WEATHER AND CUM ATE 



are several ways in which this cooling may take place, 

 but the most common is that which results from the 

 expansion of an air mass as it rises. The great white, 

 fluffy cloud masses which every one has seen on a 

 warm summer day are a striking illustration of this 

 principle at work. These clouds usually stop at the 

 peak of an ascending mass of warm, moist air. These 

 air masses are warmed in contact with the earth, and 



FIG. 60. MICKOPHOTOliKAPH OF SNOW CRYSTALS 1 



as a result of their increased water-holding capacity, 

 they absorb large quantities of moisture. As soon as 

 the mass begins to expand it cools, and soon the mois- 

 ture begins to condense as a cloud. 



Clouds are also formed when wind moves warm 

 moist air into a region of cooler air where the mois- 

 ture condenses when the cooling has proceeded below 

 the dew point. The well known fogs off Newfoundland 

 are a result of such a cause. Air which has been 

 warmed by the Gulf Stream becomes moisture laden 

 and then blows into the region of the cold Labrador 

 Current. 



The fogs which frequently occur over certain cities, 

 of which London, England, is the best known, are 

 partly condensed moisture and partly smoke. They 

 hang over an area because atmospheric conditions are 

 such as not to move them out. Such fogs as these 

 are a menace not only to trade and shipping but also 

 to health. 



There are many types of clouds. Scientists recog- 

 nize several classifications of cloud forms. Some of 

 these are illustrated in Figure 59. 



Cirrus clouds are wisp-like, delicate, and feathery, 

 usually white and at great altitudes. At times cirrus 

 clouds are composed of ice crystals because of the 

 low temperatures at the high altitude's in which they 

 are formed. 



Stratus clouds, as their name implies, are those 

 which are formed in layers. These clouds are usually 

 formed near the earth and somewhat resemble fog in 

 their appearance. The dark days of late fall and win- 

 ter are often caused by stratus clouds. 



Cumulus clouds are the familiar "fluff-balls" or 

 "wool packs" seen on warm summer days. These 

 cloud forms are usually very thick at the bottom 

 and taper to a peak at the top. At times it is not un- 

 usual to see cumulus clouds that have been torn by 



1 From Snow Crystals by Bentley and Humphreys. McGraw- 

 Hill Book Company. Reproduced by permissioa 



the wind into cotton-like masses which may cover 

 large areas. 



Nimbus clouds are dark, irregular masses from 

 which rain or snow usually falls. They usually have 

 very ragged edges and are almost shapeless. 



Dew and frost result when the air near the earth 

 is cooled below its dew point. On still, clear nights 

 which are cool, the earth cools off very rapidly, losing 

 its heat by radiation. Soon the air immediately in con- 

 tact with the earth has been cooled below its dew 

 point, and the moisture begins to condense on grass 

 and other objects. This moisture is known as dew. 

 Dew is formed only when the temperature is above the 

 freezing point of water, that is 32 degrees Fahren- 

 heit. If the air is cooled below this temperature the 

 moisture is then deposited as frost. Frost forms are 

 often fantastic and beautiful. Snowflakes, too, may 

 have very beautiful shapes. Many photographs of 

 snowflakes have been made and many varied appear- 

 ances found, but always they conform to some type 

 of a six-sided pattern or hexagon. Figure 60 shows 

 some types of snow crystals. 





FIG. 61. HAILSTONES BESIDE TENNIS BALL 



At times in a storm raindrops may 

 pass through a layer of air which is 

 below the freezing point. They are ''-'*& 



frozen into ice pellets and fall to the X^ff 

 ground as sleet. Hail results when 

 raindrops are frozen and then caught FIG 62 CROSS 

 in an upward current of air which SECTION OF A 

 causes the ice pellets to melt on the HAILSTONE 



surface. As they again fall through 

 the colder layer of air another layer of ice is added. 

 This process may repeat several times, building up 

 several layers of ice in the hailstone. Figure 62 shows 

 the layered structure of a hailstone. 



REFERENCES FOR FURTHER STUDY 

 Texts 



Caldwell and Curtis, Science for Today, Chap. 13 

 Clement, Collister, and Thurston, Our Surroundings, Chap. 9 



