34 FOREST INFLUENCES. 



tliau transpiratiou; altogether, the cooliug effect of transpiration up to this time 

 seems to have been overestimated. 



(d) Tlie warming effect of the upper crown cover upon the temperature during the 

 day in a beech growth extends only about 2 m. (6^ feet) above the crowns, and 

 indeed is most pronounced in the warmest mouths. 



(e) The absolute values of the temperature differences effected by the forest are 

 not very important, but depend to a great exteut upou the species of the trees on the 

 one hand and the kind of soil cover of the open country on the other. 



(/) It is remarkable that in the forest the relative humidity increases and decreases 

 with the absolute humidity, whereas it is known that in general, and also at the sta- 

 tion in the open country, these two climatic elements arc inverse. This is accounted 

 for by the fact that the forest is a source of atinosplieric ac^ueous vapor as well as 



of cooling. 



(r/) The results of the investigations at Ried show that the aqueous vapors from 

 the forest often affect the air to a great height. With this increase in the humidity 

 of the atmosphere the chances of preciiiitatiou are increased. 



From tliese observations Dr. Liburiiau draws the following conclu- 

 sions regarding the influence of forests upon the general climate of the 

 region in its vicinity: 



(h) Since the temperature in the forest, to above the crown-cover, is in most cases 

 lower than it is beloAV in the open country, it follows that modifying currents can 

 arise from the forest at any time of the day, especially in a generally still atmos- 

 phere. In summer, duriug the night and toward murniug. differences of tempera- 

 ture can produce a current of cool air from the forest and from the air strata imme- 

 diately above the forest; in the warmer part of the day, however, such currents can 

 be looked for only from below and in the crown cover. 



((■) In the summer half of the year, winds coming from a distance and crossing a 

 forest can be cooled below their original temperature by the influence of the forest 

 considerably in the night and toward morning, but less duriug the day. 



(j) The conclusions which Dr. Li burnau arrives at concerning atmosi)heric humidity 

 are based for the most part on the results of the observations at Ried. It was est.ab- 

 lished that Aviuds from the forest (at least J'rum a beech forest) convey a greater 

 water supply into the neighboring open country. It depends, however, upon the 

 original humidity of winds crossing a forest and upon the distribution of tempera- 

 ture whether or not their humidity will be increased and they will subsequently in- 

 crease the humidity of the adjacent open country. 



Hence Dr. Liburnan concludes that the humidity of warm and dry 

 winds will be increased by the presence of a forest, but that such winds 

 as are already quite thoroughly saturated cau receive no essential 

 addition of aqueous vapor, but only an increase in i-elative iiumidity. 

 It was also observed at Ried that winds coming from the forest to the 

 station in tlie open country have a higher humidity, botli absolute and 

 relative. 



(A) Finally, Dr. Liburnan remarks with reference to the distant influence (of the 

 forest) that this absolute humidity, as increased by the effect of the forest,* remains 

 in the atmosphere as long as it is not lost by condensation, and that, on the other hand, 

 the Increased relative humidity decreases as soon as the forest air reaches the warmer 

 open-country. 



^Within the last two months a further contribution from these stations has been 

 published, discussing more in detail the influence of forest areas on the climatic con- 

 dition of the neighborhood, which publication, unfortunately, could not be obtained 

 in time for a review in this report. ^- E. F. 



