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I am always amazed when there is a rain cloud moving across the country dropping inches per hour of rain as it goes. In my imagination I try to visualize how the cloud could have carried all that water so far inland from the ocean...

Some clouds pass overhead, casting a shadow but leaving me dry. Some clouds only sprinkle, hardly enough to impact my out door activities. Other clouds, well there have been sometimes that I really wish I had known sooner to look for shelter.

How can I tell, how much if any rain is going to fall from a cloud heading in my direction?

Note: I am not looking for suggestions to check the local radar on my smart phone. I want to know how to look at a cloud and decide how much rain it is going to drop.

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  • James, would you mind clarifying exactly what type of measurement you're seeking? Does "how much rain" mean something like a total liquid volume? It's not difficult to find out which types of clouds will produce types of rain, like light, moderate, or heavy, but you seem to be asking for something more specific. Am I correct? Thanks!
    – Sue Saddest Farewell TGO GL
    Jul 30, 2017 at 21:05
  • @Sue I want to be able to look at a cloud coming towards me (A few seconds observation is usually sufficient to determiner direction) and make a good estimate of of the amount and rain that will fall where I am. Does that clarify what I want?
    – James Jenkins
    Jul 31, 2017 at 12:10
  • Yup, that does clarify what you want. Thanks for responding!
    – Sue Saddest Farewell TGO GL
    Jul 31, 2017 at 15:24
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    This is actually quite fascinating. A quick Google search shows other people asking the same question. It's not easy to answer, but it seems that with some types of clouds in certain situations it can be at least estimated. Unfortunately, it's a lot of mathematics and physics I don't understand, so I don't think I can help, but I'd love to see an answer!
    – Sue Saddest Farewell TGO GL
    Jul 31, 2017 at 16:09
  • The question title is more suitable for earth science SE but your ultimate question is "how can I tell how much if any rain is going to fall from a cloud heading in my direction?". I suggest changing the title accordingly, as some good answers so far only/mostly address the title's question
    – cr0
    Aug 4, 2017 at 19:05

3 Answers 3

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Randall Munroe has the single best article on this I have ever seen, over on his What-If site where he discusses what would happen if all the water in a cloud formed one giant raindrop.

I recommend reading the whole thing, even if just for the destructive vision he has, and especially the final line, but a useful excerpt from the page here is:

We’ll imagine our storm measures 100 kilometers on each side and has a high TPW content of 6 centimeters. This means the water in our rainstorm would have a volume of:

100km×100km×6cm=0.6km3

That water would weigh 600 million tons (which happens to be about the current weight of our species). Normally, a portion of this water would fall, scattered, as rain—at most, 6 centimeters of it.

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Isolated clouds are going to be easier to judge than gradually thickening cloud cover, as cloud height is important. I assume you've already established that the cloud is approaching you, though you might still be lucky.

Heavy rain is often visible as distortion of the view below clouds at some distance, particularly if there's something distinctive that gets obscured (like a tree line, hillside, or prominent building). Lighter rain appears more like a faint mist. This can help you judge when the rain is going to arrive as well, from the rate that features are obscured. Pronounced darkening of the ground below an approaching cloud is another sign of heavy rain.

But if you're uphill from what you're looking at, the rain is likely to get heavier, or even start from a cloud that otherwise wasn't producing rain.

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    I plan to expand this from a book I've got at home.
    – Chris H
    Jul 29, 2017 at 11:21
  • Did you find that book at home?
    – James Jenkins
    Aug 2, 2017 at 12:28
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    ... but Liam wrote an answer similar to what I was thinking of getting from the physics book in work
    – Chris H
    Aug 2, 2017 at 15:42
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    sorry @ChrisH :)
    – user2766
    Aug 4, 2017 at 13:34
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    @Liam not at all, I had something else in mind when I wrote that - less physics and more observation
    – Chris H
    Aug 4, 2017 at 13:54
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There's an assumption here that a cloud is a static thing that fills up and then empties. This isn't the case. A cloud is merely a manifestation of the moisture in the air. The water condenses slightly to form a vapour. It's part of the water cycle. So as it's raining it could also be filling up from evaporation, etc.

enter image description here From Nasa

I suppose the short answer is the (maximum) amount of rain in a cloud is dependant on the air temperature, relative humidity goes up as the air increases in temperature. Here's a graph:

enter image description here

From here

So to pick a slightly arbitary figure at 60f(15c) every m3 of water in the air (can) hold just under a pound of water (I don't know why they choose pounds and meters here, meters and KG would be a better option but there you go).

Obviously cloud temperature can very considerably and clouds can be many km2 in size. Clouds can reach a height of 20,000 feet and come in lot's of forms:

enter image description here

source

If you look at the cumulonimbus cloud (a common "rain type" cloud) the temperature gradient from the top and the bottom (50,000 feet in total!) can be very large indeed as is the m3 of moisture.

If we take our cumulonimbus and assume it is 50,000 feet (15240 m) high. Let's assume the average temperature of this cloud is (picks a number at random) 33f(1c) that means every m3 holds a bout 0.3lbs of water. So a 1x m block 15240m high can hold about 4,572lbs of water. You then need to times this by the area (m2) of the cloud (if you can figure this out...).

Obviously we make a lot of assumptions here and it will not be a static amount (pretty much ever). Also bear in mind that not all this moisture will turn to rain.

So tl;dr a lot and it's complicated.

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    loved the tl;dr :D
    – WedaPashi ♦
    Aug 1, 2017 at 8:47

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