Atmosphere around Goatfell close to hydrostatic balance
Created on Wednesday, 21 September 2011 Written by pre0fmp
Written by Cumulus
The atmosphere around Goatfell on the Isle of Arran in Scotland has been found to be close to hydrostatic balance, following the analysis of field measurements taken by a team of PhD students ascending the mountain.
The measurements were taken as part of the 2011 National Centre for Atmospheric Science (NCAS) Summer School. Students from various universities made measurements whilst hiking up the hill. The aim was to form a vertical profile of the atmosphere up the mountain every twenty-metres in altitude.
The students worked in five groups, all groups took a range of atmospheric measurements every hundred metres in altitude. Each group was also assigned a section of the mountain in which to take measurements every twenty metres, so that the results from all groups combined gave a vertical profile of the mountain at twenty-metre resolution.
The hydrostatic balance approximation states that the pressure at a given altitude is equal to the weight of the air above it, so that altitude can be calculated from just temperature and pressure. The Cumulus group, who analysed the data, calculated altitude from their own pressure and temperature readings every hundred metres and found the height of the mountain, assuming hydrostatic balance, to be 853 metres.
Using temperature and pressure readings every twenty metres, they then found the height to be 880 metres. The reason for the difference is that hydrostatic balance assumes constant temperature in the layer of the atmosphere being measured; thus, the division into smaller layers generates less error. These figures suggest a convergence towards 874 metres, the known height of the mountain, as the vertical resolution of measurements is increased and thus confirmation of the near-hydrostatic balance around the mountain.
These results are surprising, given the changeable flow observed around the mountain. Wind speed and direction varied drastically, depending on the specific location. While the group was in the forested section of the mountain, there was little wind but, after passing through it, wind speeds of up to ten metres per second were measured. Winds were also observed from a wide range of directions.
Weather conditions changed a great deal on the mountain; at the foot, there was sunshine and clear skies but higher up the group found itself frequently ascending through cloud and rain. These changes on such a small spatial scale are indicative of a highly turbulent atmosphere caused by the interaction of the large-scale flow with the mountain surface. As pointed out by Professor Alan Blyth of NCAS, "The hydrostatic balance approximation should hold less precisely in the presence of a highly turbulent atmosphere where the flow is deformed horizontally and vertically on such a small scale." It is thus useful for researchers and forecasters alike to know that such a simple balance approximation holds even in mountainous areas with highly complex surface geometry.