Science Questions

Why does water cool more quickly outdoors, even when the weather is hot?

Sun, 12th Sep 2010

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Question

Pam Giles asked:

We did an experiment where we had water that was 117 degrees Fahrenheit. We placed two containers inside in the air conditioning at 78 degrees and two containers of the same temperature, same quantity of water outside in the sun and two outside in the shade.

 

We were shocked to find that the water outside in 90 degree weather in the shade cooled much quicker than the water in the cool classroom.

 

Someone suggested it was evaporation and so we covered the tops the next day and re-did the experiment and still got the same results.

 

Weíre wondering why is the water in the hotter air cooling quicker than the water in the cool, air-conditioned classroom?

 

Answer

Dave -   Thatís very interesting.  The answer I was going to give immediately was evaporation because the maximum amount of heat which water looses is generally due to evaporation and therefore if youíve got wind blowing across the top, all those hot molecules evaporating off the top will get blown away quicker, you can get more evaporation and itís going to lose heat quicker.  It could still be: I would have thought the only big effect is going to be the wind and it could just be that even just losing heat to the air around it is a lot faster if wind is blowing past something because otherwise, if the air is stationary, the hot object just heats up the air around it and then that insulates it from the rest of the air.  If thereís a wind blowing past, it will keep moving that hot air away and so you can keep losing heat much more quickly.  Certainly the only thing I can think of is that itís to do with the wind but possibly due to conduction as well as evaporation. 

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Scott - I think Dave's hypothesis about the wind is right, and I think that there are some useful experiments that could be run.  Pam says that she put the outdoor water in the shade.  That suggests to me that there was probably both shaded and unshaded areas nearby.  That would likely create a temperature differential between the sunny area and the shady area. The air in the hotter sunny area would rise, sucking in air from the cooler shaded area.  That would generate the wind, which disrupts any insulating warm air that might build up immediately around the glass in the shade.   

The thing is, there doesn't have to be much wind to keep disrupting the surrounding insulating warm air from forming, and it might be hard to notice it without sensitive instruments.    But there are a couple of experiments that might help check out this hypothesis that would be easy to do without sensitive instruments.  In particular, you might want to create an experiment that prevents air flow in both locations, and another experiment that creates a standard air flow in both locations.

To eliminate air flow, get a two identical large styrofoam insulated boxes like you might take on a camping trip, the bigger the better.  Put one in each location and leave the lid open.  Put a thermometer  inside each box, laying it on the center point and wait until the temperature seems to have stabilized at the ambient temperature. Then move the thermometer slightly and put the glass of water on the center point and  then close the lid Wait an hour and check water temperatures in the box stored in the class room and the one stored in the shade, you might also find it interesting to compare the change in the air temperature in each box too. 

If you use the wireless garden thermometers you can get these days you can put the sensor pieces into the box and into the water and take measurements continuously outside the box.  Just make sure that the sensor you put inside the water is sealed so the water doesn't short out the electronics. You can do that by sliding putting a  condom snugly around it, and then tying it off.

Alternatively, if you get wired temperature sensors such as you would use in an oven, you can thread the sensor wires through the drain hole, and then connect them to your monitoring instrument or computer and keep a digital record that you can graph over time.

Similarly, you might want to run an experiment to see what happens if you have a constant flow of air in each setting.  To check out this case, get two of fans used to for cooing tower computer cases from a computer supply store for each box.  Cut holes in the boxes that are the right size and install the fans so that one draws air in one side, and the other blows it out the other side.  Hook the fans up to an appropriate power supply and run them continuously.  Now rerun the same experiments.

If the wind explanation is right, you should see that in the first experiment the glass in the insulated box stored in the shade doesn't cool off as fast as your exposed in the shade glass did last time, and that now the water in the glass that was stored in the insulated box in the air conditioned class room cools faster than one in the glass in the warmer insulated box outdoors in the shade.

Furthermore, in both cases, the water in the glasses should cool faster when put in the boxes with the fans running in the second experiment, than in the insulated still air of the first experiment.  Try it and let us know your findings! mcgregor94086, Wed, 15th Sep 2010

The only possible reason is the wind... Evaporation is negligible because the temperature of the water is not high enough (far from boiling point).

I have a question (just for fun): You have just prepared a cup of coffee and you like it with a lot of cream, but you want to begin to drink your coffee in 7 minutes. In order to keep it warmer, is it preferable to add the cream right now or to wait and add it only before you drink your coffee? CPT ArkAngel, Wed, 15th Sep 2010

Heat up the cream in the microwave (much faster since it is a small volume you can do that fast), then pour it in right away.  Not recommended for powdered creamer.   Even if the cream is not heated, add it right away . See http://web.cecs.pdx.edu/~graig/me323/Solutions-Practical-Problems.pdf item 14 for an explanation. mcgregor94086, Wed, 15th Sep 2010

I suspect the humidity of the air in the two locations could also be a factor. I would not assume they would be the same in both cases. Geezer, Wed, 15th Sep 2010

I should precise that you don't heat the cream... Yes, your right, because the cooling rate is proportional to the difference in temperature between the cooling object and its surrounding (temperature gradient). There is other factors like insulation, but in this case, we can assume that the insulation is fixed.

http://en.wikipedia.org/wiki/Temperature_gradient CPT ArkAngel, Thu, 16th Sep 2010

  In answer to your question, Right now.  Anything cools rapidly at first and it is better to get the cooling effect immediately as it cools more slowly later.  Thanks for comments.  Joe L. Ogan Joe L. Ogan, Mon, 20th Sep 2010

Take a look here for the cooling effect of wind.
I definitely expect it to be the wind doing it. It steals the heat from any body, and steam might be cooled even quicker?
yor_on, Sat, 25th Sep 2010

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