It’s a cliché among political commentators that heavy rain on polling day reduces turnout and particularly hits left-leaning parties.

There’s even an episode of the West Wing where fictional Democrat President Bartlett worries that poor weather in California could ruin his chances of re-election.

With rain predicted across the country this Thursday, could there be a drop of truth in the theory? And does it mean storm clouds ahead for Labour?

FactCheck investigates.

Evidence from America: “Republicans should pray for rain”

In 2006, US researchers published a study on how weather affects American presidential elections. They found that “compared to normal conditions, rain significantly reduces voter participation by a rate of just less than 1 per cent per inch [of rainfall]”.

Voter turnout is about convenience. There are some “core” voters that will head to the ballot box come rain or shine. But there are those on the “periphery” for whom factors beyond pure politics play a part.

The suggestion is that people who haven’t decided whether to vote or not could be put off by the added hassle of making their way to polling stations in the rain.

Even more interestingly, the study concluded that “poor weather is also shown to benefit the Republican party’s vote share”, because it tends to put off more Democrat voters than Republicans.

They attribute this to “the conventional wisdom” that voters inclined to vote Democrat are more susceptible to the “last straw” effect. In other words, it wouldn’t take much for these voters to decide against voting altogether.

According to the study, “for every one-inch increase in rain above its election day normal, the Republican presidential candidate received approximately an extra 2.5 per cent of the vote”.

The researchers say this effect may even have contributed to the outcomes in the 1960 and 2000 presidential elections. Quite a compelling claim.

Does that mean a washout for Labour this Thursday? Maybe not.

There are lots of factors that might make UK voters behave differently to their American counterparts.

When we’re talking about the weather, the distance that voters travel to reach polling stations is important. The further you have to go, the wetter you’re going to get – if you’re walking or taking public transport. Unless you happen to enjoy being drenched by a passing storm, a long, rainy trip to the polling station is going to make casting a ballot much more inconvenient.

19.3 per cent of Americans live in rural areas, and are likely to be further away from their nearest polling station than urban dwellers. Census data on rural fringe populations puts the UK figure at 9.2 per cent. That means Americans probably have to travel further to their nearest polling station than Brits.

By that logic, the added inconvenience of rain for potential voters in America is even higher than it is for Brits. It’s difficult to tell whether UK voters would be put off by rain to the same extent as their American counterparts.

So the US trend that rain disadvantages left-leaning parties might not translate directly to this side of the Atlantic.

On the flipside, if you live really far away from a polling station, you might have to drive anyway – whether it’s raining or not. On that interpretation, Americans might not care as much about rain as UK voters. So it could be the case that rain suppresses voter turnout even more in the UK than in the US.

In other words, there are arguments on both sides. We should be wary of copying and pasting the findings of the American study into UK election predictions.

As veteran British election commentator and academic John Curtice says: the supposed link between rain and voting behaviour in UK elections is “one of the most common theories of turnout but nobody has ever found evidence to back it up”.

Only very few places will see as much as an inch of rain on Thursday

We asked our weatherman, Liam Dutton for the low-down on Thursday’s conditions. He said that polling day “will see heavy rain and a brisk breeze sweep northwards across the UK”.

Crucially, he also said, “despite being an unseasonably unsettled day for June, very few places will see an inch of rain – probably only the hills of Wales, north west England and south west Scotland. Everywhere else is likely to have half an inch of rain or less, with some eastern parts of England seeing nothing at all”.

So even if weather conditions do affect UK voters, the rain we’re due on Thursday is unlikely to be enough to make a big splash.

FactCheck verdict

There’s evidence to suggest that voter turnout in US elections is lower when it rains, and that bad weather favours Republicans. But there are many reasons why the American experience would not be replicated in Britain. There’s no strong data either way in the UK, and there won’t even be that much rain on election day this year. Labour have a lot to think about before Thursday, but rain is unlikely to be a significant factor.