Precipitation has trended either wetter or drier in many areas of the United States over the last 50 years, and newly released maps show exactly where these changes have occurred.

Dr. Brian Brettschneider, a climatologist at the University of Alaska Fairbanks, compiled the map above, showing the trend in annual precipitation over the 50-year period spanning 1970 to 2019 – the most recent 50-year period available for analysis.

Brettschneider noted that he mapped the trend of statistical significance, known by statisticians as "Pearson's R," providing a hypothetical example that a 0.2-inch increase in precipitation in Arizona – a dry climate – is more significant than a 0.2-inch increase in Florida – a wet climate. Therefore, that Arizona increase would be plotted in green (moderately or significantly wetter), while the same increase in Florida would be plotted in gray (little change).

The map indicates that areas from the Plains to the Midwest and Northeast have generally trended wetter, with higher amounts of rain and snow, since 1970. Portions of Central and South Florida, as well as Alaska, have also trended wetter.

Meanwhile, much of the West and South have trended drier, with lower amounts of rain and snow, over the last 50 years.

(MORE: Average January, February Temperatures Trending Warmer Nearly Everywhere – Except in Central North America)

Breaking it down by season rather than the year as a whole, there are both similarities and differences among the data.

In the spring (March to May), the Pacific Northwest, upper and mid-Mississippi Valley, Great Lakes and Ohio Valley have trended wetter since 1970.

The Desert Southwest and parts of the Southeast have trended moderately to significantly drier over the last 50 spring seasons, however.

In the summer (June to August), the Plains, Mississippi Valley, Northeast and Florida Peninsula have the most widespread wetter trends in the last 50 years.

After a wetter spring trend, the Pacific Northwest has trended moderately to significantly drier during the summer since 1970, but summer is already the dry season in that region, so rainfall is typically limited anyway. Other parts of the West and South Texas have trended drier during the summer as well.

In the fall (September to November), there are only a few areas that have trended at least moderately wetter in the last 50 years, including northwestern Washington, northern Montana, parts of the Northern Plains, northern New England and the eastern Carolinas.

The reason the eastern Carolinas stick out like a sore thumb along the East Coast is due to tropical activity in recent years, including Hurricane Florence in September 2018, which set two state rainfall records in both North and South Carolina during its agonizingly slow crawl. When removing the last five years and only analyzing the September-to-November period from 1970 to 2014, the strong wet trend in the Carolinas no longer exists.

Much of the West, Southwest, Southern Plains and lower and mid-Mississippi Valley have trended drier during the fall since 1970.

In the winter (December to February), much of the Northern Plains and Midwest trended wetter since 1970. Parts of the Southern Plains and mid-Mississippi Valley trended wetter as well.

Parts of the Northwest, Gulf Coast, Southeast and mid-Atlantic coast trended drier over the last 50 winters.

Climate change projections are in close agreement that precipitation will increase at higher latitudes and decrease toward the subtropics as the planet continues to warm.

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