Chapter 15 Chapter 15

Earth’s ClimatesEarth’s Climates

Climate refers to the statistical properties of the atmosphereand is concerned with the long-term behavior,

or expected (typical) conditions.

The most widely used climatic classification scheme isthe Koeppen system based on world distribution

of natural vegetation boundaries and thecombinations of monthly mean temperature

and precipitation associated with those boundaries.

Climate types according to Koeppen.

World map of Koeppen climates.

The Tropical Climates consist of three groups,each of which is warm year-round.

The tropical wet climate has significant rainfallevery month of the year,

the tropical wet and dry climate has apronounced dry season,

and the monsoonal climate undergoesrelative dryness for 1 to 3 months but receivessufficient moisture that vegetation need not be

adapted to seasonal drought.

Tropical Wet (Af) climates have no dry period and precipitation is almostalways convectional with strong solar heating of the surface triggering

brief but heavy thundershowers in the mid to late afternoon.

Monsoonal (Am) climates usually occur along tropical, coastal areassubjected to predominant onshore winds that supply warm, moist air

to the region throughout most of the year.

Tropical Wet and Dry (Aw) often occur along the poleward margins ofthe Tropics and border dry climates on one side and tropical wet climateson the other. They undergo much greater seasonality in precipitation and

temperature than do the tropical wet and the monsoonal climates.

Tropical wet and dry climates are associated with the savanna.This vegetation consists mainly of grasses interspersed with

widely separated trees or clumps of trees.

The lack of forest results from numerous factors,including recurrent fire, waterlogged soils, and the

development of hard layers within the soil.

The Dry Climates of the world can be divided by the level of aridity and by their latitudinal position.

Semi-deserts are transitional zones that separate thetrue deserts from adjacent climates. They are also

called steppe climates, with reference to the associated vegetation type consisting of short grasses.

True deserts are so dry that only sparse vegetation consisting entirely of xerophytic species can take hold.

Subtropical Deserts (BWh) tend to exist in the subtropical regions,particularly within the western portions of the continents as a result

of subsidence associated with the Hadley circulation.

Subtropical Steppe (BSh) are marked by aridity, high year-to-yearvariations in precipitation, extreme summer temperatures, large annual anddaily temperature ranges, and commonly border the subtropical deserts.

Mid-latitude Deserts (BWk) result from extreme continentality inregions deep within continental interiors or downwind of orographic

barriers that cut off the supply of moisture from the ocean.

Mid-latitude Steppe (BSk) accounts for most of the arid regions ofwestern North America and has the same temperature characteristicsas the mid-latitude deserts. The primary difference between the two

is the greater amount of precipitation in the steppes.

The Mild Mid-latitude climates are located in parts of thelatitude range between 30° and 60° in either hemisphere.

The term mild refers to the winter temperatures and not necessarily those of the summer.

Mediterranean climates can be found along thewest coasts between about 25° and 40° latitude.

At about the same range of latitude on the easternside of continents are the humid subtropical climates.

The marine west coast climates lie adjacent toand poleward of the mediterranean climates.

Mediterranean (Csa, Csb) climates are the only ones that have a distinctsummer dry season and a concentration of precipitation in the winter.

Annual precipitation increases with latitude and with elevation along windward slopes in mediterranean climates.

Humid subtropical (Cfa, Cwa) climates lie within the lower middle latitudesof eastern North America, South America, and Asia. They have a distinct

tropical feel during their long summers and receive abundant precipitation.

Marine West Coast (Cfb, Cfc) climates normally occur poleward of mediterranean climates. Both summers and winters are typically mild

with low annual temperature ranges and a wide range of precipitation.

The Severe Mid-latitude climate group includes two climates, humid continental and subarctic, both of which are marked by

very cold winters. These climates require large continental areaswithin the high-middle latitudes between about 40° and 70°.

Both of the severe mid-latitude climates receive precipitationthroughout the year and have no true dry season.

Humid Continental (Dfa, Dfb, Dwa, Dwb) climates are found between40° N and 55° N in the eastern parts of continents with warm and oftenhot summers followed by cold winters. The coniferous forest found here

is referred to as the boreal forest in North America and the taiga in Asia.

Subarctic (Dfc, Dfd, Dwc, Dwd) climates occupy the northernmost extentof the severe mid-latitude regions. Summer temperatures are lower

than humid continental regions and winter mean monthly temperaturescan be extremely low. Precipitation is greater in the summer than winter.

Polar Climates exist in the highest latitudes poleward of about70° at the high latitude boundaries of the subarctic climates.

The most equatorward and milder of the twois the tundra. At the most poleward regions of the

globe lie the true ice cap climates.

Tundra (ET) climates are named for the associated vegetation type thatconsists primarily of low-growing mosses, lichens, and flowering plants,

with a few woody shrubs and trees. Winters are severe with a perennially frozen layer below the surface called permafrost.

Polar Ice Cap (EF) areas exist where ice covers the ground the entire year.The mean temperature of the warmest month does not rise above 0 °C.

Most areas of ice cap receive little precipitation because of the intense cold.

Highland Climates (H) are governed by topography and found in large mountain or plateau areas.

This group contains an extremely rich collection of climates.

In high mountains there can be large changes inmean temperature over short distances while

mountain slopes can enhance precipitation on their windwardsides and simultaneously create a rainshadow downwind.

The next chapter examinesThe next chapter examinesclimate changes: past and future.climate changes: past and future.