Lecture 7 Global Climates

Weather is almost changing all the time. But if we consider the weather at any one place over many years, a clear patterns emerges. So climate is the average weather of a region.

Climate is so diverse that no two places on Earth’s surface experience exactly the same climatic conditions; in fact, Earth is a vast of collection of microclimates. However, broad similarities among local climates permit their grouping into climatic regions.

7.1 Climatic Classification

Climate is a result of a number of weather elements such as temperature, atmospheric pressure, precipitation, moisture, wind, etc. However, observations as detailed as these are not made regularly at most weather stations around the world.

To study climate on a worldwide basis, we must turn to the two simple measurements that are made daily at every weather station-temperature and precipitation.

There are many climatic classification schemes, among them the most popular one is the Koppen System.

The Koppen classification system is widely used for its ease of comprehension. It was designed by Wladimir Koppen (1846-1940), a Russian born German climatologist.

Classification Criteria.

In Koppen climatic classification system, three criteria are selected to devise its spatial categories and boundaries. They are:

(1) average monthly temperature;

(2) average monthly precipitation;

(3) total annual precipitation.

Koppen’s Climatic Designations.

Each climate type in Koppen’s climatic system uses two or three letters to represent. The first letter stands for categories. Second letter represents the type and the third letter stands for sub types.

The Koppen system uses capital letters (A,B,C,D,E,H) to designate climatic categories by latitude. Of the six categories, all but B are based upon purely temperature criteria:

A: Tropical climate ( average monthly temperature above 18 C or 64 F.)

C Mild midlatitude climate(Cool winters, coldest month avg. > -3 C)

D Severe midlatitude climate(Cold winter, coldest month avg. < -3 C)

E Polar climate (cold year round)

H Highland: significant climatic changes in short horizontal distances due to altitudinal variations.

B is the only category based on moisture

B : arid and semi-arid climate (PE>ppt)

There are second and third letters which are listed in the textbook in detail. I will not talk about them here. No need to remember those. I could not remember either.

Climagraph

Probably the most useful tool in a general study of world climatic classification is a simple graphic representation of monthly temperature and precipitation for a specific weather station. Such a graph is called climagraph. On a climograph, horizontal axis represents the months from J to D. there are two vertical axis, left side represents for monthly temperature and right side represent monthly precipitation. Average monthly temperatures are connected by curved line in the upper portion of the diagram, and average monthly precipitation is represented by bars extending upward from the bottom.

The value of a climagraph is twofold: (1) it displays precise details of important aspects of the climate of a specific place, and (2) it can be used to classify the climate of that place

6.2. World distribution of major climatic types and subtypes

Koppen’s climate classification system has been modified by many people.

We are now using one of the modified classification system to discuss the basic climate patterns.

In discussing the climatic types and subtypes, we focus on three aspects.

(1) the global circulation features

(2) their location

(3) their typical climate patterns.

Low-latitude Climates

It can be further divided into several types.

1. Tropical rain forest (Koppen: Af)

(1) Global circulation features: ITCZ dominates year round. Wet, unstable, frequent afternoon and evening thundershows

(2) Geographic distribution: straddles the equator, generally within 10° N/S.

a. Amazon Basin of South America

b. Congo Basin of central Africa

c. Malaysia/Indonesian in SE Asia

(3) typical climate pattern:

a. abundant rainfall each month of the year. Typically, annual rainfall totals exceeds 80 inches.

b. Very little seasonal change in mean monthly temperature. In fact, more diurnal temperature change than seasonal

2. Tropical Monsoon (Am)

(1) Global circulation feature: ITCZ in summer, STHP in winter. Very wet in high-sun season, but often dry in low-sun season.

(2) Geographic distribution: Tropical coastlines, usually 10-20 N/S. Especially common in southern Asia, but small areas on most continents.

(3) Typical climate pattern:

(a) generally, as much precipitation as tropical rainforest, except that it is more strongly seasonal. At least 60, often 80 inches or more annual precipitation, but dry season can generate moisture deficits.

(b) Temperatures, like tropical rain forest, show modest seasonal variation.

3. tropical savanna (Aw)

(1) global circulation features: ITCZ in summer, STHP in winter. Same basic seasonal precipitation regime as in tropical monsoon, but much lower mean annual precipitation totals. Generally drier climate.

(2) Geographic distribution: think of savanna as regions of transition between tropical rain forests (ITCZ year round) and tropical/subtropical deserts (STHP year round).

Found across continents from 10-20 N/S

a. Caribbean, Central America, Venezuela

b. Central and southern Brazil

c. Much of tropical Africa

d. Drier parts of tropical India, SE Asia, and northern Australia.

(3) Typical climate pattern

a. High temperature year round, like other tropical climates

However, there would be a distinct contrast between the more humid wet season (summer) and the less humid dry season (winter)

b. precipitation is strongly seasonal. Annual totals range from as low as about 25 inches to upwards of 60 inches. Pronounced moisture deficits and drought accompany the dry season in most years.

4. Tropical semi-arid and arid climates

Arid/semi-arid climates cover more than 1/3 of global land area

4-1. tropical and subtropical steppe (BSh)

(1) global circulation features: STHP most of the year.

(2) Geographic distribution: on the margins of tropical/subtropical deserts, from 15-35 N/S latitude

a. Texas, NE Mexico

b. Northern Argentina

c. Surrounding the deserts of the Middle East, North and South Africa, and Australia.

(3) typical climate pattern:

a. somewhat more precipitation than deserts, but less than adjacent tropical savanna (on the equatorward side) or Mediterrean (on the poleward side).

b. Mild winters, hot summers, much like tropical/subtropical deserts.

4-2. tropical and subtropical desert (BWh)

(1) Global circulation features: STHP year round. Hot, dry cT air masses dominate throughout the year.

(2) Geographic distribution: 20-30 N/S latitude, especially on western side of continents.

a. southwestern US and northern Mexico

b. Atacama desert of Peru, no. Chile in South America

c. Sahara Desert, Arabian Peninsula, Middle East

d. Kalahari/ Namib Desert of so. Africa

e. Interior Australia

(3) typical climate pattern

a. some temperature seasonality, mild winters and very hot summers. The highest temperatures recorded on earth (Sahara Desert: 138 F)

b. Little precipitation (generally < 10-20 inches). Driest climates on earth.

c. Along some western coastlines, cold ocean currents can create very stable, fog desert. Cloudy, surprisingly mild temperature, but exceedingly dry, e.g. Baja California.

Midlatitude Climates

1. Mezothermal climates (C )

1-1. Humid subtropical (Cfs, Cwa)

(1) Global circulation features: unstable side of STHP in summer, PFW in winter. Fairly wet and unstable year round. Summer wet from frequent thunderstorms; winter wet from migrating cyclones along polar front.

(2) Geographic distribution: generally 30-40 N/S latitude, on the eastern side of continents

a. Southeastern U.S. in North America

b. So. Brazil, no. Argentina in South America;

c. No. India, so China, so. Japan in Asia

d. Small areas in southern Africa and SE Australia.

(3) Typical climate pattern

a. fairly wet year round.

b. Winters are mild to cool. Average monthly temperature remains above –3 C all year.

1-2. Mediterranean (Csa, Csb)

(1) G.C.F.: stable side of STHP in summer, PFW in winter. Wet winters and dry summers.

(2) G.D.: generally 30-40 N/S latitude, on west side of continents.

a. central and southern California;

b. Central Chile in South America

c. Mediterranean Basin of so. Europe, no. Africa, Middle East;

d. SW coast of Africa (Capetown) and SW/SC Australia.

(3) T.C.P.:

a. disctinctive seasonal precipitation regime, wet winters from migrating cyclones along polar front; summers very dry, large deficits. Only climate with pronounced wet season in winter.

c. temperatures much like humid subtropical, except more moderated right along coast lines (cool currents off shore).

1-3. Marine west coast (Cfb, Cfc)

(1) G.C.F: PFW year round. Wet, unstable conditions throughout year.

(2) G.D.: generally 40-60 N/S latitude, along west coast of continents.

a. Pacific Northwest, from SE Alaska, British Columbia, Washington, Oregon, northern California.

b. So. Chile in South America

c. British Isles, much of W. and C. Europe

d. Tasmania, New England

(3) T.C.P.:

a. wet year round, precipitation totals can reach 100+ inches in some coastal settings.

b. Marine influence on temperature seasonanlity is pronounced. Stays mild-cool in summer; mean temepratures remains above freezing in winter. Least annual temperature range of any mildlatitude climate.

2. Microthermal climates (D)

2-1 Humid conditional (Dfa, Dfb, Dwa, Dwb)

(1) G.C.F. PFW year round. Wet, unstable conditions from migrating clcones throughout the year.

(2) G.D.: generally 40-55 N latitude, interior or eastern side of continents. Effectively blocked from moisture of mP air masses. Strong continental influence on climate.

a. Northeastern and midwestern U.S., SE Canada

b. Eastern Europe and western Russia

c. Northern China and Japan

(3) T.C.P.

a. Uniformly wet year round, although precipitation totals are lower than marine west coast or humid subtropical climates. Often 20-40 in./yr.

b. Large seasonal temperature range. Can be hot and humid in summer, but cold in winter (well below 0 C)

2-2: Subarctic climate

(1) G.C.F.: summer PFW, winter Polar High Pressure (PHP)

(2) G.D.: 55-60 N

(3) T.C.P.:

T. short cool summer, long bitterly cold winter

P.: annual precipitation small, mostly in summer, dry in winter

3. Midlatitude desert and steppe (BWk,BSk)

(1) G.C.F.: none. Caused by rainshadow effects in the interior of continents.

(2) G.D.: generally 35-55 N/S latitude. On the leeward side of major mountain ranges.

a. great basin desert of Nevada, Utah

b. Great Plains of central U.S. and Canada

c. Patagonia in so. Argentina

d. Deserts of central Asia, from Turkmenistan to Mongolia.

(3) T.C.P.

a. same precipitation regimes as tropical/subtropical deserts and steppes. Dry to very dry, generally < 15 in/yr.

b. Summers hot, but winters get much colder than deserts at lower latitudes. Subfreezing monthly mean temperatures and snowfall are common in midlatitude deserts.

Polar and Highland Climates

1. Polar climates (E)

1-1. Tundra (ET)

(1) G.C.F.: PHP year round. Cold and dry year round, with a brief thaw in the summer.

(2) G.D.: 60-80 N/S latitude, circumboreal

a. Rim of Arctic Ocean

b. Southernmost tip ofSouth America

(3) T.C.P.

a. meager precipitation totals (<10 in./yr), mostly falling as snow. Little precipitation seasonality.

b. Long, subfreezing winter. 1-3 months of tem. Above freezing. Results in permanentaly frozen layer of soil-permafront.

1-2. Ice cap (EF)

(1) G.C.F.: PHP year round. Cold, dry year round.

(2) G.D.: 65-90 N/S

a.Greenland and adjacent ice pack over Arctic Ocean.

b.Antarctica

(3) T.C.P.

a. meager precipitation (5-10 in/yr.)

b. cold all year, no months with mean temp. above freezing. Antarctica records coldest temp. on earth (below –100 C)

2. Highland Climate (H)

(1) G.C.F.: none characteristic.

(2) G.D.: High mountain ranges from equator to high latitudes.

a. Rocky Mt., Sierra Nevada, Alaska Range

b. Andes in South America

c. Kilimanjaro, Ethiopian Highlands in Africa

d. Alps in Europe

e. Himalaya in Asia

(3) T.C.P.:

a.temperature cold like tundra/ice cap climates

b much higher precipitation totals, due to orographic lifting.