925 mb questions

1. What's the approximate height of the 925 mb level ?
2. What kind of Td's are needed at 925 mb for good storm developement ?

Thanks - Jon Miller
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:

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



     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]
Thanks for the info. I'm still curious about what 925 mb dew points are desirable for storm developement - i'm trying to get a handle on how to evaluate depth of moisture.
Originally posted by Jon Miller
Thanks for the info. I'm still curious about what 925 mb dew points are desirable for storm developement - i'm trying to get a handle on how to evaluate depth of moisture.

It all really depends on location. Typically, you want the moisture depth to be at least 75mb, preferably near and more than 100mb. So, even for those locations near sea-level, the 925mb level should still fall in the lowest 100mb (1010mb-100mb=910mb, above the 925mb level). Assuming a well-mixed boundary layer, the temperature profile decrease dry adiabatically with height while the moisture profile follows the mixing ratio (well-mixed implies constant mixing ratio with height). The constant-mixing-ratio line almost follows the constant-temperature line. In other words, in a well-mixed boundary layer, the dewpoint at 925mb should be a few degrees less than the dewpoint at the surface. If you're looking at a place which has a surface pressure of 940mb, the 925mb dewpoint should very well be very near the surface dewpoint. Therefore, look for 925mb tds near the same magnitude as sfc tds (minus a few degrees).

For what it's worth, many mets prefer looking at moisture on isentropic surface rather than constant-pressure surface. Since unsaturated air follows isentropic surfaces (not constant pressure surfaces in cases of temperature gradients), one can visualize moisture advection and moisture depth considerably easier by the use of isentropic maps rather than 925mb, 850mb, etc, maps. Unfortunately, isentropic charts are not easy to come by, though NCEP model forecasts (NAM, GFS, etc) of isentropic surfaces can be found on Earl Barker's website ( http://www.wxcaster.com/isentropic.htm ).