18 



METEOROLOGY AND CLIMATOLOGY: 



comparison to that of cumulonimbus. The upper 

 surface is dome-shaped and exhibits rounded protu- 

 berances, but the base is nearly flat. In general, each 

 cloud resembles a puff of cotton. 



When the cloud is viewed against the sun, it looks 

 dark except for a bright edge. When light comes from 

 the side, it shows strong contrasts of light and shade. 

 When opposite the sun, the side appears brighter 

 than the edges of the protuberances. 



One variety results from wind "tearing." It has 

 the same general appearance as cumulus but is 

 ragged looking and constantly changing its form. 

 Such clouds are called fractocumulus. Over the land, 

 typical cumulus clouds develop on days of clear skies. 



Cumulonimbus is a heavy mass of cloud with extreme 

 vertical development. The tops are cumuluslike, re- 

 sembling mountains or towers. The uppermost parts 

 often have a cirrus cloud, spreading out in the shape 

 of an anvil. The base is frequently torn (like nimbo- 

 stratus) and gives rise to a layer of very low, ragged 

 clouds {scuds) below it. 



Cumulonimbus clouds are generally associated 

 with rain or snow and sometimes hail. They are 

 often associated with thunderstorms. 



WEATHER AND CLIMATE 



Weather and climate are two aspects of the same 

 thing. Weather refers to the local atmospheric con- 

 ditions at a given time and climate to the sum total of 

 these conditions over a period of time. Both are a 

 direct reflection of the waves of air masses which pro- 

 duce both "High" and "Low" pressure areas, as well 

 as fronts. An examination of these phenomena will 

 provide some understanding of what produces 

 weather and climate. 



PRESSURE AREAS 



Pressure areas are constantly forming and chang- 

 ing because of ever-varying winds. The world winds 

 are produced by the heat of the sun and are directed 

 by the rotation of the earth and position of oceans 

 and continents. These winds, in turn, produce pres- 

 sure areas. 



Pressure areas are either "Highs" or "Lows." 

 "Highs," also called anticyclones, are characterized 

 by winds that spiral outward in a clockwise direction 



in the northern hemisphere. "Lows," collectively 

 called cyclones, feature winds that spiral inward in 

 a counterclockwise movement in the northern 

 hemisphere. Directions of wind spirals are just the 

 opposite in the southern hemisphere (Figure 2.6); 

 "Highs" are counterclockwise and "Lows" are 

 clockwise. 



"Highs" can form anywhere that air cools, sinks, 

 and compresses. As a result of winds spiraling out- 

 ward from a "High," upper air sinks to replace the 

 lost air, the replacing air warming as it sinks and 

 compresses. Under these circumstances, the skies are 

 clear. "Highs" usually occur in polar and horse 

 latitude areas (see Figure 2.4, p. 15). 



top views 



side views 



LOW 



HIGH 



Figure 2.6 "High" and "Low" pressure areas. Arrows suggest the 

 direction of air movement and the vertical lines imply the precipitation 

 often occuring in a "low." 



"Lows" can result from a horizontal, wavelike ac- 

 tion between "Highs" of unlike temperatures. The 

 winds spiraling into these "Lows" rise to colder 

 elevations, cool, and contract. Under such condi- 

 tions, the atmospheric water vapor usually condenses 

 into clouds, and snow or rain may result. "Lows" are 

 characteristic of the doldrums. 



"Lows" of the tropics may develop into severe 

 storms, the tropical cyclones. These cyclones are 

 of less diameter but greater energy than are the 

 extratropical cyclones. In fact, the energy of the 

 tropical cyclones is so great that these storms draw 

 considerable attention from man. Peoples in various 

 parts of the world have special names for them; for 

 example, Americans have hurricanes and peoples of the 

 Far East have typhoons. 



