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Does moving air read cooler than still air on thermometer?

Does moving air read cooler than still air on thermometer? - Physics Forum

Does moving air read cooler than still air on thermometer? - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 07-26-2006, 09:51 AM
Chris
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Default Does moving air read cooler than still air on thermometer?



I am in the UK and we have hot and humid weather at the moment.

Of course. a room fan which moves air does not actually cool the room
but it increases evaporation from the skin which reduces its temperature
a little.

However, does moving air affect the temperature shown by an ordinary
digital thermometer for use as a domestic room thermometer?

At first glance the answer is no, but the air humidity is about 70 to 75
(percent at 26 C) and I wonder if there is an evaporative component to
temperatures read by such a "dry bulb" thermometer?

Anyone?
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  #2  
Old 07-26-2006, 10:08 AM
The Natural Philosopher
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Default Does moving air read cooler than still air on thermometer?

Chris wrote:

Any improvement in cooling that comes from waving a *dry* object about
in a breeze, only happens if the object is hotter than the surrounding air.

A thermometer should end up in total equilibrium with the air, waving it
gets it there quicker thats all surely.
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  #3  
Old 07-26-2006, 10:47 AM
Bob Mannix
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Default Does moving air read cooler than still air on thermometer?


"Chris" <[Only registered users see links. ]> wrote in message
news:Xns980C6E791458271F3M4@127.0.0.1...


You have answered your own question. IF there were an evaporative component,
it could only be if the bulb were not dry. For wet bulb readings (which,
when compared with a corresponding dry bulb reading allow you to calculate
the dew point and relative humidity), the most accurate readings are
accomplished by a "whirling hygrometer" which is a wet bulb thermometer (and
the dry one for convenience only) in a football rattle type object which you
whirl round to get the stabilised evaporative cooling and then take a
reading. (Well actually it's all done with electronics these days).

The effect of non still air on bodies that are hotter than the surroundings
is down to Newton and his law of cooling. The rate of cooling is
proportional to the temperature difference if forced air is passing over the
object in question. As it is nearly always the case that we are hotter than
the air in the UK, we always feel cooler with a fan. It is also the origin
of wind-chill factors. When the wind-chill factor on a day with an air
temperature of say 0degC is quoted as -5degC, it just means you feel as cold
as you would on a still day at -5degC. All of this is because, if the air is
still, it forms an insulating layer next to the skin - this is whisked away
by moving air. An inanimate (or even cold blooded) thing will feel 0degC at
0degC whatver the wind, as they are not losing heat by trying to stay
hotter.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)


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  #4  
Old 07-26-2006, 05:52 PM
DarkStar
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Default Does moving air read cooler than still air on thermometer?

It is my understanding that any moving gas
such as air represents a lower pressure
if this true then lower pressure no
matter how slight will have a lower temperature.

pv=nrt

ideal gas law might be of interest to you.

you can tell things like air density and
lots of other nifty things from this
seemingly simple formula in all its various forms.


"Chris" <[Only registered users see links. ]> wrote in message news:Xns980C6E791458271F3M4@127.0.0.1...


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  #5  
Old 07-29-2006, 03:05 PM
evo
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Default Does moving air read cooler than still air on thermometer?


"Chris" <[Only registered users see links. ]> wrote in message
news:Xns980C6E791458271F3M4@127.0.0.1...
......
Experience, not physics, has proven the fan motor will increase room temp.
Fans are always switched off when room is unoccupied: with one exception,
window fans at opposite ends of house, one set in and the other out.
No AC in the deep south, Charleston, SC.


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