Have you read this?

http://server.gladstonefamily.net/pipermail/wxqc/2013-August/018190.html

Yes, that is part of the reason I started this exercise, the other part is that *I* need true 'raw' barometric sensor 'station pressure' (P

_{stn}) to calculate wetbulb temperatures...and I can NOT get it from either one of my Davis units (VP2 with WL-IP; Envoy8X with WL-USB)!

So, I am forced to "roll" my own, hence this mathematical exercise of trying to 'pull' a

*station pressure* **RABBIT** 'out' of a Davis

*sea level pressure* **HAT**!

Backing 'raw' station pressure (P

_{stn}) out of barometric altimeter pressure (P

_{alt}) is relatively easy because it is a single-function reduction factor that is based

*solely* upon

**height** and

*typical* pressure "lapse-rate" (pressure change vs. height change). There are SIMPLE "lapse-rate"

*approximations*, such as these:

•

**P**_{stn} ~ P_{alt} × (h_{m}/773) ..."lapse-rate" of "Hg-per-

**meter**.

•

**P**_{stn} ~ P_{alt} × (h_{ft}/2536) ..."lapse-rate" of "Hg-per-

**foot**.

•

**P**_{stn} ~ P_{slp} - h_{ft} x (0.001"Hg/ft) ..."lapse-rate" of "-0.001 in.Hg-per-foot[/b].

When starting from sea level pressure (P

_{slp}), there are more precise "lapse-rate"

*approximations*, such as these:

•

**P**_{stn} = P_{slp} - (h_{m} / 8.5) ...Docbee calculation.

•

**P**_{stn} = P_{slp} - (h_{m} / 9.2) ...Sandhurst Weather, UK calculation.

•

**P**_{stn} = P_{slp} × [(288-0.0065×h_{m})/288]^{5.2561} ...NOAA calculation (assumes 15ºC).

•

**P**_{stn} = P_{slp} × [(293-0.0065×h_{m})/293]^{5.255876} ...WMO calculation (assumes 20ºC).

•

**P**_{stn} = P_{slp} × EXP[ -0.119*(h_{m}) - 0.0013*(h_{m})^{2} ] ...where h

_{m} = meters/1000; GLOBE approximation.

However,

none of the above approximations account for temperature, humidity, gravity, or latitude compensations typically applied in SLP calculations.

So far, I've found this equation incorporating temperature compensation (in degrees Kelvin) for converting SLP back to station pressure:

•

**P**_{stn} = P_{slp} × EXP[ -h_{m}/(Kº × 29.263) ] ...where:

**Kº = Cº+273.15º****Source**: Sandhurst Weather, UK, document:

http://www.sandhurstweather.org.uk/barometric.pdfThis equation is *very close*...for example, when Steve Hatchett's

**Vantage Pro Pressure Calculator** gives a station pressure of

**933.9mb** (for conditions: T

_{now} = 100ºF; T

_{12hr} = 72ºF; RH = 14%; H = 2330 ft; VP2 SLP = 29.85"Hg), the equation gives a station pressure of

**935.0mb** -- a difference of just

**1.1mb**!

An 'expanded' version of the above NOAA equation incorporates "two-point

**12-hour ***average* temperature" for converting SLP back to station pressure and yields a station pressure of

**933.7mb** using the same setup conditions:

**Kº = 273.15º + (T**_{now} + T_{12hr})/2 ...both T's in ºC.

**Z = 0.0065×h**_{m} ...height in meters.

•

**P**_{stn} = P_{slp} × (Kº / (Kº + Z))^{5.2561}