Sunday, 6 May 2012

April showers

We have an official drought here in the south of England; ironic in a country famed for its rain.  Of course, that's part of the problem: why bother to store something that's in constant supply.  So it takes only two dry years in succession to cause a problem. However, recently we've also had a lot of very heavy rain, or so it seems.  Is it really much wetter than usual, or have we simply  forgotten how wet it usually is?

Last year was the driest since records began.  Well, since our records began in June 2005, when we started collecting rainfall statistics in our Cambridgeshire garden.  The annual rainfall (in mm) since then shows last year very dry, with 2010 on the dry side.

annual rainfall, in mm
Actually, it isn't the total rainfall over the year that's important for droughts, it's the rainfall over the winter.  Summer rain evaporates more; winter rain soaks in.  So here's the rainfall month by month, averaged since (our) records began:

average (mean and median) monthly rainfall, in mm
So the wettest month by far is August, closely followed by May: neither exactly winter months. Also, the driest month by far is April: so much for it being famed for "April showers".  Yet it is this April that has been so seemingly wet.

However, plotting both mean and median points out an anomaly in the April value.  Its mean is much higher than its median, indicating a skewed distribution.  Let's look at April in more detail:

April rainfall, in mm
So, for three years we detected no rain at all in April, and in 2010 only 1mm.  Two further years were around 10mm.  Then this year, 72mm!  That's some outlier.  Not only has it been wet this April, it's been significantly wetter than the average for any month, and it's been over three times wetter than all those other Aprils put together! 

So yes, it really has been wet. And it's not only our garden, and our short measurement span.  It has in fact been the wettest April in the UK for over a hundred years.  But it's still officially a drought.

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