88 THE STUDY OF PLANT COMMUNITIES • Chapter V 



made up of droplets of water suspended in the air because they are 

 so minute that they do not settle out. Clouds are frequently formed 

 when air is carried upward by convectional currents and is cooled 

 to the dew point as it rises. Cooling and condensation with con- 



FlG. 35. Ocean fog pouring over crest of Coast Range, Oregon.— Photo 

 by IV. S. Cooper. 



sequent cloud formation also result when air is forced upward 

 over a mountain range and from cyclonic disturbances. 



Clouds are classified on the basis of form and position, the termi- 

 nology being derived from an early simple classification in which 

 four types were recognized : cirrus (curly), cumulus (piled up), 

 stratus (flat), and nimbus (rain or storm). Modern systems divide 

 clouds into families, each with its own type of clouds distinguished 

 by descriptive names that are combinations of the old terminol- 

 ogy. 265 For details about clouds and cloud forms, an illustrated 

 manual should be consulted. 261 ' 128 



Precipitatio72.—Fogs and clouds reduce intensity of solar radia- 

 tion that reaches the earth and may thus be of constant, though 

 minor, ecological significance in certain areas. But, of more gen- 

 eral importance, they are the source of precipitation when, be- 

 cause of rapid condensation, their tinv droplets increase in size 

 sufficiently to respond to gravity and fall to the earth. Not all 

 clouds produce rain because convection may not be rapid enough 

 or persistent enough to produce drops of sufficient size. Summer 

 rains are frequently short and heavy because of local, vertically 

 ascending air currents of high velocity. During cooler seasons, rain 



