Heat, Humidity and Crop Yields

At the ASSA meetings next month I'm going to present some new research wherein we (Wolfram Schlenker, Jon Eyer and myself), incorporate vapor pressure deficit (VPD) into earlier regressions linking crop yields to weather. Vapor pressure deficit, a close cousin to relative humidity, has a linear relationship with evaporation, and is a key input in many crop models. For this meeting we're only presenting evidence on Illinois, which has been our testing ground for fine-scale data development. If you haven't been following, I've written a lot about the strong and robust association between extreme heat and crop yields. This puzzles some crop scientists who focus on soil moisture and precipitation as the key impediments to higher yields. But in comparison to any precipitation or soil moisture variable we've constructed, extreme heat, measured as degree days above 29C, is a far better predictor of yield. And the underlying relationship is similar across widely varyi