Sea-Level VS Altimeter

[B Ozanne]

We are having a heated discussion about the difference between sea-level pressure and the altimeter setting.

Can anyone "clearly" explain the difference? I would really appreciate it if you know the difference and not just think you know.

Thanks a lot.

 

[mikegeukes]

altimeter setting—Value of the atmospheric pressure used to adjust the subscale of a pressure altimeter so that it indicates the height of an aircraft above a known reference surface.

sea level pressure—The atmospheric pressure at mean sea level, either directly measured or, most commonly, empirically determined from the observed station pressure.

In regions where the earth's surface is above sea level, it is standard observational practice to reduce the observed surface pressure to the value that would exist at a point at sea level directly below if air of a temperature corresponding to that actually present at the surface were present all the way down to sea level. In actual practice, the mean temperature for the preceding 12 hours is employed, rather than the current temperature. This “reduction of pressure to sea level†is responsible for many anomalies in the pressure field in mountainous areas on the surface synoptic chart.
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Other terms:
actual pressure—The atmospheric pressure at the level of the barometer (elevation of ivory point), as obtained from the observed reading after applying the necessary corrections for temperature, gravity, and instrument errors.
This may or may not be the same as station pressure.

atmospheric pressure—(Also called barometric pressure.) The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the “column†of air lying directly above the point in question.

pressure altimeter—(Also called barometric altimeter.) An aneroid barometer calibrated to convert atmospheric pressure into altitude.

Altimeters use standard atmosphere pressure–height relations in converting pressure into altitude. Therefore, the altimeter shows indicated altitude, which may, and frequently does, differ from the actual altitude. An altimeter may be set to measure altitude from an arbitrarily chosen level. It is common practice to use mean sea level; the level of the constant-pressure surface of 29.92 in. of mercury is also used; and, less frequently, the constant-pressure surface of the pressure at airport height. See altimeter setting, pressure altitude.

station pressure—The atmospheric pressure computed for the level of the station elevation.

Source: AMS Glossary

Mike

 

[B Ozanne]

That's the answer that everyone gives but nobody understands. Can anybody shed some light on that definition?

 

[Glen Romine]

Bill, guess I'll give this a shot and see if I can satisfy your curiosity

First - let's start with what is actually measured at weather stations - atmospheric pressure. As you know, the air pressure descreases rather rapidly with increasing height - so if we were to make a contour plot of station pressures - what we'd really see is a contour map of the terrain - with lower pressures at higher elevations. As you can guess - this isn't very practical for analyzing the surface weather patterns - so a method needed to be created which could remove the effect of the differences in elevation.

Two methods exist for achieving this - first is the altimeter setting, second is reduction to sea level pressure. The latter uses a math equation that requires knowledge of what the mean temperature would be of the air below the station all the way down to sea level elevation - essentially what we think the measured pressure would be if we could dig a hole down to the height of sea level and measure the station pressure there. Bad guesses as to what this temperature profile should look like can results in big errors in this calculation - so in regions of sharp terrain SLP values can still vary markedly and make analysis difficult. The altimeter setting used to be the normal method used before SLP - because temperature readings weren't available 24/7 which were needed to make reasonable guesses for the SLP reduction. So, the altimeter setting has no temperature dependence, but again uses a fancy math formula to try and guess what the air pressure measurement would be if we could move the site down to sea level elevation. It is generally thought that the SLP measurement is a better way to do this adjustment than the altimeter setting method - but not all agree.

Hope this explains it better - but maybe I just made it worse.

Glen

 

[mikegeukes]

Federal Meteorological Handbook No.1
CHAPTER 11: PRESSURE
http://www.ofcm.gov/fmh-1/pdf/ch11.pdf

Mike

 

[B Ozanne]

Glen, thanks I think that does explain it very well....just two different ways to get pressure to "sea-level."

I know exactly why we do this. I am a meteorologist afterall, I work at the Mount Washington Obs (we're at about 23.00" on average). You'd be hard pressed to find an easy way to explain the difference between altimeter setting and SLP.