Latent Heat

January 16, at 9:00 a.m.  Track made by hauling the SS Von Guerard across the now-frozen inlet of its namesake stream.  By the next day, the Von Guerard was unnecessary.

Sensible heat is heat that changes the temperature of something.  I think it is so called because its exchange can be sensed, but it could just as well be because it makes sense–when you add or remove sensible heat the temperature of a material goes up or down, respectively.  When you turn on the burner of your stove, the water in the teakettle gets warmer.  Latent heat is a bit more mysterious.  Latent heat is exchanged without a change in temperature.  Once that teakettle starts to boil, its temperature stays the same, even though the burner on your stove is adding heat to it.

Latent heat and sensible heat are not different forms of heat, even though I might be leaving that impression.  Rather, it is the behavior of the materials that are exchanging heat that determine whether a particular exchange of heat involves latent heat or sensible heat.  Thus, we speak of the latent heat of fusion of water, which is the amount of heat that must be added to ice to make liquid water, once the ice has been brought just to its melting point (0° C or 32° F).  It takes nearly 160 times as much energy to turn ice into liquid water, still at o° C, as it does to raise the ice from -1° C to 0° C, and it takes takes nearly 80 times as much energy to melt ice as it does to raise the resulting liquid water from 0° C to 1°C.  That is why a single ice cube can cool an entire glass of water.

The same thing happens at the boiling point of water, but the the latent heat of vaporization of water is far higher–to boil water it takes 540 times the amount of heat as it does to raise the the water from 99° C to 100 ° C.  The latent heat of vaporization of water is the basis for the heat engine that drives our weather, especially thunderstorms, but because we are working in a land of ice, the latent heat of fusion is more relevant to us.  (This may be too much information for some readers, but it water can evaporate directly from the ice on our frozen lakes, even in the dead of the Antarctic winter, a process called sublimation.  This is the sort of like boiling water at temperatures far below freezing.  Strange as it sounds, sublimation is quite familiar to all modern folk–it is what makes our ice cubes shrink in the freezer.)

We measure water that melts from glaciers.  During our summer,  sunny, warm days (warm by our standards) had given us abundant stream flows.  We had a little cool spell, that I characterized as autumn, which slowed but did not stop our streams.  But, in mid-January, autumn turned into full fledged fall.  Temperatures dropped, it snowed, and the sun was hidden by clouds for several days.  But, the ice in the glaciers and the water in the moat did not get colder like the air had, at least not much colder.  Instead, a lot of water gave up its latent heat of fusion and froze.

We had been boating across the moat for quite a while, and then in just a few days we could walk across it again.   At the end of our January cold snap, the moat was now ice, two or three inches thick, and we could cross it comfortably, even carrying packs.  Even though sunny weather returned, the moat continued to gain strength because temperatures were low, the sun’s angle was just a little lower, and the thin coating of snow on the ice increased the albedo of the ice (the amount of incoming sunlight that is reflected without adding heat to the ice).  And, snow is a pretty good insulator, so when the air temperature did get above freezing the snow protected the moat ice.

This made our lives much easier–now we could walk out to the ATV and drive to the mouth of a stream and walk up to the gauge.  But, it also threatened to put us out of business–the physical behavior of the water in the moat of the lake was the same as that of the water on and in the glaciers.  A lot of latent heat had to come out of our moat to freeze three inches of ice.  Roughly the same amount of heat had to come out of the glaciers.  Water everywhere in the valleys was freezing.  Our streams dried up.

Images of fall in the Taylor Valley.



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