The featured chart presents the daily average Vapor Pressure Deficits (VPD)s for Ames for this year and a climatology based on period of record observations. Large VPD values indicate a larger demand from the atmosphere for water. This demand is based on the availability of the air to hold more water vapor. Saturated air will have a low VPD and thus a low demand for water vapor. The plot for this year tells an interesting story with plenty of days with a larger than average VPD. So even while we have seen a wet spring, there was also been plenty of days with dry air present. The net result becomes worrisome going into June with plenty of drying out of the soil having happened and some areas lacking precipitation as of late. This sets up for a flash drought type situation and undoing the drought busting progress that was done this spring season. The forecast has plenty of chances for rain, but the coverage of these events over Iowa remains in some doubt.

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