This report gives details of a series of experiments carried out in the Bavarian Alps by Drs. Kirschner and Rimkus of the Forschungsinstitut für Wasserbau und Wasserkraft in Munich. The apparatus used was a specially designed snow-balance which was built in a pit. The “pan” of the balance consisted of a sampling table (6.25 m.2) exactly level with the surface of the ground. The table was insulated from below and so arranged that the melt water could run through a pipe into a sampling vessel also standing on the balance. The authors claim that with this instrument they have, so far as is possible, reached natural conditions for evaporation and melting.
The weight of the snow on the table together with the melt water was measured every day at 7.30 a.m. Measurements were usually made only on days without precipitation, when conditions for evaporation were most favourable.
If the weight of the snow on the table one day is W 1 and 24 hours later W 2, then
where +G represents the evaporation which has taken place, −G the condensation during the same interval.
The authors found that only an extremely small and practically negligible amount of snow was evaporated during the seven winters (1937–38 and 1944–45). The condensation was also quite small. They also found that throughout all the winters, independently of temperature, the snow cover was melting from below, which means that there is a run-off of melting water during the whole winter. From this they conclude that the common conception, that the snow will keep its water content until the spring thaw, is wrong, since an appreciable amount of water had run off during the cold season.
These experiments are interesting, especially from a hydrological viewpoint, but one would have liked them to have been carried out in conjunction with meteorological observations. We know from many experiments that under special conditions evaporation from snow can be considerable. G. Seligman, for instance, in his book Snow Structure and Ski Fields, Chapter 5, has discussed his own experiments and reviewed some others which certainly show that evaporation under special circumstances is very high. We also know on theoretical grounds that evaporation can only take place if the vapour pressure is decreasing from the snow surface upwards. If the opposite is the case condensation will occur.
As we do not know anything about the temperature and moisture conditions under which the experiments discussed above were carried out, it is very difficult to determine the reasons for the low evaporation. It seems reasonable to assume, however, that the temperature was generally below 0° C. Since 680 calories are needed to evaporate 1 gramme of snow it is understandable that the lower the temperature the better must be the convectional or insolation conditions to obtain any evaporation at all. To get evaporation we need relatively dry air above the snow surface and also heat from insolation, generally from a clear sky or by a strong wind causing great turbulence. It is of course quite possible that such conditions would not occur in the region where the experiments were carried out until the spring, when stronger insolation would undoubtedly favour evaporation.
The authors cannot, therefore, draw the general conclusion that there is no snow evaporation in the Alps. It is not even possible to state that it does not take place during the winter, because such conditions as strong föhn winds must cause evaporation even in winter. Therefore if a thorough understanding of the results is to be reached, experiments for the study of the factors involved in ablation must go hand in hand with the study of the meteorological conditions.
It can therefore be said that the experiments made by the German scientists do not contradict any of the previous results that have been obtained in glaciological experiments. For hydrological purposes the results may of course be of practical value, but because of the intimate relationship between evaporation and other meteorological factors mentioned above, it seems dangerous to suggest that the results are valid for regions outside a rather small area centred around the place where the experiments were carried out.
