We all know that on a hot day in dry air we feel cooler than at the same temperature in humid conditions. It is simply that in dry air, our sweat evaporates and cools us down. As long as we keep drinking lots of water, we will be fine.
For plants and soil, hot, dry air is much more of a problem. Scientists call it vapour pressure deficit, or VPD. Plants and trees have the water they need to flourish and grow sucked out of them by the dry air, and the ground dries out too. The result is poor crops, dry vegetation and a much greater risk of wildfires. Irrigation, and lots of it, is the only way to keep crops flourishing.
Sixty-eight researchers looked at 400 years of tree ring data in Europe and came to the conclusion that the problem of dry air has got far worse this century. The increase in greenhouse gases from fossil fuels has made VPD more frequent and extreme, particularly in the central European lowlands, the Alps and the Pyrenees. It was worst in 2003, 2015 and 2018.
So far northern Europe has suffered least from VPD because the air is cooler and can take up less water, but in time the problem is expected to grow.
