Absolute drought frequency in the British Isles

Figure 1 – Courtesy of the Met Office ©Crown Copyright

I was interested in the average number of droughts they were each year across the British Isles and came across an article in an old Met Mag (vol 76 No 903. September 1947) with the above maps of droughts for the period 1906 to 1940. These might be old climate stats but that doesn’t mean they have no value. Back in 1887 the British Rainfall Organisation introduced the following definitions for absolute and partial drought:

  • An absolute drought is a period of at least 15 consecutive days, to none of which is credited 0.01 inches of rain or more.
  • A partial drought is a period of at least 29 consecutive days, when the mean daily rainfall does not exceed 0.01 inches.

I assume that the partial drought limits the total of rain in any 29 day period to less than 0.29 inches (~7 mm) but I’m guessing, the definitions will have no doubt been changed since 1947, but I have no idea to what. The chances of an absolute drought of 15 days or more (which some stations this April have at the moment) looks to be highest, as you would expect, across the southeast of England, and south of 52° north the average looks to be around 1 or 1.2 occurrences a year. Curiously, there doesn’t look that much overall difference between the map of absolute drought and the map of partial drought (fig 1).

It’s refreshing to see that climate maps in 1947 included the whole of Ireland as well as the rest of the UK, it’s a shame the head of international climate service development at the Met Office can’t get around to talking to his counterpart at Met Éireann and share our climate data! Perhaps if we make this a stretch target or SMART goals for Dr Chris Hewitt (who I believe is the present incumbent in this role) he will make it happen. Weather and climate know no man-made political boundaries, or it didn’t the last time I looked.