Educational - Sea level

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Sea level reductions of pressure

Before I begin, the bottom line, is that you can probably figure out how to do this yourself with very little difference from the NWS calculation.

Yes, the NWS uses an average of the current temperature and the temperature 12 hours ago ('ts' in the discussion below). If no temperature was observed 12hours ago, then the msl pressure is coded as missing (this is why part time stations do not report msl pressure likely).

The temperature and lapse rate are the more interesting aspects of the calculation.

Virtual temperature is used in order to compute the ratio between the pressures at the top and bottom of a column of air using the hypsometric equation. i.e...

Po/P = r = 10^(KH/Tmv)

Where:
Po = pressure at column bottom (msl pressure)
P = pressure at column top (station pressure)
r = "pressure reduction ratio"
K = hypsometric constant=0.0266895 deg R/gpm
H = depth of the air column in geopotential meters (station elevation)
Tmv = MEAN virtual temperature of the AIR COLUMN in degrees Rankine,
where T (deg R) = 459.67 + T (deg F)

(From what I've read, I think they used deg R instead of deg C for no other reason than to avoid using deg C in the tables!)

Four terms are used to compute Tmv, each term is in deg F.1. ts=station temperature argument (12 hour average temperature discussed abv) 2. standard lapse rate correction = aH/2, where a is the lapse rate = 0.0117 deg F/gpm = 6.5 deg C/km H is the column depth (station elevation)3. humidity correction = es*Ch, to account for moist air, where es is the surface vapor pressure in mb, calculated operationally as a function of ts. The function is computed using climatological values of e at various ts values at nearby, representative stations. Ch is correction factor used to extrapolate es to the mean e of the column. Ch is a known function of the station elevation (H). A brief table follows:

                H (gpm)         Ch (deg F/mb)
                -------         -------------
                0               0.1935
                100             0.1975
                200             0.2017
                500             0.2148
                1000            0.2387
                1500            0.2657
                2000            0.29624. 

4. "Correction for Plateau Effect and Local Lapse Rate Anomaly", F which is a function of ts and the annual normal station temperature. This term can become fairly large in areas with high, irregular elevation and very high or low ts values. Only stations in mountainous areas use this correction. A short table for stations above 1000 ft follows:

            ts= -20     0       20      40      60      80      100 (deg F)
Annual  -----------------------------------------------------------------
normal  45      24.9    18.6    11.1    2.4     -7.6    -18.8   -31.1
temp    50      26.9    20.7    13.4    4.7     -5.1    -16.2   -28.4
(deg F) 55      29.0    22.9    15.7    7.2     -2.5    -13.6   -25.7
        60      31.0    25.1    18.0    9.6     0.0     -10.9   -23.0
        65      33.1    27.3    20.3    12.1    2.6     -8.2    -20.3

The 4 terms are combined to produce Tmv in deg R:

       Tmv = 459.7 deg R + ts + aH/2 + es*Ch + F(ts)

Plug this back into the above equation to get Po (msl pressure), that's it. In practice, the tables for values of r given ts are made in advance for eachstation by NWS Headquarters, so the observer only has to do a simple multi-plication (r*ts) to get msl pressure.