Saturday, 23 April 2016

Wetter winters, drier summers

I am a typical Scot, totally obsessed by the weather. But just in case you share my interest, here is a trend:

April Showers
This graph compares 2006-2008 (three years, inclusive) with 2013-15. It shows how much wetter or drier it was in the later period by comparing the same six month periods in each three year group, ending in the month shown on the graph. I hope you are following at the back of the class because there will be a test later.

In simpler language: in the three six months periods from July to January 2006, 2007 and 2008 we had an average total 404mm of rain over those six month periods. But in the three six month periods from July to January 2013, 2014 and 2015 we had just 338 mm. In other words the late summer and autumn 2013-15 was a bit drier than the late summer and autumn 2006-8. The difference was that the later period was 16% drier, so the first point on the graph is below the zero line. But the six month periods ending in April (i.e. November-April) were much wetter in the later (2013-15) period. In fact the later period was 42% wetter than the earlier. Geddit? 

This data does not include the very dry winter we had this year.

There could be many explanations for this shift in the weather (which is consistent, by the way, whichever pair of three year periods you use). Whatever the cause, winters and springs have been much wetter at the Croft during 2013-15 than they were 2006-8, and summers and autumns are much drier. Overall rainfall in the two periods has not varied much; total rainfall 2013-15 is only 3% lower than 2006-8.

There you are. The weather is a wonderful topic for conversations, a subject for hours of debate...

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