Caption: A phase diagram of water on a semi-log plot.

Features:

1. The horizontal axis is temperature in Celsius degrees (C) on the bottom and kelvins (K) on the top.

2. The vertical axis is pressure in pascals (Pa) on the left-hand side and in bars on the right-hand side: 1 bar = 10**5 pascals (Pa) = 100 kilopascals (kPa) = 0.987 atmospheres (atm) ≅ the air pressure near the surface of the Earth.

3. It helps to understand the plot by considering the horizontal level of ordinary Earth-surface air pressure which is at about 100 kPa = 10**5 Pa.

   As one goes to the right, one passes through the three phases: ice, liquid water, water vapor.

   Water vapor dissolved in air exists in the regions where water is ice or liquid water as we well know.

4. At different pressure, condensation and vaporization happen at different temperatures.

5. Note the triple point (61173 Pa, 273.16 K) (where all three phases of water can co-exist at once) and the critical point (about 22.064 Mpa, 647 K) (beyond which the distinction between liquid water and water vapor ceases to exist.

6. The phase diagram applies to thermodynamic equilibrium: that timeless state where the thermodynamic variables are NOT changing.

7. There are 15 ice (sub)phases numbered in Roman numerals: I, II, III, ... , XV.