Why rainfall is measured in length units and not volume units

Why rainfall is measured in length units and not volume units

Does the size of the rain collecting vessel matter? In other words, does its diameter matter?

Nope, any diameter will produce the same result.

Why? the height of the water in the collector is equal to the volume of water in it divided by the area of its opening. The volume of water in the collector is proportional to the rate of rainfall (measured in something like drops per unit area) times the area of the can. If we put these together, we find that the area of the rain gauge cancels from the top and bottom of the ratio—meaning the area doesn’t matter!

The only meaningful measure left is the height of the rain collected. Hence only mm or inches.

Another way to think about it is – if all the rain that fell didn’t disappear (absorption in the ground, evaporation, etc.), and if there were a way to keep it in one location like it is in a vessel then it’s height would stand at x mm, or inches, if you are like me :), in that location.

So, stating the measurement as volume would be misleading unless you also specified the amount of area on which that volume fell.


1mm=1 liter/M ^2
if the measuring container has an area 1M ^2
To increase resolution one may use a collecting area larger than the measuring cylinder dia, thus allowing finer marking on the cylinder. The standard ratio is 10:1 in the standard rain gauge.

Some interesting additional detail of rainfall measurements for agriculture:

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@punkish. I took me sometime to swallow this. I’ve always thought that that its 100mm of rainfall over 1 square meter. But now I realise that it doesn’t matter.

So what’s left as important is over how much time. As in x mm of water over 1 hour. That would help determine flood situations to an extent. Typically the amount of rainfall is stated over a 1 day period. So if you have 150mm every hour for 3 hours and then no rain the rest of the day it would still be 450mm in the day. Which doesn’t quite tell you about the flood in the morning. :wink:


Older mechanisms used a drum plotter. Newer ones use electronics. Tipping bucket is the most common. But weight measuring devices are also used.

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Equally as important, it also matters where the rain gauge is located. As can be well imagined, in a municipality, this is as much of a real estate decision, as one of scientific or even social relevance.

To get around it, the Maker Lab at #HBCSE has been engaged in developing diy weather stations. The key to creating am affordable device, or, to be precise, a family of devices, is to create a doable construct around relatively affordable and accurate sensors, and linking them up with a data recorder. The latter, fortunately, need not only be quite smart, it is also network ready, meaning that data can actually be gathered in a weather resistant location for safety and security, and minimising the need to physically inspect the station for this purpose.

The challenge is therefore to make the station durable and service proof, allowing it to be mounted in a place that makes the most sense for the collection of evidence about the weather, or the climate. Doing it oneself also opens up the possibility of innovative thoughts about new kinds of data that can be productively gathered regarding the climate or the local ecology.

With homemade stations, the number of locations for placing one increases phenomenally. It becomes possible to obtain very grainy information, apart from getting very many more people, especially youngsters, in the process of getting to understand climate.


This question was mumbling in my mind while reading @punkish’s post and scrolled down to see this :slight_smile:

Question: given the fact, based on personal experience, that in big city like Mumbai rain fall happens very unevenly across the city why/how the reportage say " Chennai receives over 100 mm rainfall in 24 hours" or “Mumbai has already recorded its wettest monsoon since 1954. From June 1 to September 17, the rainfall recorded in the city was 3,467.6 mm, surpassing the record of 1954 when 3,451.6 mm average seasonal rain was recorded.”

Am wondering if IMD etc has multiple weather stations or rain guages across the city/district and then average of all these multiple gauges is taken to conclude city/district/state wide rainfall measurement or something else is done to arrive at these numbers?

I think HBCSE maker lab’s DIY weather-station, if actualised, has great potential to democratise, decentralise and enhance the accuracy of weather related understanding.

Here I have explained the math and reason behind it go check it out.

volume of rainfall per unit area = length
Since cm^3 / cm^2 = cm (it is basic dimensional analysis)

So, this is quite puzzling. When I scroll up this thread, I find @punkish’s original post was the first of 7 till now, and it didn’t include that sentence. It starts here:

Are there more posts in the thread that, for some reason, are not visible to me?

@vvcstemplay that was just an aside, my main puzzle was the area or dia of rain gauge doesnt matter but the location matters. So, how do city/district/state wide rain falls get measured and how accurate they are?

I suppose we should try and get a comprehensive answer.

For Mumbai, there are two reporting stations, Colaba and Santa Cruz. They definitely provide rainfall measurements, that is public information. Exactly what else do they report, and to whom?

How does the reporting data vary for other cities?

Several cities nowadays have large display signs on roads, reporting pollution data, for instance. Where is this information gathered, at how many locations in each city?

Actually the rain gauge is not always one unit consisting of a collection area with a cylinder whose base area is the same as the collecting area.
This is so to increase resolution. Thus the measuring cylinder in many commercial gauges has an area 1/10th of the collecting area. Therefore 1cm on the cylinder will represent 1mm.