THE ATMOSPHERE, WEATHER, AND CLIMATE 



15 



pathways. Any object, natural or man-made, causes 

 a barrier that in various degrees modifies wind path- 

 ways. 



Winds are controlled on a planetary scale by the 

 heat of the sun and the rotation of the earth. Solar 

 heat causes pressure differences and winds to equalize 

 the differences. The rotation of the earth steers the 

 major patterns of winds over the globe. Although 

 these patterns show various modifications, winds of 

 the northern hemisphere generally move clockwise 

 and winds of the southern hemisphere counterclock- 

 wise. 



As one might expect, world winds have a pro- 

 nounced effect upon the environment of our planet. 

 However, it amounts to no more than the mixing of 

 warm tropical air and cool temperate air, a condi- 

 tion that prevents violent temperature extremes be- 

 tween the poles and the equator. 



Examination of the pattern of world air circula- 

 tion indicates how temperature and rotation bring 

 about this pattern (Figure 2.4). Warm air of low 

 pressure rises at the equator and moves pokward. 

 These equatorial doldrums, winds with little lateral 

 movement, are influenced by the earth's rotation. 

 When rotation moves high levels of these winds east- 

 ward, they are piled up, creating a high-pressure 

 area near latitude 30° Some of the air is immediately 

 forced downward, reaches the surface, and is modi- 



■^ STRATOSPHERE ^- 



TROPOPAUSE 



TROPOSPHERE' 



Figure 2.4 Lower atmosphere layers and world winds. Arrows give 

 the directions and interrelationships of surface and above-surface winds 

 in the troposphere. 



fied by the earth's rotation. In the northern 

 hemisphere, the earth's rotation produces a con- 

 tinuous air movement to the right, in clockwise 

 motion, resulting in the northeastern trades south of the 

 high-pressure area and the prevailing westerlies to the 

 north. South of the equator, counterclockwise mo- 

 tion results in the southeast trades and another band 

 of prevailing westerlies. In both hemispheres some of 

 the upper air continues poleward, cools, and sinks in 

 the vicinity of the pole. The prevailing northern 

 clockwise and southern counterclockwise motions re- 

 sult in the polar norlheasterlies and the southeasterlies, 

 respectively. 



CLOUDS 



A cloud is visible water vapor, occurring at various 

 heights regulated by winds, temperature, and the sun. 

 However, clouds contain little water by weight, owing 

 to the vapor form of the water. In other words, there 

 is potentially little liquid water in clouds. If all the 

 clouds covering the earth were to shed their moisture 

 simultaneously, the resulting rainfall would add only 

 about an inch of water to the entire surface. In spite 

 of the insignificance of their size and moisture con- 

 tent, clouds are so distributed that they prevent dust 

 storms and temperature extremes from killing us, al- 

 low no more than a 1 5 per cent loss of solar radiation 

 to space, and announce future weather with some de- 

 gree of accuracy. Weather forecasting is possible be- 

 cause clouds are often found in weather phenomena 

 called fronts (see below). 



There are two general methods of cloud formation: 

 condensation and convection. Condensation of water 

 fairly close to the ground produces the so-called 

 "bathroom type" of clouds. The word "bathroom" 

 alludes to the fact that water vapor condenses in the 

 same general manner in both situations. These 

 clouds form when warm air is blown over a cool sur- 

 face or up a mountain slope, when precipitation falls 

 through warm air, or when heat is lost rapidly from 

 the land (e.g., the blowing of warm air from the land 

 over cool water or ice, and the cooling of air over the 

 land after sunset). Clouds formed by condensation 

 usually are found below 20,000 feet and most clouds 

 below 20,000 feet are formed in this manner. 



Cloud formation by convection involves the rising 

 and cooling of air masses. Convection clouds are 

 usually over 20,000 feet in altitude, and frequently 

 their water vapor has frozen to ice crystals. 



