188 FORERT INFLUENCES. 



cent studies, and many different conditions tliat are generally ennmerated by the 

 books as prodnctive of rain have been set aside. 



The following quotation, from a recent paper by Blandford, sets forth with great 

 clearness the opinions that meteorologists are now adopting: 



"As a result of a long study of the rainfall of India, and perhaps no country affords 

 greater advantages for the purpose, I have become convinced that dynamic cooling, 

 if not the sole caiise of rain, is at all events the only cause of any importance, and 

 that all the other causes so frequently appealed to in popular literature on the sub- 

 ject, such as the intermingling of warm and cold air, contact with cold mountain 

 slopes, etc., are either inoperative or relatively insignificant." (Nature, xxxix, 583). 



The ascensional movement of the air requisite to dynamic cooling maybe brought 

 about by three different processes: (1) by couvective currents; (2) by hills and 

 mountains; (3) by cyclonic circulation; and I classify rainfall as convective, oro- 

 graphic, or cyclonic, according as it is due to the first, second, or third of these 

 causes of ascensional movement. Of course in some cases all of these may be opera- 

 tive at the same time, but for the purposes of analysis they will be separately con- 

 sidered. 



In a region of purely convective rainfall, the circulation is primarilj^ vertical, and 

 the moisture evaporated is largely precipitated before being carried away by hori- 

 zontal currents. Therefore an increased evaporation will be followed by an in- 

 creased rainfall. Consequently anj- change in surface condition which increases or 

 diminishes the evaporation will, in such a climate, be followed by a corresponding 

 increase or decrease in precipitation. 



In the case of orographic rainfall, currents that are essentially horizontal are 

 forced to become locally ascending. The moisture evaporated in the region lying in 

 the patli of the current is partially or entirely precipitated over the region where 

 the ascensional movement is developed. Consequently an increased evaporation 

 will, to a greater or less extent, be restored to the basin by an increased precipita- 

 tion. The extent to which this will take place, that is, the proportion of the in- 

 crease of moisture that will be returned to the basin, will depend on the extent and 

 height of the mountains and the relative frequency of the orographic rain-bearing 

 winds. For example, if these winds prevail only half of the time, only half of an 

 increased evaporation can, in general, l)e precipitated as orographic rainfall. It 

 seems quite possible, therefore, with observations showing the amount of change in 

 the evaporation and with observations of the relative frequency of wind direction, 

 to compute with considerable closeness a maximum value which the resulting 

 change in rainfall can not exceed. 



The second point to be considered with respect to orographic rainfall is whether a 

 forested hill or mountain can originate or divert to itself moisture-laden currents 

 which, without a forest cover, would not exist or would give their rain to other 

 localities. Mountains, by means of their heated surfaces, develop upward currents, 

 but when they are forest covered the observations presented by Mr. Fernow in this 

 bulletin show that the air is less heated than when the surface is bare. Forests 

 will, therefore, tend to diminish rather thaji to augment the diurnal currents which 

 set in upwards toward the summit, and which, by dynamic cooling, precipi- 

 tate their moisture. Likewise, there is lacking any sufficient reason to supjiose that 

 lateral currents moving in some other direction will more likely be deflected from 

 their course and diverted toward this mountain summit because it is covered with 

 forest and hence relatively cool. 



The general class called cyclonic rainfall includes a great variety of rain types 

 related to a cyclonic circulation, some of which are as yet by no means well under- 

 stood. 



In the ordinary progressive area of low pressure the cyclonic circulation is largely 

 horizontal, but with an upward component. This upward component produces the 

 usual rainfall of our cvclonic storms. In these storms the horizontal component of 



