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 high- pressure 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.