Snow in winter below 0 °C, still sublimed very slowly over time. If I’m not mistaken, sublimation is an endothermic process. Where is this absorbed energy coming from when the surrounding temperature is below 0 °C?
$\begingroup$
$\endgroup$
3
asked Nov 21, 2019 at 3:38
-
2$\begingroup$ Water vapor has a finite value even at temperatures below 0 C, thus there is an equilibrium between water's vapor pressure and the temperature of the ice. $\endgroup$– MaxWCommented Nov 21, 2019 at 3:59
-
3$\begingroup$ As long as the temperature is above 0 K and the air is sufficiently dry, it’s fine. $\endgroup$– Karsten ♦Commented Nov 21, 2019 at 4:11
-
1$\begingroup$ Above 0°C or below 0°C, sublimation (as well as melting) is endothermic all the same. What's so special about 0°C? $\endgroup$– Ivan NeretinCommented Nov 21, 2019 at 5:19
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
Evaporating liquids and sublimating solids have temperature lower than the surrounding.
That is because the most energetic molecules escape to gaseous phase, cooling down the condensed phase, as it's average energy per molecule decreases.
The standard psychrometric method of air humidity measurement, used in meteorology/climatology, is based on this effect, evaluating the temperature difference between "dry" and "wet" thermometers. Note the the dry/wet temperature tables to calculate the humidity are provided even for the frozen "wet" thermometer".
Therefore, during ongoing sublimation or evaporation, the surrounding continuously provides thermal energy to sublimating/evaporating matter, as it is permanently colder ( unless 100% vapour saturation is reached and sublimation/evaporation stops )
If you have ( in interior ) a big enough freely hanging piece of washed up wet cloth, put your hand just below it. You may feel very slight, somewhat colder downward air flow, as air near cloth surface gets little colder and flows down.
