One hundred years of metric rainfall measurement

– 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.

9 thoughts on “One hundred years of metric rainfall measurement”

  1. That is amazing. Rainfall measured for 100 years in metric is definitely worth knowing – shows that Britain has been using metric in something at least for a very long time.

    How much easier it would be to disseminate information only in metric – for everyone, no more conversions and a lot of time saved.

    I do wonder why the media haven’t bothered reporting this. Perhaps they feel that reporting this news would pretty much undermine their propaganda in a single stroke?

    That we have been using metric for 100 years would certainly upset the anti-metric lobby and their supporters in the media, the corporate world (at least those who are indifferent or who would obstruct change) and government.


  2. Metric Views’ readers may not all be aware that one litre of rain falling on a square metre of a flat surface will cover that area to a depth of one millimetre. This is no doubt why German and French weather forecasters often refer to rainfall in terms of x number of litres falling per square metre. In that formula the number of litres is the same as the number of millimetres of rain which will be measured.

    I think this is a neat way of describing what actually happens and it also embraces the beauty and simplicity of the metric system (one litre on one square metre gives one millimetre of water). I don´t know if there is a formula for x number of pints/gallons falling on one square foot/yard giving a cover of one inch, but I am pretty sure there isn’t and it’s probably a hideous calculation to do, like most calculations in imperial units.


  3. Jake,
    It can be done, but much more inconveniently. 1″ of rain falling on 1 sq yd is 1296 in³. Using the American (formerly Queen Anne Wine) gallon of 231 in³, that’s 5.61 gallons ( about 4.67 Imp. gallons).

    Of course for larger areas, we just “invent” new units like acre-inch, which (along with acre-foot) is actually used in irrigation studies.

    The metric version is simpler. That same 1 mm falling on a hectare is 10 m³ of water.


  4. John Steele:

    Thank you for showing how it can be done ‘the American way’, but I see absolutely no logical, easily memorable link between any of the figures. I also note that you express the gallon figure as a decimal. I have never heard of a decimal gallon.


  5. Jake,
    Not sure, but maybe Americans like decimals better than Brits, or feel more comfortable with them. Our currency has always been decimal based, gasoline (petrol) is sold in decimal gallons on the pump. Under labeling law (FPLA), manufacturer may use decimal fractions with largest whole unit, or using cascading compound units. Both exist, but there seems to be a trend toward decimalized largest whole unit.

    As an example, a 160 fl oz container (US gallon has 128 oz) could be labeled either 1.25 gal, or 1 gal 1 qt; it may NOT be labeled as 5 QT. Total ounces (160 oz) may be given optionally, but is not sufficient alone. Additionally, metric, 4.73 L, must be given. Typically the decimal version gives a cleaner appearance and takes less room on the bottle. Unfortunately for metric advocacy, it is simpler and looks more like the metric.

    I think our tendency to decimalize things partially offsets one of the advantages of metric.

    I do agree there is absolutely no useful memory aid. You simply have to work it from basic principles and know or look up a bunch of conversion factors. The fact I can struggle through it should not be taken as advocacy.


  6. @Jake, you say … I also note that you express the gallon figure as a decimal. I have never heard of a decimal gallon.

    That Jake, is the American way. No problem with decimal inches either. They may be behind with metrication but they are way, way ahead of us with decimalisation.
    For that reason the USA will overtake us if and when they decide to go metric in the public domain. (MHO)


  7. For all the reasons given above the millimetre is a beautifully simple way of measuring rainfall because of how it relates to the volume of water over surface area.

    The problem is how is this relevant to people’s perception? The popular one is to think only in terms of one dimension which reduces it to a simple scale of comparison. In other words what does the number of mm or inches over period of time mean in terms of how heavy it is? (I guess we could answer that one a bit facetiously by saying 1 mm of rainfall is 1 kg of water per square metre).

    I’m afraid it comes down to the old chestnut. Yes the metric system is a great one and far better than traditional units but people don’t appreciate it because they don’t calculate very often or explore the differences in potential.


  8. Decimal versions of US Customary units may appear on some product labels and decimal inches may be used in certain trades but in everyday speech and in the media it’s still the good old fashioned way (non-decimal US Customary) here in the USA for the most part.


  9. Over 6 years and 5 months further down the road.
    1st May 1914 to 3rd October 2020, 110 years, 4 months and 2 days.

    Today the BBC online report on the flooding in UK actually translated the met office rainfall figures into inches, just so us old folk know its wet outside.

    Sometimes I just wonder what it will take to get used to the idea that we use metric.


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