Does moving air read cooler than still air on thermometer?

[Chris]

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?

[The Natural Philosopher]

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.

[Bob Mannix]

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.

[Bernhard Kuemel]

For low speeds the difference is negligible. At higher speeds the temperature will rise which is well known from space ship or asteroid entries into the atmosphere.

Bernhard

PS: alt.sci.physics was removed because it is unknown at my news server.

[DarkStar]

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.

[Andrew Gabriel]

Your skin is more like a wet bulb thermometer, which others have described. Providing the relative humidity is below 100%, it will be cooled by evaporation, and a breeze helps this.

I wrote a long article on this some time back, which you might want to refer to:

[evo]

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.