Found in today's Accuweather WeatherWhys® feature, accompanied by this explanation:
You will notice that snow melts quicker when the air is moist than when it is dry. Evaporation of any snow that melts when the air is dry readily cools the snow's surface, slowing the melting process. On the other hand, evaporation is slowed when the air is moist. The snow's surface cannot cool itself, causing melting to occur at a quicker rate.This is a bit counterintuitive for me, since I would have thought that dry air would lead to faster sublimation of snow - but perhaps above freezing the other principle dominates.
One fact that might have lead partly to your misunderstanding is that the ice is probably not appreciably sublimating at atmospheric pressure. The water molecules in ice are pretty tightly bound and only appreciably sublimate at really low pressures.
ReplyDeleteSee this diagram and you can follow the line at 1atm http://serc.carleton.edu/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg
Really what is likely happening is that a little bit of water is melting on the surface of the ice and then evaporating off. This means that the cooling effect of evaporation is really important because if there isn't a little bit of melting on the surface it is really hard for the water molecules to leave the ice.
This is just me remembering back to college chemistry so take it with a grain of salt.
Well, if that's the case, then the freezer in my refrigerator must be below atmospheric pressure, because ice cubes left in a tray lose half their mass within a period of weeks.
DeleteThe same thing happens in my freezer. But A. Fischer might still be right. Whenever you open the door, or your automatic defrost cycle goes, the surface of those cubes will likely melt a bit. And that water evaporates. A clue that liquid water forms on your ice cubes sometimes is that they get stuck together.
Delete-Chuck
I found some discussion of this at a USGS page -
Deletehttp://ga.water.usgs.gov/edu/watercyclesublimation.html
Atmospheric science is so consistently counterintuitive (to me) that I should start going against my gut every time.
ReplyDeleteOne little factoid that seems to trick people is that a parcel of moist air is less dense than a parcel of dry air. Intuitively moist air is hard to breathe and thus feels heavy. But water vapor has a much lower molecular weight (18 g/mol) than air (something like 28.8 g/mol I think).
Science!
Wow - that was totally new to me. Had to look up more info in Wikipedia -
DeleteThis occurs because the molecular mass of water (18 g/mol) is less than the molecular mass of dry air (around 29 g/mol). For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume (see Avogadro's Law). So when water molecules (vapor) are added to a given volume of air, the dry air molecules must decrease by the same number, to keep the pressure or temperature from increasing. Hence the mass per unit volume of the gas (its density) decreases.
Related - but only vaguely, is the unexpected (to me) fact that air emerging from a trompe [1] is dry. This is because the water is generally quite a lot colder than the air and vapour condenses onto the walls of the bubble leaving dry air behind.
Delete[1] http://en.wikipedia.org/wiki/Trompe
I had never heard of a trompe. Thanks, Skipweasel.
DeleteThink of it as sweating. When it's really humid you can't lose heat to the air easily because your sweat doesn't readily evaporate carrying away the energy required to change phase. Conversely, when the air is very dry it's easy for sweat to evaporate and cool you effectively.
ReplyDeleteSame with ice.