Rain: Chasing Sheffield's rain shadow

I lived in Crookes for a year in 2011. An attic bedroom gifted me a panoramic view of Sheffield, looking out towards the east. You could see right down into the valley of the city centre, then out towards Rotherham and beyond. I’d lean out of the skylight to soak up the view. More often than not, the clouds would track straight over the house travelling east. They’d pour over from the Peak District, having come from Manchester, and before that the Irish sea. A few times I saw it rain without clouds - blue skies and crystal clear rain. A couple of years later, I was working behind the bar at The Hallamshire House and studying for a degree in Science Communication. There was an opportunity to make a documentary for my final project. I wandered around the city looking for ideas. It clicked. Winds flowing east would often leave Sheffield dry and Manchester soaked. I found the first clue in the archives at the Central Library. In a book from 1956, Climate in Sheffield and its Region, there’s a study by Alice Garnett, a Professor of Geography at the University of Sheffield. It contains the earliest mention of a ‘rain shadow’ describing our city’s climate, the reason for its relative dryness in comparison to Manchester and the Peak. As it goes, air filled with moisture reaches a mountain range and gets buffered upwards. There’s less pressure and it’s colder the higher you go, so the water vapour condenses into a liquid as clouds. When the clouds get too heavy, it rains. On the other side of the mountain range, the air descends and warms up again. The water in the clouds evaporates, leaving a ‘shadow’ of rain where it should have fallen before the descent. Looking for answers, I spoke with friends, family and the locals at the Hallamshire. Everyone had a story to tell about the rain shadow. A retired RAF pilot taught me about atmospheric waves generated by undulating terrain that he’d hitch a lift on to gain speed. The beekeeper at the Sheffield Honey Company told me about the challenge of caring for his colonies in the heavy rainfall they get out in Dungworth. A local brewer told me about the Rain Shadow ale from Buxton Brewery. A couple of dog walkers shared a trip to Redmires, which saw them get drenched before returning to a city that was dry as a bone. But the key that unlocked the secret to Sheffield’s rain shadow was held by one of the scientists that I interviewed for the documentary. David Schultz, a Professor of Synoptic Meteorology at the University of Manchester, argued that a rain shadow is too simple an explanation to entirely account for the difference in rainfall between our cities. To explore the idea, we looked into the wind and rainfall records from 54 weather stations around Sheffield and Manchester between 1981 and 2010. The results of our research were published this May in the Bulletin of the American Meteorological Society. We saw that the weather travels into Sheffield from the west on 207 days each year. During these days, on average, Manchester gets 3.9mm of rain, the Manchester side of the Peak District 4.5mm, and Sheffield 1.7mm - a classic pattern of enhanced rainfall over elevated terrain. Wind aside, this levels out over a year to around 1200mm of rain in Manchester and 700mm in Sheffield. We also identified 34 days each year that showed a distinct rain shadow pattern - throughout the day, rain in Manchester but none in Sheffield. That’s just over an extra month of full rainy days. Although these 34 days look like a rain shadow, the picture might not be so clear. There are other interactions between land, wind and air that can cause a rainfall difference between the cities. As David sums up: “An explanation for the rain shadow [in Sheffield] is easily demonstrated and intellectually satisfying […] However, such an explanation may not be so obvious.” As for the blue skies rain, reading into the science suggests that it could be due to ‘spillover precipitation’ drifting over to Sheffield as the Peak holds up the rainfall. I’d even go as far as to suggest that this could be the reason why, until recently, Sheffield held the crown for the world’s longest-lasting rainbow. ‘Quantifying the Rain-Shadow Effect: Results from the Peak District, British Isles’ was published in the April edition of the Bulletin of the American Meteorological Society. )