MadSci Network: Earth Sciences

Re: How do I compensate an altimeter for changes in temperature ?

Date: Mon Sep 26 07:19:55 2005
Posted By: Ken Harding, Science and Operations Officer
Area of science: Earth Sciences
ID: 1127144413.Es

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. 
( 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:

Hope that helps...


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