I remember reading a long time ago about some experiments measuring evaporation rate of water in large open containers (tubs) sitting on open ground. It was a simple experiment, each day the water level would be checked and recorded. From what I remember the experiment was in the 1950's or 1960's in Israel.

Results generally made sense, the rate of loss was highest on hot sunny days, but something about the analysis lead to the conclusion that in addition to thermal effects (hot water evaporates faster than cold water) there was also a component to the evaporation rate related to the brightness of the light itself; a non-thermal or light-induced evaporation process.

I've been searching and so far come up empty-handed. I've even asked a related question in Physics SE (Rate of direct (non-thermal) photon-induced evaporation of water? ) several months ago, but so far no bites.

I can't remember where I read this, but it may have been as an anecdote in a text about either climate change or long term variability of the solar constant or the power output of the Sun.

  • $\begingroup$ Non-thermal? Heat transfer — conduction, convection, radiation. Am I missing something? $\endgroup$ – Rodrigo de Azevedo Feb 27 at 23:50
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    $\begingroup$ @RodrigodeAzevedo for energy to be thermalized it needs to reach some kind of local thermal equilibrium. If a photon comes in and kicks one or a few atoms out immediately for example, it would be a non-thermal process and you wouldn't use the word "heat" to discuss it. There's no temperature or change in temperature to be defined, no $\exp(-k_B T)$ Local equilibration takes time, this asks about a process that happens faster than that time. $\endgroup$ – uhoh Feb 28 at 0:43

Israel in the 60's: these experiments were probably from Gerald Stanhill (one of the discovers of global dimming). The "tubs" were probably class A evaporation pans (a fancy name for tubs!). Some of his publications on this topic include:

  • The control of field irrigation practice from measurements of evaporation (1962)
  • The use of Class A evaporation pan data to estimate the irrigation water requirements of the cotton crop (1963)
  • A field test of the control of cotton irrigation practice from Class A pan data (1964)

But all three were published in the Israel journal of agricultural research and seem very hard to find online... You might have to try to get a paper copy from a science library, or maybe send him a request via ResearchGate.

  • $\begingroup$ Excellent! Wow, thank you very much for tracking this down! I will check with librarians here and see if there is some way to get copies of these paper somehow. I'll let you know if I have some success. $\endgroup$ – uhoh Mar 4 at 16:05
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    $\begingroup$ You're welcome. Yes please, if you find something about the non-thermal evaporation process, I'd like to know (out of pure curiosity)! $\endgroup$ – Jean-Marie Prival Mar 5 at 8:36
  • $\begingroup$ 65; S. Cohena, A. Ianetzb & G. Stanhilla (2002) Evaporative climate changes at Bet Dagan, Israel, 1964–1998, neither of which I can see beyond the abstract at the moment. The first paper indeed says "Widespread reductions in pan evaporation during the second half of the twentieth century were first reported for the former Soviet Union and much of the northern hemisphere [62, 63]. These reports were considered evidence of global warming, which was thought to be increasing regional evaporation but decreasing... $\endgroup$ – uhoh Mar 7 at 5:54
  • $\begingroup$ ... pan evaporation due to a feedback influence of increasing regional humidity on local (or pan) potential evaporation [64] (see below). However, Stanhill and Cohen [23] considered decreasing evaporation to be evidence for decreasing solar radiation and Cohen et al. [65] showed that in Israel's arid conditions the overwhelming influence on evaporation is solar radiation." (my emphasis). I assume that going backwards in citations these will eventually link back to the early works you've found already. I'll see what I can find on those next week. $\endgroup$ – uhoh Mar 7 at 5:54

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