# how conductive is rain water?

**URL:** <https://boards.straightdope.com/t/how-conductive-is-rain-water/593202>\
**Category:** Factual Questions\
**Created:** [August 18, 2011, 6:23pm UTC](https://boards.straightdope.com/t/how-conductive-is-rain-water/593202 "2011-08-18T18:23:44Z")\
**Posts on this page:** 2\
**Page:** 2

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**Author:** ![WarmNPrickly](https://avatars.discourse-cdn.com/v4/letter/w/d78d45/32.png) [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)\
**Post date:** [August 19, 2011, 10:37pm UTC](https://boards.straightdope.com/t/how-conductive-is-rain-water/593202/21 "2011-08-19T22:37:23Z")

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I read something recently that suggested some of the high resistance in ultrapure water was due to dissolved gases. When these gases were removed, it was more like 5 MegaOhms.

I’m speaking from memory, and have no cite. We use 18.6 MegaOhm water in our lab.

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**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\
**Post date:** [August 20, 2011, 12:36am UTC](https://boards.straightdope.com/t/how-conductive-is-rain-water/593202/22 "2011-08-20T00:36:47Z")

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Resistivity is commonly given in ohm-cm, not ohm/cm. This is known as “volumetric” as you can derive the resistance based on a given volume. You would take this value and multiply by the length, since length increases resistance. Then you divide by the cross-sectional area, as more area decreases resistance. Technically measured values are probably defined on a standard volume at a specific temperature as well.  
And of course it’s not uncommon to incompletely specify the units when everyone knows what they signify (as in the 18.6 Megaohm figure **WarmnPrickly** gave).

Referring to all water as conductive to a degree isn’t a problem in itself, as long as you understand it apples to every material. By this reckoning, insulating materials do not exist. I’m not trying to be facetious either; it can be useful to remember what can happen under the right circumstances.

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