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Well 10 millimetres of rain is different that 10 millimetres thickness for say branch...

kqr 2 days ago | parent | next [-]

At first I was going to contradict you but then I realised 10 mm of rain is actually a measure of volume or mass, corresponding to 10 kg, right?

And of course, people use it mainly as a rate, i.e. 10 kg per hour, or per four hours, or six hours, or 24 hours.

And it gets worse when we start talking about snow, the density of which can vary a lot!

	▲	Ekaros 2 days ago \| parent [-]
		Well depending on area you are counting it on. So yes per square meter it would be 10 kg. Your rain gauge might for example not have 10mm spaced markings...

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margalabargala 2 days ago | parent | prev [-]

Could you elaborate on this?

If 10mm rain falls, and you have an open collection container, then the depth of the water in that container would be more, less, or the same as the thickness of the branch?

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camtarn 2 days ago | parent [-]

...as long as the container has straight sides. The area of the opening needs to be the same as the area of the container all the way down, otherwise the two areas don't cancel out.

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margalabargala 2 days ago | parent [-]

Sure you can do that, and now you're measuring something different. The volume of an exotic container is measured in different units than the depth of rain falling from the sky evenly.

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camtarn 2 days ago | parent [-]

I have no idea what you mean here. Exotic containers? I'm talking about something like a measuring cylinder or a straight-sided glass or mug.

This StackOverflow answer probably does the subject more justice than I can: https://earthscience.stackexchange.com/questions/14587/what-...

but I'll try and explain it succinctly:

What you're measuring is the volume of rain falling per unit area in a given time (usually 24 hours). If you're collecting rain in a vessel, you divide the volume collected by the area of the vessel's mouth to get the volume collected per unit area. And in order to measure the volume in the container, you measure the height of the water and multiply by the area of the vessel's bottom.

If the bottom and mouth have the same area, those cancel out and you can just specify the rain height regardless of the size of the container. That is, if you have 1ml of rain falling per square mm over 24 hours, it will produce the same height of water if you set out a big container or a small container, as long as the containers have straight walls.

If they don't have the same area, then you can't use the handy mm unit.

	▲	margalabargala 2 days ago \| parent [-]
		I couldn't have explained it better myself. Glad you understand! Aside from the last bit. The rainfall per unit area is not relevant to the shape of a particular container. 5mm of rain falls regardless of your container shape. Whether that 5mm of rain falling, also means that the water in your container is 5mm deep, is a function of your container shape. It's not a unit problem. If you're trying to measure rainfall in a conical vessel, you can do that, and the conversion from collected volume to fallen rain will still yield the same 5mm out of the sky.