Spiny Norman said:

This is the triple point of water where it melts, boils, and freezes at the same time. It occurs at 0.01°C and 611.657 pascals, where all three phases of water coexist in perfect equilibrium.

https://www.reddit.com/r/BeAmazed/comments/1kisi85/this_is_the_triple_point_of_water_where_it_melts/

If you travel upwards to 27.563 km above sea level, the air pressure will be about 611.657 pascals.

If I have my calculations right.