Make your own free website on


Chapter 18


I. The climate system

A. Climate is an aggregate of weather

B. Involves the exchanges of energy and moisture that occur among the

1. Atmosphere

2. Hydrosphere

3. Solid Earth

4. Biosphere, and

5. Cryosphere (ice and snow)


II. World climates

A. Every location has a distinctive climate

B. The most important elements in a climatic description are

1. Temperature, and

2. Precipitation


III. Climate classification

A. Brings order to large quantities of information

B. Many climatic-classification schemes have been devised

C. Köppen classification of climates

1. Best known and most used system

2. Uses mean monthly and annual values of temperature and precipitation

3. Divides the world into climatic regions in a realistic way


Climate classification continued

4. Boundaries Köppen chose were largely based on the limits of certain plant associations

5. Five principal climate groups

a. Humid tropical ( A )

b. Dry ( B )

c. Humid middle-latitude with mild winters ( C )

d. Humid middle-latitude with severe winters ( D )

e. Polar ( E )


Climate Classification continued

6. A, C, D, and E climates are defined on the basis of temperature characteristics

7. Precipitation is the primary criterion for the B group


IV. KÖppen climates

A. Humid tropical ( A ) climates

1. Winterless climates, with all months having a mean temperature above 18 degrees C

2. Two main types

a. Wet tropics

1. High temperatures and year-round rain

2. Luxuriant vegetation ( tropical rain forest )

3. Discontinuous belt astride the equator

4. Influence by the equatorial low pressure

b. Tropical wet and dry

1. Poleward of wet tropics and equatorward of the tropical deserts

2. Tropical grassland ( savanna )

3. Seasonal rainfall


B. Dry ( B ) climates

1. Evaporation exceeds precipitation and a constant water deficiency

2. Boundary determined by formulas involving the three variables

a. Average annual precipitation

b. Average annual temperature

c. Seasonal distribution of precipitation

3. Two climatic types

a. Arid or desert ( BW )

b. Semiarid or steppe ( BS )

1. More humid than arid climate

2. Surrounds desert


4. Causes of deserts and steepes

a. In the low latitudes

1. E.g., North Africa to India, Mexico, southwestern U.S.

2. Coincide with the dry, stable, subsiding air of the subtropical high-pressure belts

b. Middle-latitude deserts and steppes

1. Due to their position in the deep interiors of large land-masses and/or the presence of high mountains

2. Most are found in the Northern Hemisphere


C. Humid middle-latitude climates with mild winters ( C climates )

1. Average temperature of the coldest month is below 18 degrees but above - 3 0C

2. Subgroups

a. Humid subtropics

1. Eastern sides of continents

2. 25 - to 40 - degree latitude range

3. Hot, sultry summers

4. Mild winters

5. Winter precipitation is generated along fronts

b. Marine west coast

1. Western ( windward ) side of continents

2. 40 - to 65 - degrees north and south latitudes

3. Onshore flow of ocean air

4. Mild winters and cool summers

c. Dry-summer subtropics

1. West sides of continents between latitudes 30 and 45 degrees

2. Strong winter rainfall maximum

3. Often called a Mediterranean climate


D. Humid middle-latitude climates with sever winters ( D climates )

1. Average temperature of the coldest month is below - 3 0 C and the warmest monthly mean exceeds 10 0 C

2. Land-controlled climates

3. Absent in the Southern Hemisphere

4. Subgroups

a. Humid continental

1. Confined to the central and eastern portions of North America and Eurasia between 40 and 50 0 N

2. Severe winter and summer temperatures

3. High annual temperature ranges

4. Precipitation is generally greater in the summer than in the winter

5. Snow remains on the ground for extended periods


b. Subarctic

1. North of the humid continents climate

2. Often referred to as the taiga climate

3. Largest stretch of continuous forests on Earth

4. Source regions of cP air masses

5. Frigid winters, remarkably warm but short summers


E. Polar ( E ) climates

1. Mean temperature of the warmest month is below 10 0 C

2. Enduring cold

3. Meager precipitation

4. Two types of polar climates

a. Tundra climate ( ET )

1. Treeless climate

2. Almost exclusively in the Northern Hemisphere

3. Severe winters, cool summers

4. High annual temperature range

b. Ice cap climate ( EF )

1. No monthly mean above 0 0 C

2. Permanent ice and snow


F. Highland climates

1. Usually cooler and wetter than adjacent lowlands

2. Great diversity of climatic conditions


V. Human impact on global climate

A. Humans have been modifying the environment over extensive areas for thousands of years

1. By using fire

2. By overgrazing of marginal lands

B. Most hypotheses of climatic change are to some degree controversial


C. Global Warming

1. Water vapor and carbon dioxide absorb heat and are largely responsible for the greenhouse effect of the atmosphere

2. Burning fossil fuels has added great quantities of carbon dioxide to the atmosphere

3. Models project a 1.5 to 4.5 0 C increase in the mean global surface temperature by the second half of the next century


Global warming continued

4. Trace gases that add to global warming

a. Methane

b. Nitrous oxide

c. Certain chlorofluorocarbons

5. Polar regions will have a greater temperature response to global warming than the global average


6. Some consequences of global warming

a. Global mean surface warming

b. Global mean precipitation will increase

c. Reduction

d. Polar winter surface warming

e. Summer continental dryness

f. High latitude precipitation will increase

g. Rise in global mean sea level


The End

Back To:  348 Syllabus