Jökull


Jökull - 01.12.1953, Síða 27

Jökull - 01.12.1953, Síða 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

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