Transcript Midlatitude and High-Latitude Climates

Midlatitude and High-Latitude
Climates
Chapter
10
Overview of Midlatitude Climates
The midlatitude climates extend from the subtropical zone
to approximately 55° N and S latitudes.
The midlatitude climates range from those with strong wet
and dry seasons to those with precipitation that is more or
less uniformly distributed through the year.
Temperature cycles also are quite varied (windward west
coasts experience low annual ranges but annual ranges
are large in the continental interiors).
The Dry Subtropical Climate
(Köppen BWh, BWk, BSh, BSk)
The dry subtropical climate is a poleward extension of the
dry tropical climate, but because of the midlatitude location,
the annual temperature range is greater.
There is a distinct cool season at the time of low sun, which
is accentuated by invasions of cold continental polar (cP)
air masses that also bring frontal storms to the region.
As is the case for the dry tropical climate, the dry
subtropical climate type has both arid and semi-arid
subtypes.
The Moist Subtropical Climate
(Köppen Cfa)
Circulation around subtropical high-pressure cells provides
a flow of warm, moist maritime air onto the eastern side of
continents (flow of mT air dominates the moist subtropical
climate).
Summer is usually a time of abundant rainfall, much of it
convectional (occasional tropical cyclones augment
summer precipitation).
Strong monsoon effect in Southeast Asia, with much more
rainfall in summer than in winter.
The Moist Subtropical Climate
(Köppen Cfa)
Summer temperatures are warm, with persistent high
humidity.
Winter precipitation in the moist subtropical climate is
similarly plentiful, and is associated with the passage of
midlatitude cyclones.
Invasions of cP air masses are frequent in winter, bringing
spells of below-freezing temperatures. However, no winter
month has a mean temperature below 0 °C.
The Mediterranean Climate
(Köppen Csa, Csb)
The Mediterranean climate is located along the west coasts
of continents in the latitude range 30° to 45° N and S
(distinguished by its annual precipitation cycle - wet winter
and very dry summer).
This is caused by the poleward movement of the
subtropical high-pressure cells during the summer season,
which brings dry cT air into the region.
The Mediterranean Climate
(Köppen Csa, Csb)
In terms of total annual precipitation, the Mediterranean
climate type varies from arid to humid, depending on
location.
Generally, the closer an area is to the tropics, the stronger
the influence of the subtropical high-pressure cells, and
thus the drier the climate.
The Marine West-Coast Climate
(Köppen Cfb, Cfc)
The marine west-coast climate occurs in midlatitude west
coasts where prevailing westerlies bring moist air onshore
from the adjacent oceans (latitude range 35° to 60° N and S).
Precipitation comes mainly from low-pressure systems
(where the coast is mountainous, annual precipitation is
increased substantially by the orographic effect).
Precipitation is plentiful in all months (often a distinct winter
maximum).
The Marine West-Coast Climate
(Köppen Cfb, Cfc)
In summer, the poleward extension of the subtropical highpressure system leads to a reduction in rainfall.
The annual temperature range is relatively small for
midlatitudes (marine influence keeps winter temperatures
mild, compared with inland locations at equivalent
latitudes).
The Dry Midlatitude Climate
(Köppen BWk, BSk)
The dry midlatitude climate, at a latitude range of about 35°
to 50° N, is limited almost exclusively to the interior regions
of North America and Eurasia.
Typically the region lies in the rain shadow of mountain
ranges (maritime air masses are effectively blocked).
In winter, the climate is dominated by cP air masses; in
summer, a dry continental air mass of local origin is
dominant.
Summer rainfall is mostly convectional and is caused by
occasional invasions of maritime air.
The Dry Midlatitude Climate
(Köppen BWk, BSk)
The temperature cycle is strongly developed, with a large
annual range. Summers are warm to hot, followed by cold
to very cold winters.
Annual precipitation in the dry midlatitude climate is quite
variable and there may be large differences in precipitation
from year to year.
Drought is an ever-present threat in the dry midlatitude
climates, and its occurrence has serious implications for
the region.
The Moist Continental Climate
(Köppen Dfa, Dfb, Dwa, Dwb)
The moist continental climate is restricted to the northern
hemisphere, occurring at lat. 30° to 55° N in central and
eastern parts of North America and Eurasia, and at lat. 45°
to 60° N in Europe.
These regions lie in the polar-front zone; seasonal
temperature contrasts are strong, and day-to-day weather
is highly variable.
Ample precipitation throughout the year is increased in
summer by invading mT air masses.
The Moist Continental Climate
(Köppen Dfa, Dfb, Dwa, Dwb)
In winter, cold cP and cA air masses dominate these
regions.
The moist continental climate becomes progressively drier
westward toward the continental interiors of North America
and Asia.
Overview of High-Latitude Climates
The high-latitude climates are mainly restricted to the
northern hemisphere.
They occupy the northern subarctic and arctic latitude
zones.
Only the ice sheet climate of the polar zones is present in
both hemispheres.
The dynamic wind pattern at high latitudes brings mP air
masses from the northern oceans into conflict with cP and
cA air masses on the continents.
Overview of High-Latitude Climates
The Rossby wave system transports lobes of warmer,
moister air toward the poles in exchange for colder, drier air
that moves toward the equator.
The result of these processes is frequent wave cyclones
produced along a discontinuous and constantly fluctuating
arctic-frontal zone.
In summer, tongues of mT air occasionally reach subarctic
latitudes to interact with polar air masses and yield
significant amounts of precipitation.
The Boreal Forest Climate (Köppen
Dfc, Dfd, Dwc, Dwd)
The boreal forest climate is a continental climate with long,
bitterly cold winters and short, cool summers.
The latitude range for this climate type is from 50° to 70° N.
The boreal forest climate represents the source region
for cool cP air masses, but invasions of cold, dry, stable cA
air masses are frequent at these latitudes.
The annual temperature range in the boreal forest climate
is the largest of any climate type.
The Tundra Climate (Köppen ET)
The tundra climate is dominated by polar (cP, mP) and arctic
(cA) air masses (winters are long and severe).
The nearby oceans moderate winter temperatures, preventing
from falling to the extreme lows found in the continental interior.
There is a short mild season.
Precipitation generally increases in the summer, when more
moisture is available from the ice-free polar seas.
The Tundra Climate (Köppen ET)
Arctic tundra lying beyond the arctic treeline describes both
an environmental region and a type of vegetation
dominated by herbaceous plants, such as grasses and
sedges, together with mosses and lichens, and in some
places small woody shrubs.
The tundra is a treeless zone that borders the open lichen
woodland at the extreme range of the boreal forest.
The Tundra Climate (Köppen ET)
Arctic Permafrost
Soils of the arctic tundra are poorly developed and consist
of freshly broken mineral particles and varying amounts of
partially decomposed plant matter (peat bogs are
numerous).
Because the soil remains permanently frozen not far below
the surface, the summer thaw saturates the soil with water,
especially in areas where drainage is limited by lack of
slope (permafrost).
The Tundra Climate (Köppen ET)
Arctic Permafrost
Continuous permafrost, which extends without gaps or
interruptions under all surface features, coincides largely
with the tundra climate.
Discontinuous permafrost, which occurs in patches
separated by frost-free zones under lakes and rivers.
The Ice Sheet Climate
(Köppen EF)
The ice sheet climate coincides with the source regions of
arctic (A) and antarctic (AA) air masses, situated on the
vast ice sheets of Greenland and Antarctica and over the
polar sea ice of the Arctic Ocean.
The mean annual temperature is much lower than in any
other climate, and no monthly mean exceeds 0 °C. Strong
temperature inversions, caused by radiation loss from the
surface, develop over the ice sheets.
A Look Ahead
Climate exerts strong controls on landform processes, such
as those related to flowing water, glaciers, and wind and
wave action.
Soil formation and soil properties also show close
relationships with regional climate, as do world biomes.