Pressure level to altitude

[joleenf]

Hi,

How does McIDAS-V convert from pressure level to geometric altitude. I am using some pilot reports and I believe I will need to convert the flight level to the pressure level based a standard atmosphere. From there, I would like to compare to some satellite retrievals. I am not certain if I should try to convert from the plane's reported pressure level based on that standard atmosphere to a geometric altitude, or if I can let McIDAS perform that calculation.

Joleen

[Rick]

I'm not finding anything under the standard list of formulas, but did find this in the FAQ section of the Users Guide:


Q. What is the difference between the Standard Atmosphere and Vis5D pressure to height conversion?
A. McIDAS-V displays pressure surfaces by converting the pressure to a height in some reference system. There are two reference systems that come standard with McIDAS-V - the U.S. Standard Atmosphere and Vis5D. The U.S. Standard Atmosphere only computes has conversions for pressures up to about 80 km. The Vis5D coordinate system uses a logarithmic conversion:

P = 1012.5 * e^( H / -7.2 ) (^ denotes exponentiation)
H = -7.2 * Ln( P / 1012.5 ) (Ln denotes natural log)

so it can be used for heights above 80 km.

You should be able to enter that in as a formula. Let us know if that answers your question.

Rick

[mhiley]

Hi Joleen -
A little more info on the "Standard Atmosphere" option... this conversion is done in the "StandardAtmosphereCoordinateSystem" class in the following source file:

https://github.com/Unidata/IDV/blob/master/src/ucar/visad/quantities/AirPressure.java

See especially lines 400-415, where the pressures in hPa are defined that correspond to a given altitude in meters.

FWIW, if Wikipedia is to be believed, there is no difference between the 1962 version and the 1976 version below 32 km!

-Mike

[joleenf]

Thanks, I think this is good news.

Joleen

[mhiley]

You can use the "StandardAtmosphereCoordinateSystem" class like this:

Code: Select all

from ucar.visad.quantities import AirPressure
cs = AirPressure.StandardAtmosphereCoordinateSystem()
# use fromReference to convert from meters to hPa
print cs.fromReference( [[0.0]] )
# use toReference to convert from hPa to meters
print cs.toReference( [[1013.25]] )