The Rain it always Raineth...in Manchester. Posted on 04 Jun 20:12 , 2 comments

Being based in Manchester is a blessing when you're rainwater harvesting

We're often asked 'How often does it rain in Manchester?' so we decided to find out! 

Some basic research reveals a couple of studies have been carried out on this topic. The first study suggests that Manchester may have been incorrectly labelled the 'rainy city' due to an erroneous rain map entry dating back from 1926!

A second study was a little more pragmatic, the author (Alfie Yarmouth, pseudonym!) took a daily diary throughout 2013 and recorded whether it had rained or not by looking out the window! The conclusion: it rained 53% of the days on record in Manchester in 2013 (far too many by most people's standards, but in rainwater harvesting terms, it's a dream place to live...)

Finally, the Met office blog: "What is the Wettest City in the UK" couldn't make its mind up...

All good research, but we decided to take this one step further and lay this one to bed definitively-ish! 

We delved into the archives and obtained 32,000 data points between 1924 and 2013 for the Manchester area which represents the daily rainfall totals (measured in mm) over that period. 

Now for the analysis: 

 

This is what the information looks like represented as a time series.  

Not that useful to be honest. Given the length of series and number of data points it's not easy displaying the data in a meaningful way. The black line is a moving average which shows average rainfall is around 2mm/day. There 's also a spike of 66mm recorded on 30/4/1992 - long before our time -ehem! 

To make more sense of the data, it needs to be arranged to see how often it rains, or another way of thinking about it is: for how many of the days on record was rainfall recorded? The following is a rainfall exceedance curve which represents the proportion of the record (89 years) when rain was recorded above a certain level. The y-axis has been represented on a log scale to make the visualisation a bit clearer:  

  

You can see that rainfall occurs for 55% of the days on record! It should be noted at this point that rain is defined as anything greater than 0.1 mm rainfall recorded on the day. 

So over the period of the 89 year record it rained for 55% of the days on record. It seems that Alfie Yarmouth was not far off with his single year of observation! 

So there you have it folks, on average there's a 55% chance of it raining (>0.1mm) in Manchester on any day based on the existing data record or 40% if 1mm is taken as the threshold of rainfall. Which hopefully answers the question 'How often does it rain in Manchester?'. 

If you have any views or comments on the above please leave comments below or feel free to contact us: .