» Continentality

Published: 5-04-2010, 16:47

Continentality

Continentality is a climatic effect that results from a continental interior being insulated from oceanic influences. Winds and air masses of moderate temperature that originate over oceans move onshore to diminish differences in winter and summer temperatures in coastal areas of continents. Interiors of continents are too distant to experience the moderating effect. As a result, climates of continental interiors have great seasonal differences of temperatures and a mean annual temperature below the latitudinal average. Continental interior climates also tend to be subhumid to arid, as oceans are primary sources of atmospheric moisture. Alexander von Humboldt, the celebrated 19th-century German geographer, was the first person to provide empirical proof that the climate of continental interiors differs from that of coasts. Through letter writing and extensive travels, he collected enough weather station data to draw the first world map of isotherms. (Isotherms are lines that join points of equal temperatures, and their patterns on a map reveal temperature trends over distance.) After examining the map, von Humboldt concluded that continental climates are colder in winter and warmer in summer than places near oceans at the same latitude. He popularized the term continentality to describe the effect that a location’s distance from an ocean has on its mean annual range of temperature. Since von Humboldt’s time, geographers and scientists have used other climatic elements to measure continentality, including precipitation, wind, and air mass frequency. Nevertheless, the most widely accepted criterion remains the mean annual range of temperature, which is the difference between a location’s warmest and coldest average monthly temperatures. This facile calculation is a clear-cut expression of how climates of inland and coastal areas differ. The spatial variation of the mean annual range of temperature is a result of differential heating of air by land and water. The differences in summer and winter temperatures over land and water are greatest. In summer, air temperature over the ocean is cooler than over the land. There are several reasons for the difference. First, chemically, water has a higher specific heat than land does, meaning water must absorb more solar energy than rock and soil in order to be raised the same number of degrees in temperature. Water also heats more slowly than land does because solar energy passes tens of meters below the water surface before it is absorbed completely. Land heats up faster than water because heat conducts only a few inches (centimeters) and feet (meters) into the ground. Additionally, water stores heat at even greater depths than land does, as downwelling of water distributes absorbed energy hundreds of meters below the surface. Water undergoes high rates of evaporation in summer, which cools air temperatures by transferring sensible heat of water into air as nonsensible heat of vaporization. In winter, the ocean is still in the slow process of transferring heat energy stored during the summer into the atmosphere. The belated transfer makes air over the ocean relatively warmer than that over land; conversely, winter air over land is cooler because summer heat is stored closer to the ground surface, causing land in winter to cool air faster and to a lower temperature than water does. A main influence on the mean annual range of temperature on land is distance from the oceans. Three cities on the North European Plain that are progressively farther inland illustrate the effect. Antwerp, Belgium, is nearest the Atlantic Ocean; Warsaw, Poland, is a mid-distance away; and Saratov, Russia, is farthest from the ocean. In this example, latitude, which affects sun angle and therefore intensity of solar energy, does not explain differences in mean annual temperature ranges of the cities, as the three are less than 1-degree latitude apart (between 51 degrees N to 52 degrees N). Therefore, distance from the ocean is the only major factor that could explain the differences. The plain’s open terrain gives all three cities potential access to the moderating effects of the westerlies, a belt winds that blows across the Atlantic Ocean and carries marine heat and water vapor inland. However, temperature data show that the size of the mean annual temperature range increases with distance from the ocean. Antwerp’s mean range is 26.6 degrees F (14.8 degrees C); Warsaw’s is 35 degrees F (19.5 degrees C); and Saratov’s range is 59.3 degrees F (32.9 degrees C). Antwerp has a moderate temperature range because it is within 50 mi (80 km) of the sea. Saratov’s range is more than double Antwerp’s because it is 1,600 mi (2,700 km) inland. Warsaw’s temperature swing is between the two extremes, owing to its middle- distance location of 600 mi (950 km) from the ocean. Mountains influence continentality by limiting the distance that maritime winds can enter continents. For example, Nevada is coastal California’s inland neighbor and not far from the Pacific Ocean. However, the Sierra Nevada of California add to the continentality of Nevada by blocking marine air of the westerlies from the ocean. The absence of the ocean’s humidity in Nevada leads to fewer clouds there. Clearer skies means Nevada has greater solar heating in summer and radiation cooling in winter than it would have if mountains were not present to block ocean air from entering. Additionally, blocking of moist ocean air causes aridity in Nevada. Latitude also influences continentality. In the tropics, annual temperature swings usually are small even in continental interiors. In middle latitudes of the Northern Hemisphere, the continental effect is an overriding factor in climates of continents, as average annual temperature ranges increase with increasing latitude there. In the middle latitudes of the Southern Hemisphere, the effect of continentality is smaller, as continental areas are less massive in that part of the world. In areas poleward of the middle latitudes, the polar night and ice cover introduce complications, so it is difficult to separate influences of land or sea in terms of temperature or other climate variables. Climate scientists have developed several formulas to correct temperature range for polar latitudes.
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