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Old 04-20-2019, 04:44 PM   #133
DickR
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Quote:
Originally Posted by chaseisland View Post
Hate to be disagreeable, but a small amount of ice will sublime to vapor and become incorporated into the atmosphere. When the conditions are right the sublimed water vapor will condense and become fog (which then eats more ice).
Further to this, ice does indeed sublime, even if well below the melting point, because it, too, exerts vapor pressure into the space above it. Visible proof if this is when driving in cold weather; if there is any thin film of ice on the windshield (from whatever source), driving along in that cold, dry air results in that ice evaporating to nothing.

The situation of ice melting in water is unique in that 32 F is the "triple point" of water, at which you can have water, ice floating in it, and a water vapor space above it (very cold steam, at that very low pressure), all in thermal equilibrium. Also, as part of that definition, the vapor pressure exerted by ice (0.08854 psi) is the same as the pressure exerted by liquid water. However, by enthalpy balance, the heat of sublimation (energy absorbed going from solid ice to vapor is the total of heat of fusion (143 BTU/lb) plus heat of vaporization (1075.8). Any ice that sublimes or water that evaporates into the air above it absorbs the heat it needs to do so mostly from the water and partly from the air in contact with the surface (that theoretically would cause some water to refreeze if done slowly). Condensing that water vapor back onto the water/ice surface would just return the energy absorbed by turning into vapor, so it's mostly a wash (I guess that's a pun, too).
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