8— NOTES ON RAINFALL IN FOREST REGIONS. 

 By W. Reid-Bell, F.R.Met.Soc. 



Professor Schubert, of Vienna, points out that the Silesian rain- 

 chart is to a certain extent a reflection of the relief map of the country, 

 the annual precipitation varying from 600 to 700 mm. in the lowlands 

 to 800 mm. as the edge of the Silesian Highlands is approached, the 

 isohyetals showing tongues projecting into the lowland districts over 

 certain wooded areas. Precipitations of over 800 mm. are only 

 found in the mountains, but probably the zone of maximum precipita- 

 tion lies below the highest summits which in winter often project 

 above the clouds. Professor Schubert combines the districts of che 

 Province into 26 groups, each with approximately equal percentage 

 of forest land, and arranging the groups in 5 divisions, arrives at 

 the following average values : — 



Percentage of Elevation above Sea Rainfall 



Woodland. Level, metres. in mm. 



II 222 650 



21 181 655 



28 271 7x5 



35 516 848 



48 157 646 



and he finds the rainfall in millimetres to be 529 + o.78p + o.57h. 

 where " p " is the percentage of afforestation and " h " the elevation 

 above sea level in metres, so that there is an increment of 0.57 mnj. 

 for each metre of elevation and of 0.78 mm. for each one per cent, 

 additional of forest land in the area. 



In the course of discussion he gives the curve of increase 

 of the rainfall due to afforestation as a curve steep at first 

 and flattening off till it becomes nearly level at 50 % afforestation, 

 so if the Silesian forests, occupying 29% of the country, were 

 increased till they occupied 50% or even more, the rainfall, 

 Professor Schubert concludes, would only be increased by 6 mm. or 

 one per cent. 



A rain chart of the forest region of Oppeln is given, in which 

 the gaugings from Sept. 1900 to June 1904 are reduced by the aid 

 of the foregoing ascertained rate of increase of .57 m.m. per metre 

 elevation, to a uniform elevation of 180 metres above sea level, the 

 results showing an increased annual precipitation within the forest 

 of 50 m.m., as compared with the rainfall in the open country, but 

 one half of this is attributed to the influence of the protection from 

 wind afforded to the gauges within the forest. 



Comparing this with the increase due to elevation, it is con- 

 cluded that the afforestation corresponds to an additional elevation 

 of the land surface of 40 metres, and that probably the woodland 

 exercises no other influence upon the rainfall. 



The author discusses the manner in which the air currents 

 passing over forest areas are diverted upwards and checked, and the 

 consequent effects upon the temperature and pressure resulting in 

 increased precipitation, and concludes that the effect of increase is 



