Voter turnout in the US presidential election could be affected by a 3,000 kilometer long zone of rain and storms

In a country where voting is not compulsory and the margin of victory is small, the weather can influence the outcome of the election.

Research has shown that fair weather in 1960 benefited Democrat John F. Kennedy, while rain in 2000 helped secure Florida and eventual victory for Republican George W. Bush.

This year's race for the White House between Donald Trump and Kamala Harris is a classic case in which the atmosphere could be the deciding factor, with a 3,000-kilometer-long strip of heavy rain and thunderstorms forming from Texas to Canada, which forecasts predict Heavy rain and thunderstorms will bring severe weather in several states over the next 48 hours.

Professor Benjamin Reilly, senior fellow at the United States Studies Center at the University of Sydney, said the competition between candidates was one of the toughest in US election history.

“It’s too close to name. Opinion polls come very close to that,” he said.

Rosalind Dixon, a law professor and director of the Gilbert + Tobin Center of Public Law at the University of NSW, said the race was “extremely close”.

“I think all the polls suggest it's down to a few states, most of which are within the margin of error,” she said.

Professor Reilly said that while the heavy rainy weather would have a “negative” impact on voter turnout, it was too difficult to determine how significant the impact would be on the current election.

So could the weather make a difference? Bad weather such as rain or snow affects the turnout of marginal voters who are less politically active.

And because people on the fringes of politics are more likely to be working class, the decline in voter turnout is higher for one party than another.

Two swing states could be influenced by the weather

Even without the background of a US election, the current weather system is newsworthy.

A very moist stream of air from the Gulf of Mexico flows into a low pressure area over the central United States.

This system is producing severe weather with excessive precipitation and severe thunderstorms across most of the south-central United States as early as Monday, causing flooding in areas.

This low is slowly moving east and will extend a line of rain and storms from Canada to Texas on Tuesday (US time).

The heaviest rain is expected to fall over relatively safe states, including Republican-leaning South and Democratic-leaning Illinois.

However, in the critical swing states of Wisconsin and Michigan, up to 50 millimeters is also possible.

Professor Dixon said these two states were the ones to “keep an eye on” for this reason.

While Detroit should remain east of the main rain band, northern Michigan and southern Wisconsin, including Milwaukee, are expected to be covered by the rain band for most of the voting hours on Tuesday.

How a wet day affects voter turnout

Several studies examine the influence of weather on elections around the world and the results are clear: rain and snow have a significant impact on voter turnout.

A study of Denmark's 2023 elections found that rainfall reduced voter turnout by 1 percent per 10 mm.

A 2007 study analyzed U.S. elections and found that rain reduces voter turnout by just under 1 percent per inch, or 1 percent per 25 mm, compared to normal conditions.

That could mean a 2 percent drop in participation in Michigan, Wisconsin and other counties affected by the heaviest falls (and even higher in states further south, where 100mm is possible).

A separate weather system that drops snow in Montana will also likely reduce voter turnout, but is unlikely to prevent a certain victory for Trump.

Republicans are praying for it to stop raining

So does rain or snow favor one party over another? The old adage “Republicans should pray for rain” has proven true in past US elections.

The theory is that bad weather could be the final straw for marginal voters, particularly young people who have not developed voting habits and were disproportionately inclined to support the Democratic candidate.

This relationship was tested in the Gomez study, and the data showed that for every 25 mm increase in rainfall, the Republican presidential candidate received just over 2 percent of the vote – easily enough to sway a state and therefore an entire election.

Professor Reilly said since the generally staunchly Republican states in the South – Texas, Oklahoma and Arkansas – will be hardest hit by the rain band, it would impact the party's overall vote share.

However, based on academic studies, including Gomez's study, Professor Reilly said there had been “speculation” that the Republican Party had benefited more from bad weather in the past.

Professor Dixon said she suspected the weather would have a “very modest impact”.

“If you asked me to bet on it, I would say it won’t matter. Could it be? Absolutely,” she said.

Professor Dixon said regardless of the weather, “more determined” and “politically engaged” voters would turn out.

“There are definitely motivated voters on both sides, and I think it's very important to say that weather is a factor and we should pay attention to that.”

“But I think it’s very unwise to make any firm predictions about what it’s going to be.”

In fact, weather may have contributed to the results of two electoral colleges in the past.

It is estimated that in a very closely contested election in 1960, with very rainy or snowy weather, Richard Nixon would have won 106 additional Electoral College votes over John F. Kennedy.

The reverse results – if it had been dry instead of wet – showed two cases that would have changed the Electoral College outcome – Bill Clinton won North Carolina in 1992 and Al Gore won Florida in 2000.

This latter change would have tipped the incredibly close election of 2000 in favor of Mr. Gore over George W. Bush.

However, recent evidence suggests that this pattern may have reversed, with working-class voters now leaning more toward the Republican Party and college-educated voters toward the Democrats.

A 2023 article by Spencer Goidel and co-authors showed the shift in voter turnout by examining U.S. federal elections from 2010 to 2020.

“Our results show that increasing voter turnout greatly benefited the Democratic Party in the pre-Trump era. However, this pattern has changed drastically. In 2016, 2018 and 2020, the Democratic Party sees much smaller increases in vote share than simulated turnout. “Growths, but also a large vote share advantage when turnout is extremely low.”

That means Mr. Trump might want to pray for sunshine instead of rain over Michigan and Wisconsin on Tuesday.