Jökull - 01.12.1953, Blaðsíða 27

Max. 36.8 ft./sec.
Min. 17.2 ft./sec.
Mean of 14 Tests: Roughness Parameter 0.31
cms.
The wind speeds were plotted on a log. scale
of height with linear scale of velocity and again
with logs of both height and velocity. It was con-
cluded that the log. law was applicable for
wind over each of the surfaces.
When plotted on a log of height the lines
through the points cut the ordinate each at a
slightly different value of height but there was
a definite grouping between very close limits for
this value. The mean of these heights at which
the graph shows zero wind velocity is the rough-
ness parameter of the surface and is generally
J/30 of the height of the obstructions on the
surface (Bagnold 1938).
TEMPERATURE.
The 6 thermocouples measuring dry bulb
temperature were read in less than 20 seconds,
which was less than the lag given by the brass
bullet in which each thermocouple was set. .4
mean of 3 sets of readings was used as the
profile of temperature for one test.
Over all areas:
Temperature at 2 m.
Max. 8.6° C.
Min. 2.2° C.
Temperature at 3 cms.
Max. 7.2° C.
Min. 1.1° C.
The figures indicate the relatively small in-
crease in temperature with height in the top
197 cms. of the 2 m air layer. The ice surface was
melting so that it remained at 0° C. through-
out the summer. From the figures it seems
that the very high temperature gradient is con-
tained in the lowest 3 cms. of air above the ice
surface but from the graphs plotted it is obvious
that the greatest temperature difference is to
be found in the lowest one or two millimetres of
air, the plotted curves of temperature appear-
ing not unlike an exponential curve with the
line of temperature merging with the abscissa
(Fig- 1).
Where the dirt covering the ice was a few
millimetres thick an attempt was made to
measure the temperature at the surface of the
dirt layer by making a very small thermocouple
of fine wires (to eliminate most of the heat due
to radiation) and applying these to the surface
using small diameter paxolin tubes as probes,
through which the wire was threaded. The wind
over the surface and the water brought to the
dirt by capillary action did not permit air
temperature to be read in this way so small
pieces of paper were put between the fine
thermocouple junctions and the dirt surface.
This permitted a reading to be taken but the
minimum three readings required to give a
mean for each test wére so difíerent from each
other that these readings have not been used
in plotting the profiles of temperature above
the surface.
The graphs of temperature plotted on a log.
scale of height are not so satisfactory as those
of wind but when plotted on log. scales for both
height and temperature the points are slightly
less coincident with a straight line so that it
has been concluded at this stage that tempera-
ture in the two metre air layer above the ice
surface obeys a log. law.
Temperature below the ice surface was not
measured since any holes drilled to take a
thermometer (or thermocouple) usually fill with
meltwater. It is extremely difficult to measure
ice temperatures from the surface while ablation
is present. Ahlmann (1947) concludes that a
temperate glacier stays at 0° C. during the
summer. It can thus be assumed that there is
no heat conducted to or from the surface of a
temperate glácier from below when the glacier
is melting.
HUMIDITY.
The 6 thermocouples measuring the depres-
sion of the wet bulb below the dry bulb read-
ings were read consecutively in less than 30
seconds but these readings gave more trouble
than any other part of the experiments. The
wet bulbs required longer time to stabilise and
required constant attention. A number of read-
ings were taken of the wet bulbs until fairly
consecutive readings were possible. Three sets
of readings were then taken along with the dry
bulb readings and the means of these were
plotted as with the dry bulbs.
As with the air temperature the greatest
change in vapour pressure was in the lowest
few millimetres. Figures for maximum and
minimum values recorded are not given here
since they would only be misleading at this
25