Heavier precipitation is a signature of climate change. For every 1°F of temperature increase, the atmosphere can effectively hold 4 percent more water vapor. So as the world warms from the increase in greenhouse gases, the amount of evaporation also increases from oceans, lakes, rivers, and soils. The extra water vapor is available to produce additional rain and snow, creating an environment ripe for heavy precipitation events which is exactly what we are seeing in the numbers. This week’s analysis, an update from our 2015 Climate Matters, shows an increase in the top 1 percent of daily rainfall events across the vast majority of states in the U.S.
States in New England and parts of the Midwest have the most substantial increases in heavy precipitation events, whereas the trends in a couple of western states are slightly down. This follows a general rule about a supercharged water cycle in a warming world — wet places will get wetter, and dry places will get drier.
Missouri, home to intense flooding this week, is one of the states with the strongest trends in heavy precipitation events. You find heavy rain trends for Missouri and the rest of country at our archive.
Heavy rain is the root cause of a number of inland flooding events. Topography, land use, levees, and dams also play a role. As runoff is also projected to increase in a warming world, the threat for flooding via heavy rain is likely to increase, putting additional property and farming interests at risk.
For daily rainfall at over 3,000 stations across the U.S.:
- Calculate the inches of rainfall representing the top 1% of rainfall totals (from days with non-zero rainfall amounts) at each station.
- Note: The vast majority of this heavy precipitation came as rain, although in a few rare instances, major snowfalls also count toward these large events.
- Count the number of days per year exceeding this level at each station.
- Average the number of events per year across climate division.
- Weight the climate division total by its size as a proportion of the state.
- Sum the weighted climate divisions total across each state per year.
- Above calculations are the source of our Downpour Index