The featured chart displays a metric called Vapor Pressure Deficit (VPD) computed from observations by the Ames Airport weather station. While relative humidity measures the normalized difference between the amount of water vapor in the air and what it could potentially hold at saturation, VPD expresses this difference in an absolute value of pressure. The application is that the higher the VPD value, the more suction power that the atmosphere will exert on surface reservoirs of water (plants, soils, etc). The top chart shows the daily range of values, the simple average, and values this year. The bottom chart shows the difference between this year and long term average. Differences in red are days with above average VPD, so above average atmospheric demand for water. For this June, you can see above average demand just about each day so far.

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Tags:   vpd   vaporpressure