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At Modernist Cuisine, Nathan Myhrvold states that "Coffee with cream cools about 20% slower than black coffee". He then gives three reasons, two of them related to radiation and the third one to the influence of viscosity on the evaporation rate.

My question is twofold:

  1. Is his conclusion correct?
  2. Are those reasons relevant?

The question is somewhat open-ended, because "20% slower" could refer to the instantaneous cooling rate at $t=0$ or to the time needed to reach some lower temperature. It would be great to consider both parameters in the answer (will they behave the same?)

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Compare the partial pressure of water (which is 99.99% of coffee) to water with X% cream. Assume for simplicity that the container is a perfect insulator so only surface radiation and evaporation cause cooling. Then also look at thermal transfer and fluid flow (top-bottom-top) in the cup for various viscosities. –  Carl Witthoft Dec 10 '13 at 12:37
    
Do you want to repeat the experiment at one of our famous questions? physics.stackexchange.com/a/5510/562 –  hwlau Dec 20 '13 at 9:58
    
I do not see how repeating that experiment would answer the question above... –  Doru Constantin Dec 21 '13 at 8:22
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