Friday, July 25, 2014

Precipitable Water

As most readers of this blog know, precipitable water (PW) is a measure of the amount of moisture that is contained in a column of air extending from the surface to about 300 millibars. It also represents a measure of the volume of water that can contribute to a precipitation event. The PW amount is calculated from upper air balloon soundings released from each of fourteen sites around the state. It stands to reason that more moisture in the atmosphere will lead to more precipitation. In fact, anomalously high PW values are frequently noted in Area Forecast Discussions as indicators of heavy rain potential. So, let's put this to the test.

Since this June-July has been the wettest on record for Fairbanks (by far) I wanted to see if a correlation existed between the average daily PW value and the precipitation totals during this two month period. Using a June-July time period also has the advantage of eliminating snow events and it is also the season of peak thunderstorm activity.

When looking at it on an annual scale (June-July constituting a single year) there is no correlation between average PW over the 61-day period and the total rainfall over the same period (see Figure 1). For Fairbanks, this June-July is already the wettest on record but the average daily PW has been unremarkable.

Figure 1. Average June-July PW (orange) and total June-July precipitation (green) in Fairbanks between 1971 and 2014.

When we group days by the amount of precipitation that fell and calculate an average of those days, a strong correlation exists (see Figure 2). Days with no precipitation or only a Trace have substantially lower PW than those days with over 0.50" of precipitation. The three days this month with over 1.00" of rain in Fairbanks had a PW average of 1.16".

The apparent discrepancy between the two charts is primarily a result of the number of days with precipitation not taken into account in the first graph. If most of the rain falls in 3 or 4 days, the PW from those days is averaged with the PW from the other 57 or 58 days and thus blends in with the other values.

Figure 2. Average June-July PW value grouped by observed precipitation amounts in Fairbanks between 1971 and 2014.

Note: Here is a site ( ) that has PW climatology for all upper air stations in the U.S.


  1. It would seem that it takes at least 'two or three to tango' when it comes to major widespread rain events. Moisture (PW?) and the ability of the atmosphere to retain it as influenced by air temperature may be some participants. A non-standard lapse rate and CAPE values likely play their hand as well some days, especially if thunderstorms are about.

    So my question : Which condition(s) prevailed during the major rain events this summer? I'm going to guess typical Occlusions also played a role at times.


  2. Another idea: supposedly Alaska has been getting wetter over the last few decades. Is there any correlation with increasing precipitation and increasing PW? Basically, how much of increased precipitation is from added moisture and now much is from different precipitation drivers (more cold fronts, occulations, etc.)? Of course this is assuming the data isn't too random to make out.

  3. I had an interesting conversation with the Aquatic Ecologist linked below today. It dealt with studies currently underway on Federal Lands by NPS scientists. It ties into the potential variability of wetness Eric mentions.


  4. Those are interesting questions Gary and Eric. There are certainly a list of contributing factors for heavy rainfall events and PW, in my opinion, isn't much of a driver for precipitation frequency but it does seem to turn a 0.2" rain into a 0.3" rain. My recollection is that this July's heavy rainfall events occurred with an upper level low in the Gulf of Alaska and short waves rotating in from the east. The cold air aloft probably increased the instability and there may have been a thermal trough at the surface but I could easily be mistaken about that. As stated in the blog post, the three largest rainfall days had an average PW of over 1" (max since 1971 in June/July is 1.4").

    As for long-term PW tracking, the balloon data that I used selectively filters out a lot of data prior to 1971 so it is difficult to calculate PWs prior to then. There is an alternate, more robust data set but its interface is much more difficult and the files sizes are too large (for me) to manage as a batch file. In any event balloon data only foes back to 1948 so long-term trends would be tough to extract in any event.