Vapor Pressure Deficit (VPD) is a weather variable that isn't as commonly used to express humidity like dew point and relative humidity are. The values also have units of pressure, which makes them even a bit more obtuse. So what use are they? They represent an absolute measure of potential demand from the air of water vapor from various surfaces that may contain water. For green house operators and horticulture folks, knowing VPD can inform plant health and development. The featured chart presents the average May daily max VPD values for Des Moines. Rewording, the maximum VPD is calculated each day of each May and then averaged by year. The 2023 value is higher than a simple average and higher than the past four years. Higher values indicate increased demand for water vapor or perhaps you could consider it as a suction force. It is interesting to see 1988 show up as the highest value since 1973, which of course, was a very bad drought year.

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