The textbooks declare that for every degree Celsius increase in atmospheric temperature there should be 6 to 8 percent more moisture in the air. That's roughly the degree of global atmospheric warming in the past century. Wehner said he guessed, when he started his research, that Harvey might have dropped about 6 to 8 percent more rain than an identical storm would have dropped in 1950. But both the Dutch and Berkeley teams found the actual rainfall to be much higher than expected.