern Eurasia, Scandinavia to Siberia
May be extreme Continentality -- e.g. Siberia
Precip from westerlies in summer, brief, cool summers and long, very cold and dry winters dominated by Polar High
Where extreme continentality: large annual temperature ranges, occasional cyclonic storms, low precipitation, quite variable, semiarid, common permafrost
TEMPERATURE
Largest annual temperature ranges of any climate type, extremely cold long winters with very low insolation, January temperatures may average -40 to -50oC. Record lowest temps in No. Hemisphere in Siberia, -68oC
Short mild summers, may be ~8 to 10oC; so annual range may be as much as 60oC
PRECIPITATION
Low precipitation, depending on continentality, proximity to
ocean, but low Potential Evapotranspiration, so little if any soil
moisture deficit
Precipitation maximum in summer, may be rain or snow, from
occasional cyclonic storms in westerlies
Would be desert if farther equatorward
VEGETATION
Taiga --northern coniferous forest, spruce, lichen
SOILS
Strongly acidic, poorly leached, stirred by frost freeze-thaw cycle, permafrost in some locations
Polar Climates (ET, Tundra, EF, Ice Cap)
Highest latitudes, ET found on lands bordering Arctic Ocean in
Northern Hemisphere, Antarctic peninsula and few polar islands in
Southern Hemisphere. ET is Antarctica, interior Greenland, some
ice-cap covered Arctic islands
ET has at minimum more than nine months mean temperatures below
freezing; EF has no summer, all months below freezing. Under the
influence of subsiding air of the polar high. Long polar nights,
with less than 2-3 hours of daylight, many days of no sun at all,
depending on latitude; long summer days, with less than 2-3 hours
of twilight, many days with sun above the horizon 24 hours, no
real darkness. Sun always low in sky even in summer. So low,
diffuse insolation year round. Gale force winds are common, caused
by Katabatic winds over Antarctica
TEMPERATURE
Cold all year, with ET reaching high mean temperatures less than 4 to10oC in summer, EF below 0oC year round. Still may have extreme annual temperature range, ~30oC. Mean annual temperature may be well below freezing, e.g. -10 to -12oC in ET region, -25 to -30oC in EF region. Antarctica is coldest place on earth, with temperatures as low as -80oC in winter.
PRECIPITATION
Very low precipitation, dominated by polar high year round. Occasional cyclonic storm brings snow in summer, cold air limits evaporation to extremely low levels, so AE = PE
SOILS
Permafrost, permanently frozen. In ET region, surface layers thaw, causing strong mixing of soils, freeze-thaw cycle.
VEGETATION
Only vegetation of any sort in ET, tundra. Low growing mosses, lichens, marsh and bogs because of thawed permafrost, very short growing season.