UK rainfall variability: It never rains but it pours

Rainfall might be stable from year to year, but the story is very different day to day.

Jay Richardson

by Jay Richardson
Wednesday 29th June 2022

In an era of disastrous storms and constant fear over coastal defences and flooding, you might expect rainfall to show reliable, multi-decade, climate-influenced patterns. Or at least to swing wildly, as seasonal temperatures do. The actual picture is incredibly complex: precipitation is in fact trending in certain directions and swinging wildly at the same time. Think of a drunken emu trying to walk in a straight line.

UK average monthly rainfall, 2019-21

Data sonification of the total monthly rainfall on average across the UK. Source: Met Office. The rainfall is mapped to the pitch and volume of a recording of a thunderstorm in Cambridge in spring 2020.

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There is some evidence that UK winters are getting wetter, and summers drier. The trend is light, though, and has all but stabilised in the past 50 years. Winter rainfall from 1775 (when records began) to about 1900 usually measured around 220mm, but since then it’s come in at 250mm on average—give or take 60mm from decade to decade. Summer rainfall, meanwhile, used to be about 240mm but is now closer to 200mm, with the same wide variation. In other words, the massive seasonal and yearly variation introduces a lot of noise into precipitation data, making long-term patterns harder to spot.

And it’s not like minor, gradual shifts in weather are likely to register much in people’s lives. Spend a few days on holiday at a campsite and you might get right in tune with the wind speed and ambient humidity, but go back inside for a week and you’ll probably forget about it. That’s not a universal experience, of course, but thoughts of precipitation and temperature generally live short lives: indoorsy people need them once a day to decide what to wear and then maybe once more for small talk. If you asked me what the rainfall was like in April last year, I would think of spring and guess that it was rainy. If you then told me it was astonishingly dry and that in fact we experienced the lowest average precipitation of any month since 1995, I would shrug. Then I would think about it for 10 seconds on my way to work, and then wonder why a stranger had asked me about monthly precipitation trends.

What does matter in people’s lives is dramatic short term variation, or extreme weather, and it’s increasing with dead-ahead certainty. Rainfall events that exceed 50mm are becoming exponentially more common, having already risen in frequency by 60% between the beginning of the twentieth century and the beginning of the twenty-first.

Projecting the rainfall data onto a recording of a thunderstorm is one way to feel the data. It’s an attempt to connect numbers with the human impact of the rising tide of energy in the global weather system. In a much truer sense, though, that impact is measured in the wreckage that major storm events leave behind, and whose meaning no data display could ever capture.