Scientific summary of historic rain data

Scientists have tried to find weekly patterns in rainfall data for over 100 years as it is thought that emissions from industry and transport can affect the development of rain. No clear result has ever been found. This page presents the first results from an analysis of data from a rain gauge that has been located in Whalley Range for the last 110 years to see if there are any patterns in Manchester.

The plot below shows that if we look at the data as whole then there is no clear signal of a weekly cycle. Tuesdays and Sundays are slightly wetter than the other days and Wednesdays, Thursdays and Fridays are the driest days. These results are consistent with the theory that the increase in pollution during the week supresses rainfall by producing more, but smaller, cloud droplets but it is by no means convincing! However, if we ignore the most recent 30 years of data then Sundays were always the wettest days.

Box and whisker plots for the 110 years of data. The thick line in each box shows the median rainfall (mm). The box represents the interquartile range and the whiskers show the full range (with outliers removed).

So has anything particularly interesting happened in the last 30 years to change this pattern? Well, if we look at the total rainfall for each year over the last 110 years we see a clear increase in rainfall over the last 30 years. This is the typical climate change signal of rainfall for Northern Europe - as the atmosphere warms it can move more water vapour around. So, Manchester has got wetter over the last 30 years.

The grey bars show the annual rainfall totals expressed as a percentage of the 1961-1990 mean rainfall. The black line shows the five year running mean of the grey bars.

If we then look at this increase in rainfall in more detail, we see that most of this extra rain over the last 30 years has come on Saturdays. In particular, the biggest rain events on Saturdays have got bigger. So, this initial investigation suggests that Saturdays are becoming much wetter than they used be and this is largely a result of climate change.

The grey bars show the Saturday rainfall totals for each year expressed as a percentage of the 1961-1990 mean Saturday rainfall for each year. The black line shows the five year running mean of the grey bars.

At the festival...

To find out more and see the latest developments (in particular, the data from home made rain gauges around the city), come along to the drop in session on Saturday 31st October 2009 between 11am-3pm at MOSI!

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