# What altitude = triple point of H20 on Earth?

**URL:** <https://boards.straightdope.com/t/what-altitude-triple-point-of-h20-on-earth/525117>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [January 15, 2010, 8:23pm UTC](https://boards.straightdope.com/t/what-altitude-triple-point-of-h20-on-earth/525117 "2010-01-15T20:23:58Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Cfidel99](https://avatars.discourse-cdn.com/v4/letter/c/5daacb/32.png) [@Cfidel99](https://boards.straightdope.com/u/Cfidel99)\
**Post date:** [January 15, 2010, 8:23pm UTC](https://boards.straightdope.com/t/what-altitude-triple-point-of-h20-on-earth/525117/1 "2010-01-15T20:23:58Z")

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Just curious about this one:

Since we understand h20’s “triple point” to be 0.006 atmospheres, and the fact that elevation has an effect on atmospheric pressure, at what elevation on Earth(if any) would h20’s triple point occur?

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**Author:** ![StusBlues](https://avatars.discourse-cdn.com/v4/letter/s/dc4da7/32.png) [@StusBlues](https://boards.straightdope.com/u/StusBlues)\
**Post date:** [January 15, 2010, 8:33pm UTC](https://boards.straightdope.com/t/what-altitude-triple-point-of-h20-on-earth/525117/2 "2010-01-15T20:33:58Z")

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Are you referring to this:

> **[How can water turn to vapor below the boiling point?](https://www.straightdope.com/21343295/how-can-water-turn-to-vapor-below-the-boiling-point)**
>
> A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

Pardon my ignorance of fluid dynamics, but you’ve lost me, mate.

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**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\
**Post date:** [January 16, 2010, 11:42am UTC](https://boards.straightdope.com/t/what-altitude-triple-point-of-h20-on-earth/525117/3 "2010-01-16T11:42:54Z")

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From this table it looks like it’s close to 40 thousand meters altitude.

> **[Atmospheric Pressure vs. Elevation above Sea Level](http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html)**
>
> Elevation above sea level - in feet and meter - with barometric and atmospheric pressure - \<i\>inches mercury, psia, kg/cm\<sup\>2\</sup\> and kPa\</i\>.

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**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\
**Post date:** [January 16, 2010, 11:47am UTC](https://boards.straightdope.com/t/what-altitude-triple-point-of-h20-on-earth/525117/4 "2010-01-16T11:47:52Z")

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While according to this: [http://www.coolmath.com/calculators/atmosphere-calculator.htm](http://www.coolmath.com/calculators/atmosphere-calculator.htm)

it’s 34500m

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**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\
**Post date:** [January 16, 2010, 11:48am UTC](https://boards.straightdope.com/t/what-altitude-triple-point-of-h20-on-earth/525117/5 "2010-01-16T11:48:57Z")

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But you’d also need a heater…