925mb is nonexistant across much of the western US, as the surface (station) pressure at many western US locales is less than 925mb. For example, the station pressure at Amarillo, TX, this morning was 886mb:
Code:
Current filename: /data/upa/2005051012.upa.cvt
Date: 1200Z 10 MAY 05
Searching for kama...
Searching the city database file for: KAMA ...
Date:1200Z 10 MAY 05
Station: KAMA
WMO ident: 72363
Latitude: 35.23
Longitude: -101.70
Elevation: 1099.00
-------------------------------------------------------------------------------
LEV PRES HGHT TEMP DEWP RH DD WETB DIR SPD THETA THE-V THE-W THE-E W
mb m C C % C C deg knt K K K K g/kg
-------------------------------------------------------------------------------
0 1000 31
1 925 717
SFC 886 1099 13.8 1.8 44 12.0 7.6 220 14 297.1 297.9 285.9 311.6 4.92
3 866 1290 20.8 11.8 56 9.0 15.0 225 37 306.3 308.2 293.5 336.7 10.09
4 850 1449 23.2 -7.8 12 31.0 8.4 225 36 310.4 310.9 288.2 318.5 2.50
5 768 2322 17.4 -8.6 16 26.0 5.2 225 28 313.3 313.8 289.3 321.7 2.60
6 700 3102 10.6 -9.4 23 20.0 1.6 235 20 314.2 314.7 289.6 322.9 2.68
7 620 4094 1.0 -14.0 32 15.0 -4.8 231 17 314.3 314.7 289.1 321.2 2.09
8 615 4159 0.6 -9.4 47 10.0 -3.7 228 17 314.6 315.2 290.1 324.4 3.05
9 611 4211 0.0 -14.0 34 14.0 -5.3 225 17 314.5 314.9 289.2 321.4 2.12
10 500 5780 -12.1 -30.1 21 18.0 -16.1 215 27 318.3 318.4 288.9 320.5 0.63
Therefore, even the 850mb level is very near ground (350m per the 12z 5/10/05 sounding). It is for this reason that you really can't use the 850mb chart from any of the models west of the TX/NM and CO/KS borders. Otherwise, the 925mb is just another level consider to be "low-level". The height above ground (or "below" ground) that the 925mb level resides varies, thus, by location AND situation. For example, if you have a very strong low pressure system over the Plains, the 925mb will be very near the ground for areas near the center of the low. Likewise, in cases of a very strong high pressure system, the 925mb will be much higher.
Just remember that the majority of the "surface pressure" maps you look at use sea-level pressure (surface pressure reduced to sea-level / 1013.25 mb). You can view the height of the 925mb level on 12-hourly upper air soundings (radiosondes) for those locations with a surface pressure above 925mb. Also note that the height levels given on most radiosonde sites display the height as corrected to sea-level. For example, take again the 12z 5-10-05 AMA sounding. The SFC pressure is given to be 886mb, and the height displays 1099m. This means that, in the "standard atmosphere" (or at sea-level with a pressure of 1013.25mb), the height of the 886mb pressure level would be, for the given temperature profile, 1099m above sea-level. If you want to know the height of the 850mb layer (or any pressure level) ABOVE GROUND LEVEL (AGL), subtract the height given on the sounding for that particular level (say, 1449m on the AMA sounding at 850mb) by the the "SFC" height (1099m in this case). This tells us that the 850mb height is 1449-1099 = 350m above ground level. The same can be used for any other pressure level or location...[/code]