But the refinements of the methods employed allow the diurnal
variations both of velocity and altitude to be successfully measured.
The velocity observations confirm the results that have been obtained
from mountain stations--that, though the general travel of the middle
and higher clouds is much greater than that of the surface winds, the
diurnal variation of speed at those levels is the reverse of what
occurs near the ground. The greatest velocity on the earth's surface
is usually about 2 p.m.; whereas the lowest rate of the upper currents
is about midday.
The diurnal variation of height is remarkable, for they find at Upsala
that the mean height of all varieties of clouds rises in the course of
the day, and is higher between 6 and 8 in the evening than either in
the early morning or at midday.
Such are the principal results that have been obtained at Upsala, and
no doubt they surpass any previous work that has been done on the
subject. But whenever we see good results it is worth while to pause a
moment to consider the conditions under which the work has been
developed, and the nature and nurture of the men by whom the research
has been conducted.
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