THE ENVIRONMENT 



305 



Fifth, the angle oi the sun's rays affects the tem- 

 perature of different mountain slopes. For example, 

 the southern slope of a mountain receives more direct 

 solar rays, hence more heat, than does the northern 

 slope. Also, west slopes obtain more heat than east 

 slopes and both get more than north slopes. 



Sixth, soil surface temperatures usually are higher 

 during the day and lower at night than air or subsur- 

 face soil temperatures. This relationship is more pro- 

 nounced at higher altitudes. 



Seventh, at a given latitude air temperatures usu- 

 ally become lower with altitude, approximately 3°F. 

 for every 1000 feet. 



Eighth, wide valleys usually are hotter and drier 

 than narrow valleys. Narrow valleys are more pro- 

 tected from insolation and are more subject to cold 

 air drainage into them from higher areas. 



Ninth, at a given latitude higher and larger moun- 

 tains generally are warmer at a particular altitude 

 than are smaller mountains at the same altitude. 



Tenth, because water reduces temperature ex- 

 tremes, the closer an area is to a body of water the 

 more stable and less extreme are its temperatures. 

 This is the basis for classifying the interior areas with 

 marked temperature extremes as having continental 

 climates and coastal areas with stable temperatures 

 as having oceanic climates. 



Eleventh, the temperature of ocean currents usually 

 reduces the temperature extremes of nearby land. 

 Even cold, last-flowing streams from mountains cool 

 otherwise desert areas some distance from the stream. 



Twelfth, direction of air movement often modifies 

 temperature. Winds from the ocean stabilize tem- 

 peratures for some distance if their movement is unin- 

 terrupted. Winds from mountains into lowlands tend 

 to be cooler than those from low elevations. Winds 

 from polar areas are mostly colder than those from 

 tropical areas. 



Finally, foggy or cloudy areas often are cooler than 

 clear areas; however, foggy regions often maintain 

 heat when clear areas become cooler. 



To summarize, environmental temperature is gen- 

 erally warmer during the day, in summer, near the 

 equator, on plains, on dark-colored objects, in coarse 

 soil, on south-facing slopes, in wide valleys, at sites 

 remote from water, during warm winds, and in places 

 having a sparse ground cover of plants or other in- 

 sulating objects. Temperature is generally cooler at 

 night, in winter, near the poles, on mountains, on 

 light-colored objects, in fine soils, on north-facing 



slopes, in narrow valleys, at sites near water, during 

 cool winds, and in places having a dense ground 

 cover. Temperature is variable from time to time 

 and is higher in the day and lower at night in wide 

 valleys, places remote from water, open areas, and 

 sites featuring dry, clear or calm air. Finally, tem- 

 perature stability with warmer nights and cooler days 

 is found in narrow valleys, locales near large bodies 

 of water, areas of fine soils, sites having dense plant or 

 other surface cover, and places with moist, dusty, 

 cloudy or foggy air. 



TEMPERATURE INVERSION 



Another local temperature phenomenon, called 

 temperature inversion, causes higher elevations t6 

 have a warmer temperature than the surrounding 

 lowlands. Inversion usually takes place at night and 

 is associated with several weather conditions that 

 stimulate the cooling of surface air. Such things as 

 long nights, clear skies, dry air, calm air, and snow 

 cover cause lowlands to lose their heat faster than do 

 the nearby elevations, and the lowlands become 

 colder than the uplands. 



Temperature inversion also can be fortified or 

 caused by heavier, colder, higher air draining down 

 into lowlands and upthrusting the lower, warmer 

 air to higher elevations. No matter which of these 

 two mechanisms brings about inversion, the result- 

 mg warmer upper slopes may have distinctly different 

 vegetation, perhaps even with less resistance to cold 

 than plants in the lowlands. 



ORGANISM REACTIONS 



Because different species of organisms react dif- 

 ferently to the temperature spectrum on earth, tem- 

 perature has a pronounced role in determining which 

 habitats are usable by individual species. The tem- 

 perature range at a given site might be the single fea- 

 ture (limiting factor) that prevents a species' being 

 there. 



Most plants and animals are in various degrees de- 

 pendent upon their external environment, particu- 

 larly the sun. for a direct source of heat. The general 

 exceptions are the birds and mammals, which have 

 internal mechanisms (endothermism) to control and 

 regulate their body heat. Endothermism generally 

 allows these two groups to exist in both colder and 

 warmer places than other organisms can. Also, this 



