CLIMATIC ILLUSTRATIONS. 189 



the circulation is so large thatthe moisture evaporated over one region is precipitated 

 over another. Consequently in regions where rainfall is of this type, an increased 

 evaporation in any region will not be followed by an increased rainfall in that same 

 locality. 



In local thunder storms we have a type of rain related to a cyclonic circulation in 

 which the vertical component often becomes very large, as compared with the lior- 

 izontal component. This predominating vertical component is due to convection and 

 the accompanying rainfall is to be considered as partly or largely convective. Con- 

 vection induces and imitates a cyclonic circulation which may continue after the 

 direct convective action has ceased. It becomes desirable, therefore, to separate the 

 directly convective rainfall, whose amount can be locally increased by increase of 

 evaporation, from the cyclonic rainfall whose amount can not be so increased. An 

 approximate separation can be made when the diurnal periodicity of rain is known ; 

 for an excess of rainfall during the afternoon hours may be classed as convective. 



Secondly, we have to consider the question as to whether cyclonic rain storms can 

 bedetiectedfromone course to anotherby diiference in surface conditions. Evidently 

 great areas of low pressure can not be aftected and so the question is restricted to 

 local cyclonic storms of small area. With respect to these the question is to be 

 answered in the affirmative, for it is entirely in accord with physical principles to 

 suppose that small cyclonic storms, areas of clouds and disturbance and other un- 

 settled masses of air with a progressive motion, will tend to follow paths of high 

 humidity and high temperature, lines of atmospheric weakness which all disturb- 

 ances will seek, just as seismic disturbances follow lines of weakness in the earth's 



crust. 



Whether the air over a forested area as compared with a neighboring unforested 

 area constitutes such a preferable storm path is a question to which a dogmatic an- 

 swer can not now be given. As shown by observation the absolute humidity is in 

 general higher over the forest and the temperature lower than over adjacent open 

 spaces. These arc apparently conflicting conditions— one decreasing, the other in- 

 creasing the relative density and stability of the forest air. It seems hardly proba- 

 ble that the resultant Avill have any .quantitative importance. However, the inves- 

 tigation of the question is a fruitful direction of meteorological inquiry. 



The practical point here to be emphasized is that it is not enough to say that the 

 air over a forest is more humid and that therefore there will probably be more rain- 

 fall. The increased humidity will be quite ineffective unless this moist air can be 

 cooled dynamically over the region itself, and thus condensed and precipitated. 

 This is a necessary additional condition, and the evidence that this will occur in 

 any relative high degree, either by convection currents or by deflected storm paths 

 is yet lacking. 



CLIMATIC ILLUSTRATIONS. 



The equatorial rain belt is the most prominent region with almost exclusively 

 convective rainfall. The Brazilian forest region, the Aruwhini district of central 

 Africa, the Malaysian Archipelago, and the valley of Upper Assam in India are in or 

 near this belt. They have light winds, and the moisture evaporated from the sur- 

 face is precipitated before being carried to any considerable distance by horizontal 

 currents. Under these conditions an increase or decrease in the evaporation will 

 be followed by an increase or decrease in rainfall. But they are the very regions 

 Avhere any nuiterial change in the evaporation appears to be most difficult to effect. 



The evaporation is large. Blanford estimated that for the Aruwhini district 

 probably over half of the rainfall is duo to the direct restoration of the moisture 

 evaporated. The surface is Tnaintaincd in a continual state of saturation and evap- 

 oration proceeds uninterruptedly at a nearly uniforni rate. How will deforestation 

 affect this evaporation? In the first place, a cleared surface will have a higher tem- 

 perature, and the winds will have freer play, both of which results will conduce to 



