Rain Units

  • Hi All,

    I do not understand the rain unit
    Is it the intensity?
    Because: Intensity= Height/Time
    Usually it is mm/h
    How can we translate the unit on Windy?
    I already checked some others posts and you replied that it mm/3hrs what I saw.
    The fact is Official Website are using the height of rain or the intensity with mm/hour

    For example, today the 07.08.2019, on Windy the prediction was around 12mm/3hr on Saint Clair du rhone around 7 o'clock - France. But if we user mm/hr unit, it is 4mm/hr as intensity which is not the one we got on the city.

    Have you an explanation? please



  • Moderator

    mm/3 hrs


    You say "But if we user mm/hr unit, it is 4mm/hr as intensity which is not the one we got on the city."

    You compare forecasted values vs observed values.
    Usually those values are not the same, especially if you reduce the time window.
    e.g. let say for a location the 3hr rainfall forecast (during a 24 hr period) is:
    0 mm, 5 mm, 10 mm, 25 mm, 2 mm, 0mm, 0 mm, 0 mm (total forecasted 42 mm/ 24 hr)
    and the observed values:
    0 mm, 0 mm, 15 mm, 30 mm, 5 mm, 0 mm, 0 mm, 0 mm (total observed 50 mm/24 hr).
    Although the total 24 hr rainfall forecast is only 16% off
    if you compare a particular 3 hr period it may be 100% off.

  • Hi,

    Alright, thank you for you explanation.

    So, the 3hr rainfall forecast is not an intensity but a rain accumulation according to you in "mm" ?
    Because the intensity are defined by Meteo France (in France) as:
    1 > 3 mm /hr > light rain
    4 > 7 mm /hr > moderate rain
    8 and more mm /hr > strong rain

    if it was an intensity on the rain forecast (Arome Model), we should have had at least 24 mm/3hrs even more than 30 mm/3hrs. As it fell in 1 hour around 12 mm.

    And if I comme back to my example: the forecast was't 12 mm/ 3hr or 11 + 12 in 6 hours and in 6 hours it fell around 50 - 60 mm.



  • mm/3 hrs. Hmm. Well, I guess that's not as bad as L/100 km as units go...

  • I do not know this unit "L/100km" and I use L/m²
    I do prefer neither one nor the other, just need to understand the difference and the correspondence :)

  • Sailor Moderator

    In Windy,
    L/km2 means « lightning per square kilometer ».
    Rain amount is expressed in mm or in inches.

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