Chapter 7
Global Climates
This chapter brings together many of the ideas developed in previous chapters and
considers climate at the global scale. It introduces the processes and factors that produce
different climates on the Earth.
 The climate of an area refers to the average weather conditions over a long period of
time based on measurements of temperature and precipitation.
 Important principles to help you understand climate are:
 Low latitude locations have warmer temperatures and smaller annual temperature
ranges than high latitude locations.
 Continental locations tend to have much larger annual temperature ranges than
coastal locations at the same latitude.
 Colder locations tend to have less precipitation than warm locations, because
warm air can hold more moisture than cold air.

The basic control on temperature is latitude, while the effect of a continental or
maritime location is an important secondary control.
 Some generalizations about precipitation are:
 Pressure systems and the global circulation are the major determinants of
precipitation patterns.
 The equatorial region experiences high convectional precipitation because of
heating. These areas of high precipitation extend north and south along the east
sides of the continents because the trade winds bring moisture onto the land.
 The subtropical high-pressure cells, characterized by dry, subsiding air, produce
arid and semiarid regions.

Mountain ranges produce wet areas where air masses are forced to rise over the
mountains creating an orographic effect.
 Coastal mountains also act as barriers to moisture producing a rainshadow effect
on the lee side of the mountains.
 Continental interiors tend to be dry because they are far away from the source
areas of moist air masses.
 Three types of annual precipitation patterns are:

Uniformly distributed precipitation

Precipitation maximum during the warmest period of the year


Precipitation maximum during the coolest period of the year
Seven global precipitation regions can be identified based on these principles:
Wet equatorial
Trade wind coasts
Tropical Deserts
Midlatitude deserts and steppes
Moist subtropical
Midlatitude west coasts
Arctic and Polar Deserts
 Low latitude climates are dominated by cT, mT, and mE air masses. The position of
the ITC and the subtropical high pressure cell affect these climates throughout the
year. Weather disturbances include the easterly wave and tropical cyclones. This
group of climates includes:
Wet equatorial
Monsoon and trade-wind coast
Wet-dry tropical
Dry tropical

Midlatitude climates occupy the polar front zone where warm and cold air masses
conflict producing wave cyclones. This groups of climates includes:
Dry subtropical
Moist subtropical
Mediterranean
Marine west-coast
Dry midlatitude
Moist continental

High latitude climates are dominated by polar and arctic air masses. They are
sources areas of cP, mP, cA, and cAA air masses. Continental polar air meets cA air
along the arctic front zone. This group of climates includes:
Boreal forest
Tundra
Ice Sheet

Dry climates are those in which the potential total annual evaporation greatly
exceeds the annual precipitation amount.

Low latitude climates:

occupy the equatorial zone, much of the tropical zone, and some of the
subtropical zone

include climates that range from very wet to very dry

are influenced by the intertropical convergence zone, tropical easterly wind
systems, and the subtropical high pressure cells

experience traveling low pressure systems such as the easterly wave and tropical
cyclones
 The wet equatorial climate is characterized by:
 dominance of the intertropical convergence zone (ITC)
 mE and mT air masses
 uniform, very warm temperatures in all seasons
 ample precipitation, heaviest when the ITC is nearby
 Monsoon and trade wind coastal climates are characterized by:
 heavy rainfall with strong seasonal patterns
 a larger temperature range than the wet equatorial climate
 dominance of the ITC during the heavy rainfall period and the subtropical high
pressure system during the dry season
 trade wind coast climates are a result of mT and mE air masses blowing onto
coastal areas bringing large amounts of moisture
 the monsoon aspect of these climates is a result of the changing position of the
ITC and reversing pressure gradients
 heavy rainfall is associated with the ITC and an airflow from ocean to land, while
the dry season is associated with airflow off the Asian continent to the ocean
 The wet equatorial, monsoon, and trade wind coastal climates produce low latitude
rainforest with dense vegetation, numerous streams, and a great diversity of plant
and animal life.
 Products and resources of the rainforest include lumber, drugs, rubber, and foods
such as cassava, yams, taro, bananas, plantain, and coconuts.
 The wet-dry tropical climate is characterized by:
 a warm climate but with a more marked temperature range
 during the high sun season, proximity to the ITC brings heavy rains
 during the cooler period, the subtropical high pressure cell produces very dry
conditions
 vegetation adapts to the seasonality of rainfall and is described as rain-green
because it enters a dormant period during the dry season and leafs out and blooms
in the rainy season
 dramatic variations in rainfall are reflected in streamflow which varies from very
low flows to flood-like conditions
 agriculture experiences periodic drought
 The dry tropical climate:
 is dominated by the Subtropical High Pressure Cell
 experiences very low precipitation and intense daytime heating under
predominantly clear skies
 includes many of the world's great deserts
 semi-arid areas on the edges of the desert may have a short wet season. These
steppe areas are transitional from the desert to the wet-dry tropical climate
 Midlatitude and high latitude climates as a group:
 occupy the midlatitude zone, part of the subtropical zone, and extend poleward
into the subarctic latitude zone
 are located mainly in the northern hemisphere
 are affected by the poleward portion of the subtropical high pressure cells, the
westerly wind belt, and the conflict between warm and cold air masses that occurs
along the polar front zone
 The dry subtropical climate:
 is a poleward extension of the dry tropical climate but shows a greater annual
temperature range due to its higher latitude
 experiences a cool or cold season influenced by invasions of air from higher
latitudes
 receives occasional precipitation from midlatitude cyclones
 is divided into arid and semi-arid subtypes
 has more abundant vegetation than the dry tropical climate due to lower
temperatures and slightly more precipitation
 supports plants and animals that have adapted their life cycles to take advantage of
infrequent periods of rain
 supports agriculture only under irrigation
 The moist subtropical climate:
 is created by warm, moist air flowing out of the subtropical high pressure cells
onto the eastern sides of the continents
 has abundant summer rainfall, mainly convectional with an occasional tropical
cyclone
 in Southeast Asia experiences a strong monsoon effect
 receives winter precipitation from wave cyclones while the storm tracks are in
their most southerly position
 abundant yearly precipitation provides ample water for urban and industrial
development but may cause flooding
 supports broadleaf deciduous and evergreen forests and some needle-leaf and pine
forests
 soils are depleted by high rainfall as nutrients are washed out of the soil
 The Mediterranean climate:
 experiences a very dry summer due to migration of the subtropical high-pressure
cell into the area.
 winter is dominated by rainfall provided by CP air masses and cyclonic storms
 has a moderate temperature range
 is limited to narrow coastal zones
 is a pleasant climate for humans although there are problems with water
availability in summer and plants must adapt to the dry summer period
 The marine west coast climate:
 experiences mild temperatures with a small temperature range for its latitude
 is a moist climate with a winter precipitation maximum due to frequent cyclonic
storms; in summer the northward movement of the subtropical high pressure cell
reduces precipitation
 supports needle-leaf forests in the wet mountainous areas of Pacific North America
and deciduous trees in the relatively drier areas in Europe
 The dry midlatitude climate:
 influences the interior regions of North America and Eurasia
 in some areas, the rainshadow effect blocks maritime air masses so drier
continental air masses dominate
 summer rainfall is largely convectional associated with occasional maritime air
masses
 has a strong annual temperature range with warm to hot summers and cold to very
cold winters
 includes arid and semi-arid environments ranging from cold desert to steppes
 soils have a high natural fertility and support a vegetation of short grass prairie
with moisture being the limiting factor
 experienced serious land degradation during the drought years of the 1930s after
vast areas of land in this climate type were broken for agriculture
 The moist continental climate:
 is found in central and eastern North America and Eurasia
 exhibits large seasonal temperature variation as well as strong day-to-day variation
 receives ample precipitation peaking in the summer with mT air masses while the
winter is dominated by cP and cA air masses
 East Asia experiences a monsoon effect which increases summer precipitation
 supports a native vegetation of deciduous forest which grades into tall grass prairie
toward continental interiors
 soils show some leaching and acidity due to an abundance of precipitation
 supports large scale agriculture
 The high latitude climates:
 are located in the westerly wind belt
 are influenced by mP air masses conflicting with cP and cA air masses and wave
cyclones which develop along the arctic-front zone
 experience higher summer precipitation brought in by mT air masses
The boreal forest climate:
 has long, bitterly cold winters and short cool summers
 experiences a very large annual temperature range as a result of its continental
location
 is a source region for cP air masses, and invasions of cA air masses are common
 has low total annual precipitation with a summer precipitation maximum produced
by maritime air masses
 supports a native vegetation of needle leaf trees which supply the pulpwood and
lumber industry; agriculture is practiced along milder coastal areas
 The tundra climate:
 is found along arctic coastal areas
 experiences long severe winters dominated by cP, mP, and cA air masses
 has a smaller temperature range than expected for its latitude due to the
moderating effect of the nearby ocean
 vegetation consists of grasses, sedges, lichens, and some shrubs; species diversity
is low but the number of individuals is high
 soils are poorly developed and are underlain with permafrost
 Permafrost is permanently frozen ground overlain by an active layer that thaws
during the summer.
 The tundra climate is cold enough to create continuous permafrost, frozen ground
with few gaps or interruptions. Discontinuous permafrost occurs in patches and is
found in the boreal forest climate.
 The ice sheet climate:
 is the source region of arctic and antarctic air masses
 occurs on the ice sheets of Greenland and Antarctica and over the Arctic ocean ice
 experiences the lowest mean annual temperature; no month has a mean
temperature above freezing
 has very low precipitation which doesn’t completely melt away because the air is
too cold
 is a very harsh environment devoid of soils and vegetation, and the few species of
animals found in this climate are marine oriented