I'm trying to learn how to make sense of numbers in a forecast. I can interpret temperature and wind speed, but I don't know what to make of rainfall. What does it mean, for a human, that there will be (for example) 0.5mm precipitation? Is it a lot or not? Is there a reference online describing precipitation amounts in everyday terms? I've been looking for something like that, but found nothing, only formal definitions of measurement.

  • $\begingroup$ You have to specify why the Wikipedia explanation doesn't work for you. $\endgroup$ – Deer Hunter Dec 24 '14 at 23:11
  • $\begingroup$ @DeerHunter It's okay, just not specific enough. In my experience, there's a difference between 0.5mm and 1.5mm, both of which fall in the ``light rain'' bracket. $\endgroup$ – Jan van Bloem Dec 25 '14 at 9:45
  • $\begingroup$ @DeerHunter to be fair, I do not see an explanation on that particular link? $\endgroup$ – Isopycnal Oscillation Jan 1 '15 at 7:43
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    $\begingroup$ You won't distinguish precip intensity from storm total precip amounts. You need the precip rate for that. $\endgroup$ – casey Jan 1 '15 at 13:15
  • $\begingroup$ (See also similar What does an mm of rain mean? $\endgroup$ – JeopardyTempest Jul 11 '18 at 16:09

It literally means that the water depth will be 0.5mm. For example, if you place a container under the rain and it fills up to 0.5mm, then the precipitation is 0.5mm.

Despite the fact that a wide container requires more rain to fill up than a narrow container, the larger surface area allows it to collect the right amount of rain water to fill up to the same amount as the narrow container in the same amount of time (as long as the walls are vertical). Therefore, if the rainfall is 1 mm, every square meter receives 1 liter of rain water.


By the way, tuna cans can be distributed along your lawn to serve as a very primitive, but very cheap measurement tool that you can use to create a spatial map of what your irrigation system is doing. You can use that for troubleshooting your efficiency, 1 full can is 1 inch irrigation/rainfall.

  • $\begingroup$ My impression was the OP needed a distinction between a drizzle and a light rain, inter alia. $\endgroup$ – Deer Hunter Jan 1 '15 at 8:04
  • $\begingroup$ @DeerHunter fair enough, i think i mostly payed attention to the title and thought the OP was after an intuitive feel for this particular convention on measuring precipitation. $\endgroup$ – Isopycnal Oscillation Jan 1 '15 at 8:14
  • $\begingroup$ This is just a definition, which I already know (and in which I believe you missed that it's per hour). I'm looking for a translation of some values into everyday terms, more detailed that the light/medium/heavy categories on Wikipedia. For example, I know that 18 degrees means that I can try going out in a tshirt, or that 45kmph wind means that I need to start being careful on a bike. $\endgroup$ – Jan van Bloem Jan 1 '15 at 11:34
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    $\begingroup$ @JanvanBloem the answer correctly measures total precipitation (no time dependency). If you are interested in precipitation rates (e.g. units of mm/hr), please edit your question to address this. $\endgroup$ – casey Jan 1 '15 at 13:19

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