Survey of Northeast Greenland. 177 
mined, not by measuring the difference of temperature at an altitude 
of 14 and 16 m but by measuring the temperature in the altitude of 
0 and 30 m. If now in certain cases a sudden jump of temperature 
has taken place in the very altitude of the line of vision, + has 
been incorrectly determined, and the effect of this must be that the 
values of k varied more than the corresponding computed values 
of x. In the case of the lines of vision at the cairns IX and X this 
view can hardly be upheld; it may therefore be supposed, at least 
as far as these lines of vision are concerned, that the chief reason 
of the difference between > and К is not to be sought in the uncer- 
tainty of the determination of r. 
The presuppositions on which the computation of k is based is 
not only that the level surfaces within the distances in question may 
be looked upon as concentric spherical surfaces, but also that the 
density of the air in the case of each of these surfaces in every- 
where the same. In other words, the atmospherical stratification 
should form concentric spherical surfaces coincident with the level 
surfaces. This presupposition, which roughly speaking may be sup- 
posed te be permissible, hardly holds good in the single cases. 
A boundary between atmospheric strata may sometimes become 
visible. This is the simple consequence of a considerable inversion 
of temperature, in that the aqueous vapour condenses in the colder, 
lower-lying stratum; the upper side of the cloud thus coincides with 
the boundary. During later years a great many photographs of 
upper surfaces of clouds have been taken during balloon-trips, which 
photographs generally show a similar wave movement to the one 
to be found in water, only of much greater dimensions. 
The wave movement is often expressed in the form of the clouds, 
as seen from the surface of the earth, and in this manner it has 
become possible to measure the length of the waves, which seems 
to be able to exceed 2000 m. In the lower strata of the atmosphere 
the length of the waves is, however, smaller than in the higher 
strata: for the lowest two kilometres one has, as a mean value of 
the length of the waves, arrived at 200 m!). A special form of wave 
movement may occur where the wind meets an obstacle, as for in- 
stance a mountain range or the top of a mountain. The stratification 
may then bend upwards over the obstacle and resume its normal 
course on the opposite side, and thus it forms a standing wave over 
the obstacle, which fact is found everywhere in mountainous regions, 
but it is particularly well known in Greenland and causes the 
formation of the mushroom-shaped Füôhn-clouds. (The cloud 
1) ALFRED WEGENER: Thermodynamik der Atmosphäre, Leipzig 1911. 
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