Sunday, 16 March 2014

The science of bad glacial tripods

The weeks are flying by! ...and among other work this last week, I have been thrown back to a task which I feel reasonably inadequate at performing... that would be metal work.  The last time I was in a workshop, clutching some small metal trinket, I was about 12 years old and in year 8 at school.  I was generally useless and D+T/metalwork/woodwork and the idea of going back to that worried me somewhat.  Flopping anxiously into a workshop of the university with departmental safety boots a size too big, not entirely sure which saw would best fit my needs.  Considering the type of mans man who tends to operate in such an environment (which I guess I'm not), I felt a little useless not knowing the name of all the tools and the best technique for using them....

However, I was saved from this by the fact that I simply had to cut some metal plates in half and was able to use a guillotine to do it in seconds.  The fact that the workshop tech squeezed my bicep afterward just reinforced my discomfort with such encounters..

The point of the whole ordeal is an attempt at stabilizing the tripods I need to hold my temperature loggers in the air, as the ones the department have would likely fall over after someone sneezes on them.  The 13 kilo steel tripods seem to have some stability on a flat surface to which it can bolted to- however, given the uneven nature of a glacier (before you pile as much as a metre or two of avalanche debris on top), it becomes a bit more tricky.

The obvious answer seems to drill the whole thing into the ice and save the hassle.  However, the idea here is to capture the temperature of the air near the surface but at a height that is the intermediate between the 'free air' and the air within the glacier boundary (see awesome drawing).  This Screen height is typically measured at 2 metres above the surface.  Because the surface of ice (assuming it is bare) cannot exceed melting point (0 degrees C ), it affects the air at the near surface that is warmer during the summer melting season.  As a result, this colder surface acts to 'dampen' the temperature changes close to the surface, which does not accurately reflect what is really changing in the air.  Therefore, looking back at our tripod: drilling it in to the ice, it will rarely remain at a constant height above the surface when the ice melts and when snow disappears.

That leaves us with option of a free-standing tripod... which sucks considering the success rate of the same tripods on a previous field season at this glacier.  Furthermore, as the surface experiences a non-uniform movement/melt, tying the tripods to something, could also act to tip it.  Annoying :/ .

As a result, I have had to make my way to a B&Q (which has proved rather challenging to get to larger stores without a car in Newcastle) in order to throw myself into more situations where I feel completely unsure of what I'm doing.  NOTE:  Informing a staff member of what you plan to do does need lead to any progress on finding something suitable when it comes to temperature tripods and glaciers.  Hopefully stabilizing the tripods with half metre strips of metal on its feet proves to be successful, but we shall see between April and June.  I'm half expecting to be rather annoyed come June when I visit the glacier to see my tripods sunbathing on the glacier surface.  An example of the type of metal strip I'm referring to can be vaguely seen in the picture below where all the tripods from the indoor calibration tests were bunched together (slacking with my photos):


But all in all.... the setup of these tripods is very simple.  The loggers can be operated by a monkey (coincidence!), the radiation shields work well the protect the thermistor probes from direct radiation to give an actual indication of air temperature (see the test below), and the stuff is mainly held together with copious amounts of sturdy tape and cable ties at the 2 metre screen height.

 One of the Tinytag loggers (TT3) was tested in the outdoors with one probe housed comfortably in a radiation shield to protect from direct heating, while the other had to bake in the clear skies that we saw during the midweek.  The upper graph shows the temperatures we are referring to until the irregularities at around 10pm on the 13th where it was brought in from the garden.  The clear skies and south facing aspect of this garden mean that conditions were ideal to test the importance of the radiation shields.  As can be seen from the bottom graph, the differences were high during the peak midday hours, showing differences in excess of 4 degrees C at times.  Considering the accuracy of these loggers in stable indoor conditions last week, the differences are significant and would severely hamper any accurate understanding of temperature differences.  Lucky we have all the radiation shields we need :) .

Therefore, I'm not too worried about the accuracy of the data at this point, more the fact if it stays in the place I want it to!

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