A mosaic of satellite imagery from the continental United States, collected at 4 p.m. on Election Day 2018NASA / RAMMB / CIRA

O,Election Day! In the ruddy twilight of each year, as autumn incarnadines the republic’s plump pastures and luxuriant hills, its people gather for that most felicitous and humbling of civic offices: to choose our elect, to decide who will rule. With renewed revolutionary vigor, this mighty tide sweeps the continent, intermingling the wise Delaware and the ferocious Klamath, making so strong a torrent as to flood the ancient Appalachians and take the West’s Rocky spine in a single splash. Woe to any tyrant who tries to dam this foamy act of faith, for no virtuous power can block the citizenry from its solemn act—

—except, that is, for some scattered rain showers.

On Tuesday, as voters decided the midterm 2018 elections, rain fell across well-populated areas of the country. In the Southeast, voters faced scattered showers and dreary conditions as some of them waited in hours-long lines at the polls. Steady rains and occasional thunderstorms also soaked the Northeast.

Snow fell in parts of Idaho, Montana, and Wyoming, according to the National Weather Service.

These arbitrary weather conditions could shape politics for years to come, political researchers have found. Rainy days can depress voter turnout, discouraging people from going to the polls at all. And lousy weather may even lead a small number of voters to change their votes to more conservative candidates. Both of these effects consistently boost Republicans when elections happen to fall on rainy days.

The party knows it. Bob Hugin, running for the U.S. Senate in New Jersey, called the rain “Republican weather” on the eve of Tuesday’s vote. “This election is about who gets the vote out and who doesn’t, and I hope it rains hard tomorrow,” he told supporters. (It did. Hugin still trails by 11 points in preelection polling.)

For years, political scientists have argued that Republicans do better on rainy days from lower turnout alone. A famous 2007 paper found that every additional inch of rain that fell on Election Day decreased overall turnout by 1 percent. The effect can be profound: Had the mid-Atlantic states seen rainy weather on November 8, 1960, historians would speak of Richard Nixon’s famous victory over John F. Kennedy, the study’s authors argued. Likewise, a dry day in Florida may have let Al Gore win the state on November 7, 2000, handing Democrats a third term in the White House and possibly averting the Iraq War.

Recently, though, political scientists have gone a step further, claiming that rain doesn’t just decrease turnout but can actually change people’s voting decisions. “When weather is bad, people’s mood is affected. Then people become more risk-averse, and in the area of political elections, risk-averse people are more likely to choose Republicans,” says Yusaku Horiuchi, a professor of government at Dartmouth College whose research helped establish the new theory.

He and his colleagues found that up to 1 percent of American voters may switch their decision at the polls based on the weather. Every inch of rain appears to drive up a given Republican’s vote share by 3.08 percent, Horiuchi told me. “So that’s not so small,” he added.

Do Republicans Get More Votes in States With Rainier Novembers?

If your state tends to have a rainier November, do its voters select more Republicans on Election Day? This map, made with climate data from the National Oceanic and Atmospheric Administration, doesn’t suggest any easy answers. On the one hand, some of the most solidly Republican states in the country—Alabama, Mississippi, and Tennessee—tend to have sopping Novembers. On the other, the wettest Novembers in the country happen along the coast of the Pacific Northwest—the same area that has seen Oregon and Washington tilting toward Democrats in recent national elections.

Even if Democrats win a given race, rainy weather on Election Day may still change how they act once they arrive in Washington, D.C. In another recent paper, political scientists at Yale and UC Berkeley argue that congressional Democrats elected on rainy days tend to act more conservatively in Congress. They argue that since every additional inch of rain depresses the margin of Democratic victory by about two percentage points, Democrats tend to assume a smaller political mandate than they may actually have and make more right-leaning choices as elected officials.

In other words, even if Democrats take the House or Senate today despite the rain, their politics for at least the next two years may be moderated by a random cold front.

Not all of these trends may hold for every election. Nationally, Democrats usually win more elections when more voters turn out. But because of the changing demographics of the parties, higher turnout may actually boost the GOP in this year’s midterms in the North, according to The New York Times’ Nate Cohn. Since it was rainier in the Northeast on Tuesday, Democrats might come out ahead.

That wouldn’t control for voters who switch their decision because of the weather, though. “It’s raining harder today, so Republicans may get some advantage. But rainfall is not so huge; rainfall could be a factor only in very competitive [districts],” said Horiuchi, who declined to make a more specific prediction of the outcome.

Not that good weather always boosts the Democrats to victory, necessarily. On another arbitrary day in November two years ago, it was bright and sunny across most of the country—although there were some fast-moving showers across the upper Midwest, where Donald Trump ultimately cemented his victory in the Electoral College. So maybe there’s a lesson there, too. Democrats’ best shot at success may not be a fight over the most appealing economic message. A more winning strategy may be to turn Wisconsin into the world’s largest tanning bed.

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