|MadSci Network: Earth Sciences|
Prof. Prida, Actually, your altimeter is working perfectly. As long as it is not located in direct sunshine, temperature changes will not affect the accuracy of the instrument. As temperature changes, the atmospheric pressure also changes. When you observe your altimeter reading different values, it is because your relative height above sea level has changed. In this sense, your altimeter is simply a barometer that reads altitude instead of pressure. Changes to your relative height above sea level can be caused by pressure systems (high or low) moving through the area, changes in temperature (as you have observed), and also atmospheric tides. The American Meteorological Glossary defines atmospheric tides as: "Atmospheric tide—(Also called atmospheric oscillation.) Defined in analogy to the oceanic tide as an atmospheric motion of the scale of the earth, in which vertical accelerations are neglected (but compressibility is taken into account). Both the sun and moon produce atmospheric tides, and there exist both gravitational tides and thermal tides. The harmonic component of greatest amplitude, the 12-hour or semidiurnal solar atmospheric tide, is both gravitational and thermal in origin, the fact that it is greater than the corresponding lunar atmospheric tide being ascribed usually to a resonance in the atmosphere with a free period very close to the tidal period. Other tides of 6, 8, 12, and 24 hours have been observed." Another reference for atmospheric tides is "Chapman, S., 1951: Compendium of Meteorology, 510–530". Mountain climbers use altimeters. They note the altitude reading before going to sleep at night, and again in the morning. If the readings are similar, the pressure hasn't changed. If the readings indicate a lower altitude, the pressure has risen overnight, often a sign of good or improving weather. If the altimeter reads higher, the pressure has dropped, often indicating an approaching storm. In our office, we uas an instrument called a microbarograph. (http://www.pa.op.dlr.de/cleocd/microbar/s.htm) It measures pressure changes constantly at a very high resolution. Atmospheric pressure changes on the scale of minutes even in 'good' weather. At my location, we are 1300 feet above sea level. Yet, we report sea-level pressure. Doing this is a process called reduction to sea level. Weather observation stations all report sea level pressure as a standard so gradients in pressure are atmospheric, not just a reflection of lower pressure due to higher terrain. This reduction to sea level involves average temperatures in the proceeding 12 hours and a correction for the actual elevation of the station. More information on this may be found (including the equation to reduce pressure to sea level) at: http://www.crh.noaa.gov/un r/edusafe/mslp/ Hope that helps... Ken
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