The real reason for Manchester's weather - pollution

Manchester's reputation as the wettest city in Britain could have something to do with its industrial heritage, says an environmental scientist who believes urban regeneration and pollution can lead to local increases in rainfall.

Professor Christopher Collier of the University of Salford said that Manchester's linger-ing drizzle was probably the result of air pollutants affecting the size of raindrops and that regeneration of the city's Victorian landscape was also influencing local climate.

"The regeneration projects, in Salford particularly, have led to increases in rainfall downwind of the urban regeneration area. But in other directions the rainfall has actually been suppressed by the air quality," Professor Collier told the Science Festival.

"The explanation we are offering [for Manchester's wet weather] is that the air quality is poor in the north of the city. The change in particulates in the air has the effect of reducing the size of raindrops to drizzle drops and of course Manchester has a reputation for cloudy drizzle," he said.

Urban regeneration, with the demolition of Victorian buildings and their replacement by tower blocks and open spaces – which increases the "roughness" of the landscape – had also played a role in redistributing local rainfall, Professor Collier said.

"If we change the fabric of city areas in some ways we will change the distribution of temperature and rainfall. In some areas it might not increase temperatures or rainfall, it might have the opposite effect.

"But there is definitely going to be changes and in Manchester we found that 7 to 8 per cent of the increase in rainfall we think is due to the regeneration of the Salford area and the changing roughness there."

The "heat island" effect of cities was well known, he said. "Cities expend much more energy than surrounding rural areas, which actually lose heat, particularly in winter, at a much quicker rate. The buildings essentially hold the heat and these two work together to produce this temperature difference."

Plans to increase housing density in the South-east from about 24 dwellings per hectare to 30 or 50 might affect local weather. "These changes may have a magnitude comparable, or certainly approaching the changes due to climate changeand we need to understand the complexities of how these changes interact."