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Meteorology - Rainfall measurement

  1. Jul 22, 2011 #1
    Hello, I wasn't sure exactly where to put this thread. Figured this was the most relevant section.
    Anyway, I wanted to know what exactly do meteorologists mean when they say that a region will receive 5mm or 10mm of rain during the evening.

    I read this wiki page
    but it is all still clear as mud. I thought there was a relationship between the mm value given and the volume of water that would land over a given area but i am not sure.

    Can someone please clarify? Is there anything straightforward like every square meter will get half a liter of water over 5 hours? or something to that effect?

  2. jcsd
  3. Jul 22, 2011 #2
    They certainly mean anything precise by it because their own information is quite general. All this means is that most of the locations in the region will get somewhere between 5mm and 10mm of precipitation during the evening. Some will get more than that, and some will get nothing at all.

    As a general rule of thumb, you should know that the lower the total (and 5 to 10mm is quite low) the lower the reliability of the forecast. A forecast of 10cm would give you a far likelier chance of getting at least something.
  4. Sep 29, 2011 #3
    When rainfall is collected, it goes through a funnel into a tube. The funnel mouth is about 10x wider than the tube(I think), so the actual height of the collected rain is larger than what you would see in a box or whatever. The height on the tube is what is measured, though. The tube is shrunken to amplify differences in height, making it more accurate.
  5. Sep 30, 2011 #4
    1mm rain means that every square meter covered with water with the height of 1mm. One meter is 1000mm, so the total volume is 1000 x 1000 x 1 = 10^6 cubic mm or 10^3 cubic cm or 1 cubic dm, which is one liter. So for every mm of rain you get one liter per square meter.
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