Figure 4 shows the hourly mean values for all of the data. No clear trend is apparent, which is somewhat surprising on first inspection. However, when looking at the winter data by itself, a clear diurnal variation of about 20% is seen (Figure 5). This must be because during winter night, a sizable portion of the atmospheric water freezes out, lowering the amount of vapor. During the summer, on the other hand, the atmosphere does not cool down enough to freeze out an appreciable fraction of the vapor, and so little diurnal variation is seen. Since the summer precipitable water values are larger than the winter ones, they mask the wintertime signature in the overall average.
