STOEM-AEEAS OVEE FOEESTS : DESIGNATIONS OF CLIMATE. 265 



dry, except as upward currents may carry moist air from below, and 

 that raiu-falls are mostly derived from low clouds that have derived their 

 moisture from the earth near by ^ It is accordingly found, that the rain- 

 fall may increase or diminish in amount, as these storm-areas move along 

 over moist or dry regions. With respect to these effects, the statement 

 is made^ that "the rai^idity of movement is a physical question that 

 cannot yet be solved numerically, but, in general, it is known that for 

 the same temperature, the moister the air or the greater its relative 

 humidity, the greater will be the effect of a general diminution of pres- 

 sure or temperature in inducing condensation and a further fall of the 

 barometer." 



From these facts it follows, that as extensive areas of woodland, over 

 which the air is relatively more humid than over open fields, present 

 the conditions most needed for the precipitation of rain, upon the pas- 

 sage of areas of low barometric pressure, their i^resence tends to diminish 

 this pressure and the temperature, both of which imply the further 

 formation of rain. " Wherever the moister air exists, there the conden- 

 sation will take place the more rapidly, there the barometer will also 

 fall the most rapidly, and thither the storm will be' strongest driven. 

 Great storms naturally, therefore, move more rapidly up towards the 

 lakes, and hang tenaciously over them, and move slowly away from 

 them."^ It is found that the lovvest barometer, in a i)assing storm-area, is 

 felt after the rain has begun to fall, and that " the presence of a surface 

 area of dry air is oftentimes sufficient to dissipate these storms, or to 

 cause them to retire into the cloud-regions." 



DESIGNATIONS OF CLIMATE : — INCLINATIONS OF SURFACE. 



It is convenient to have a definition of degrees, in speaking of the 

 character of a climate in its reference to sylviculture, and to this end 

 the following terms are specified, by an approved writer upon forestry:* 



Warm climates are those in which the olive, the fig, and cork tree will 

 grow spontaneously. 



Mild climates produce the vine, the almond, and the peach in the open 

 air, with full success, as also all kinds of fruit-trees and culinary vegeta- 

 bles. Acorns and beech-nuts grow in great abundance every two or three 

 years. 



In temperate climates fruit-trees and garden vegetables generally thrive 

 well. In the forest we see all forest trees growing, except such as are 

 limited to warm climates. Acorns and beech-nuts abound once in six 

 or eight years. 



In rude climates the culture of fruit-trees and delicate garden vegeta- 

 bles is difficult. In the forest, resinous trees more abound, and years 

 of fruitful growth of acorns, &c., are rare. 



In very rude climates buckwheat, oats, and potatoes are the principal 

 harvests that come to maturity. The prevaling kinds of timber are the 

 firs, the larch, the birch, and the beech, but the latter is almost always 

 poorly developed. In plains, these designations sufficiently characterize 

 the general climate, as respects forests, and we shall but rarely have 

 occasion to mention exceptions of temperature in certain districts. 

 Upon mountains, however, everything depends upon elevation and 

 exposure, on account of the modifications which they produce, and we 

 may pass through every degree in rising a few thousand feet. 



1 Circular of Signal Office, War Department, entitled Practical use of Meteorological 

 Beports and Weather Maps, p. 25. 

 Ud., p. 36. 

 3/6., p. 32. 

 ^Cours d'Amenagement des Forets, ly R. Nanqiiette (1860), p. 12. 



