What allows all 3 states of H₂O to be present at the same time?

How is it possible for there to be a glacier floating on water but under clouds? Depending on the temperature, shouldn't there only be one state of water? 




At the [**triple-point**](http://en.wikipedia.org/wiki/Triple_point) of a substance, all three phases can be present. Since it takes additional energy to go from ice at 0 C to liquid water at 0 C, there can be a considerable overlap for the **two** substances, e.g. 1 g ice with 99 g water or 99 g ice with 1 g water, but for thermodynamic equilibrium, the *triple point* is very narrow.


BTW, the first well-know American chemist, Josiah Willard Gibbs, pioneered in defining the relationship of temperature, pressure and state. See  [Gibbs' phase rule](http://en.wikipedia.org/wiki/Gibbs%27_phase_rule) for more information.


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Unless you're at water's triple point (273.16 K, and a vapor pressure of 611.73 Pa), you must have fewer than three phases **if all phases are in equilibrium**. 


However, it's easy to have ice, water, and water vapor present simultaneously under other conditions **if the phases are not all in equilibrium with each other.**


*Note: Water has other triple points, but they involve multiple forms of ice.*


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