– not that the media have noticed. John Frewen-Lord looks into this oversight, drawing our attention to a centenary that occurs on 1 May.

Open any of the British mainstream newspapers or tune in to news broadcasts and you will still find measurements of rainfall or snowfall in imperial units (inches, and occasionally feet if the snow is especially deep). Considering that other former imperial countries (including Canada, well overshadowed by its large non-metric neighbour to the south) have completely abandoned such units in favour of metric ones, this is really quite surprising. What’s even more surprising is that official measurement of precipitation (rainfall and snowfall equivalent) in metric units is now 100 years old in the UK!

Most people know that weather data in the UK is measured and collected by the Meteorological Office (usually known as the Met Office). The Met Office is a member of the World Meteorological Organisation (WMO). Although the WMO adopted the measurement of temperature in degrees Celsius as late as 1961 (although it was likely using degrees Centigrade long before then), its predecessor, the International Meteorological Organisation, adopted the millimetre as the measurement of rainfall much earlier, in the early years of the 20th Century.

The Met Office, set up in 1905, started collecting rainfall data in 1910. I do not know if this was in imperial or metric units, but it is on record that the Met Office officially adopted the millimetre to measure rainfall on 1st May, 1914, exactly a century ago. It has used the millimetre exclusively since then, which means that every article or report in the media that relies on Met Office rainfall data and reports that data in imperial units has involved someone making the necessary conversions. Over 100 years, that has meant countless wasted hours – hours which we all pay for in the price of our newspaper or BBC licence.

So what values are reported by the Met Office in terms of rainfall? For the UK as a whole, and averaged over the past 40 years, the highest rainfall is recorded in December (125 mm), while the lowest is in May (66 mm). The highest ever annual rainfall (taking each of the four countries that constitute the UK separately) was recorded in Northern Ireland (1411 mm, in 2002), while the lowest was recorded in England (567 mm, in 1921). Some of the most extreme rainfall events were 32 mm in five minutes (in Preston, Lancashire, in August 1983), 80 mm in 30 minutes (Dumfriesshire, in June 1953), and 279 mm in a period of 24 h (Winterbourne in Dorset, in July 1955). The highest annual rainfall for any specific location was 6528 mm, in Cumbria in 1954, while the lowest average annual rainfall is 513 mm, in Essex. (As a comparison, the world’s most extreme rainfall event was probably the 31.2 mm that fell in one minute in Unionville, Maryland, USA, on 4 July, 1956, which no doubt washed out more than a few Independence Day barbecues.)

All these UK rainfall records were of course made using rain-gauges that recorded in millimetres as they have done for 100 years. All is not so rosy however elsewhere in the Met Office’s Fact Sheet 9 (from which the above figures are derived). Wind speeds are still resolutely quoted in knots with mph following in parentheses – not a km/h to be seen anywhere (although, bizarrely, in describing the least windiest place on Earth – Dome A, an Australian research station in Antarctica – it uses the expression ‘less than a few kilometres per hour’). Snow precipitation is also described in imperial units – ‘the flakes may then have a diameter of several inches’. Barometric pressure is measured in the internationally accepted hPa, while the distances tornadoes travel are measured in miles with kilometres following in parentheses. A typical UK muddled mixture.

Finally, a call to the Met Office, and a reply by email, reveals that there is no plan to celebrate the centenary of the use of millimetres for measuring UK rainfall. Which is a pity. And no doubt a continuing reason for our media to still keep using archaic inches – officially now a hundred years out of date.

( 1 ) Likes ( 0 ) Dislikes