Our internal development page also contains 850-mb climatology charts.
The point-based precipitable water plots available below were created using data from the (1) NCDC Radiosonde Database of North America CD-ROMs, 1948-1996/97 and (2) NOAA/ESRL on-line radiosonde archive, 1996/97-present (http://esrl.noaa.gov/raobs/). The radiosonde identifiers on the maps below correspond to active upper-air observing sites; however, the climatology for some of these sites includes two or more stations in order to get a lengthy time series for calculating statistics. For example, the climatology for ABR (Aberdeen, SD) was derived from upper-air observations using ABR and HON (Huron, SD), because from 1953–1995 the upper-air site was at HON and from 1995–present the site has been at ABR. For all but 16 (7) cases, differences in station locations do not exceed 60 (80) miles for these “combined” climatologies. In general, the combining of sites is not expected to be problematic since these plots represent a synoptic signal and not a mesoscale signal.
Updates for the prior year are expected each January (last update, 1/3/2013). Nine new sites were added for the Pacific Region (1/8/2013).
Monthly Top 50 PW values (1.37 MB) for all 99 locations given on the maps below (last update, 1/3/2013).
Gridded Precipitable Water Plots courtesy Scott Lincoln, from the LMRFC.
Usage Note: The data points on each graph represent monthly aggregates—and not the 1st of that month. Assuming linearity, the halfway point between the monthly tick marks along the abscissa could be interpreted as the middle of the month. The tick marks themselves could be interpreted as the end of one month and the beginning of the next month.
The plots were created as follows: (1) questionable soundings were thrown out based on gross (objective and subjective) quality control measures*; (2) precipitable water values were calculated for all available soundings for the period of record, including 00z, 03z, 12z, 15z, and special observations; (3) the data were put into monthly bins; (4) the maximum, minimum, 99th percentile, 75th percentile, 50th percentile (median), and 25th percentile values were obtained for each month; and (5) the mean and standard deviation (SD) were also calculated, and the mean plus twice the SD is plotted on the charts (+2SD). Note that for normal (or Gaussian) distributions, 95% of the values lie within ± two standard deviations of the mean/average value, so when you reach +2SD on the charts, you have a fairly rare event. Since the sample size is quite large (i.e., ~1000 per month), the Gaussian assumption is reasonable.
*Caveat: There are some soundings with bad and/or questionable data in the sounding archive. The automated QC algorithms did not catch all of them, and in fact, may have mistakenly removed some good soundings. This does not adversely affect the climatology, but has implications for extreme values. Nevertheless, the QC algorithm is continually being improved in order to make the climatologies as robust as possible.
The equations that were used for calculating precipitable water (units of cm) are as follows:
a) e = 6.112 * exp [ (17.67 * Td) / (Td + 243.5) ] -- where e is vapor pressure in mb (or hPa) and Td is dewpoint in °C [refer to Bolton (1980) for details]
b) vapor density = [ (e / (Rv * T)] * [10^5 g Pa / kg hPa] -- where e is from above, Rv = 461.5 J/K/kg is the gas constant for water vapor, T is the temperature in K,
and the vapor density is in g/m^3. The conversion factor, 10^5 g Pa / kg hPa, is needed to convert from (hPa kg / J) to (g / m^3). Recall that J = Pa m^3.
c) pw (layer) = [mean vapor density over layer (g/m^3)] * [layer thickness (m)] * [1 / 1,000,000 g/m^3] * [100 cm / 1 m] -- where the layer thickness is simply the distance
between two adjacent sounding levels, the 3rd term is the density of liquid water, and the 4th term converts the pw (layer) from m to cm
d) pw (sfc to 300 mb) = summation of pw (layer) from the sfc to 300 mb for any given sounding -- again, these equations return the pw in cm
The new gridded precipitable water plots, at the link above, were created for the maximum, 99th percentile, 75th percentile, 50th percentile, and +2SD values. A separate GIS point shapefile was created for each of these categories, based on the latitude/longitude values of the sounding locations for each month. The point shapefiles were then interpolated to the gridded raster format using the following specifications:
a) Regularized Spline interpolation method
b) Weight: 0.1
3) Number of points: 6
d) Grid resolution: 0.05 degrees
Due to increasing uncertainty in interpolated data at increasing distances from the point data source, caution should be used when taking a value from the precipitable water maps. Because of this, the border of the continentual Unites States was used to clip the completed grids.
If you have any questions or comments regarding the raw precipitable water data or the point-based plots, please send them to Matthew.Bunkers@noaa.gov. Questions or comments about the gridded data interpolations should be sent to Scott.Lincoln@noaa.gov. Thanks!