Page 325 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 10 Global Climate Systems 289
air mass over the Sahara in Africa shifts northward over the Mediterranean region and blocks maritime air masses and cyclonic storm tracks.
Worldwide, cool offshore ocean currents (the California Current, Canary Current, Peru Current, Benguela Current, and West Australian Current) produce stability in overlying air masses along west coasts, poleward of subtropical high pressure. The world climate map in Figure 10.2 shows Med- iterranean dry-summer climates along the western margins of North America, central Chile, and the southwestern tip of Africa as well as across southern Australia and the Mediter- ranean Basin—the climate’s namesake region. Examine the offshore currents along each of these regions on the map.
Figure 10.11 page 290 compares the climographs of the Mediterranean dry-summer cities of San Francisco,
microthermal Climates (mid and high latitudes, cold winters)
Humid microthermal climates have a winter season with some summer warmth. Here the term microthermal means cool temperate to cold. Approximately 21% of Earth’s land surface is influenced by these climates, equaling about 7% of Earth’s total surface.
These climates occur poleward of the mesothermal climates and experience great temperature ranges related to continentality and air mass conflicts. Temperatures decrease with increasing latitude and toward the interior of continental landmasses and result in intensely cold winters. In contrast to moist-all-year regions (northern tier across the United States; southern Canada; and eastern Europe through the Ural Mountains) is the winter-
dry pattern associated with the Asian dry monsoon and cold air masses.
In Figure 10.2, note the absence of microthermal climates in
the Southern Hemisphere. Because the Southern Hemisphere lacks substantial landmasses, microthermal climates develop only in high- lands. Important causal elements include:
• Increasing seasonality (daylength and Sun altitude) and greater temperature ranges (daily and annually);
• latitudinal effects on insolation and temperature:
summers become cool moving northward, with winters
becoming cold to very cold;
• Upper-air westerly winds and undulating rossby waves, which
bring warmer air northward and colder air southward for
humid Continental
hot-summer Climates
Humid continental hot-summer climates have the warm- est summer temperatures of the microthermal category. In the summer, maritime tropical air masses influence precipitation, which may be consistent throughout the year or have a distinct winter-dry period. In North Amer- ica, frequent weather activity is possible between con- flicting air masses—maritime tropical and continental polar—especially in winter. New York City and Dalian,
California, and Sevilla (Seville), Spain. Coastal maritime effects moderate San Francisco’s climate, producing a cooler summer. The transition to a hot summer occurs no more than 24–32 km inland from San Francisco.
The Mediterranean dry-summer climate brings summer water-balance deficits. Winter precipitation recharges soil moisture, but water use usually ex- hausts soil moisture by late spring. Large-scale agri- culture in this climate requires irrigation, although somesubtropicalfruits,nuts,andvegetablesareuniquely suited to these conditions. Hard-leafed, drought- resistant vegetation, known locally as chaparral in the western United States, is common. (This type of veg- etation in other parts of the world is discussed in Chapter 20.)
cyclonic activity; and convectional thunderstorms from
maritime tropical air masses in summer;
• Continental interiors serving as source regions for intense
continental polar air masses that dominate winter, blocking
cyclonic storms;
• Continental high pressure and related air masses,
increasing from the Ural Mountains eastward to the Pacific Ocean, producing the asian winter-dry pattern.
Microthermal climates have four distinct regimes based on increasing cold with latitude and precipitation variability: humid continental hot-summer (Chicago, New York); humid continental mild-summer (Duluth, Toronto, Moscow); subarctic cool-summer (Churchill); and the formidable extremes of frigid subarctic with very cold winters (Verkhoyansk and northern Siberia).
Churchill, Manitoba
New York, NY
Hot, humid continental Warm, humid continental
Verkhoyansk, Russia
Moscow, Russia
Dalian, China
China (Figure 10.12, page 294), exemplify the two types of hot-summer microthermal climates—moist-all-year and winter-dry. The Dalian climograph demonstrates a dry-winter tendency caused by the intruding cold conti- nental airflow that forces dry monsoon conditions.
Before European settlement, forests covered the humid continental hot-summer climatic region of the United States as far west as the Indiana–Illinois border. Beyond that approximate line, tall-grass prairies extended westward to about the 98th meridian (98° W, in central Kansas) and the approximate location of the 51-cm isohyet
Subarctic, cool summers Subarctic, very cold winters