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Heavy Precipitation, A City View

Climate scientists tell us that when April showers arrive, they may come with heavier downpours as the planet warms. It’s not just April: more water can evaporate into a warmer atmosphere at all times of the year, and what goes up must eventually come down. (Thank you, Clausius-Clapeyron.)

But when you try to measure what “heavier downpours” are, there are many ways to come at it. The National Climate Assessment, for example, examined increasing trends in the heaviest downpours across broad regions of the Lower 48 states. But since precipitation varies so widely, even over short distances, a regional analysis doesn't always tell you much about what’s happening in individual locations.

Different View

We decided to take a more localized approach, inspired by an analysis from Brian Brettschneider of Borealis Scientific, in Anchorage. We looked at rainfalls (or rainfall equivalents) of 1”+, 2”+, and 3”+ in a single calendar day since 1950. We used three different standards because a 1”+ rainfall in Miami, for example, isn’t all that extreme while the same amount in Las Vegas is highly unusual.

In the accompanying national graphic, the data come from 207 airports across the continental U.S. where records have been reliable and continuous since at least 1950. And the data show very clearly that there’s been an upward trend in rainfalls of 1”+, 2”+, and 3”+ nationwide with respect to the average from 1950 to 2014.

Then, we applied the same 1”+, 2”+, and 3”+ criteria to the city-specific data. But using one single station can be tricky with precipitation since precipitation totals often vary dramatically over a distance of just a few miles — much more so than temperature. The bar charts here shows the number of calendar days grouped into five-year totals, which should help average out the year-to-year weather fluctuations.

Note: Four sites (Laredo, McAllen, Myrtle Beach, and Yuma) did not have complete records since 1950, so their analyses have been done with available data.