Results

Electronic versions of the observers logs from the beginning of September 1990 through the end of August 1998 were parsed for recorded values of temperature and dew point. Oddball values were excluded (quite often, for example, the dew point and temperature would be switched, or the dew point would not have a negative sign when it clearly should have). A total of 26659 valid combinations of temperature and dew point were found and used. The values were used to calculate the precipitable water ($h$) according to the above formula, and this value along with the date and time were recorded for analysis. Figure 1 shows a plot of the entire set of data obtained. The seasonal variation is readily apparent in the data. Figure 2 shows the data for 1997 only. The effects of weather systems can be seen clearly at this higher time resolution (variations on the scales of a few to 10 days).

Figure 1: All precipitable water data from September 1, 1990 to August 31, 1998.

Figure 2: Precipitable water data for 1997.