Recently, Houston’s Cedar Bayou suburb had 52 inches of rain in less that a week, courtesy of Tropical Storm Harvey; a new record for the US mainland. Meanwhile, a hemisphere away, and much less reported, more than 1,000 people died in this year’s monsoon with almost 700,000 homes destroyed in Bangladesh.
As the Times reminded us (and chemists everywhere already knew) two scientists who would not be surprised by this would be the 19th century scientists Clausius and Clapeyron. They worked out the exponential relationship between the temperature of a body of gas (e.g. the Earth’s atmosphere) and its capacity to hold water vapour.
The Clausius-Clapeyron equation indicates that, that for every 1 degree Celcius the average sea surface temperature rises, the atmosphere’s water content should increase by some 7%. Since the 1970s, the US military has measured a 0.6 degree increase in average sea surface temperature and a 4% increase in atmospheric water vapour. This is just what the Clausius-Clapeyron equation predicts. This 4% means to an extra 500 cubic kilometres of water in the atmosphere, all of which enters the the water cycle with much inevitably falling as rain somewhere or other.