One of the reasons for our recent lack of rainfall has been the dearth of precipitable water. Precipitable water is a measure of depth of water in a column of air if all phases found were converted to liquid. The featured chart presents the combination of preciptable water analyzed by a weather forecast model and the total rainfall for that day. The upper chart presents the frequency of having rainfall on a given May day partitioned by preciptable water value. Increasing water content in the atmosphere increases our chances of rain. The lower chart presents the combination of daily precip observations against the precipitable water value. The one to one line shows that often the preciptable water value provides an upper bound to the amount of rainfall we may receive. Advection processes help to replenish atmospheric water content so we can get rainfall totals greater than the amount of water in the column of air ("in this house we obey the conservation of mass law").

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