I was recently on a trans-pacific flight in which there was displayed the outside air temperature which made me think of this question.
If I were attach a thermometer to the nose of the plane (in other words, not shielded from any wind), would the temperature read higher or lower than if the thermometer were stationary?
My first inclination is that the temperature would read lower due to the cooling of the wind. But then I thought about what temperature is measuring: namely the momentum of air molecules. Therefore, moving through the air faster would cause an increase in the reading.
Wind Chill is a subjective measurement. It is how cold the air is supposed to ‘feel’ to a human. For all real aspects it is meaningless. For instance, if it is 33[sup]o[/sup]F outside with a wind chill of 28[sup]o[/sup]F water will not freeze.
As to the plane I’d expect the thermometer to read warmer than the real temperature due to friction caused by the passing air. For a plane at 600 MPH it probably isn’t all that much but consider how hot the space shuttle gets on re-entry to know that air friction can be pretty significant (I’ve heard it is what limits how fast we can make planes…the SR-71 spyplane would leak fuel like crazy while on the ground till friction heated it up and caused the gas tanks to seal).
Wind chill tries to address the rate of cooling. It does not affect the actual temperature.
The idea is that if it’s, say, 40 degrees with wind of X mph, one would lose heat as rapidly as if it were 30 degrees in still air. This is sometimes stated as “it feels like 30 degrees.” Still, it actually is 40 degrees, and nothing gets colder than 40 degrees (not totally true, but a helpful simplification for this discussion). Wind chill is only significant with respect to a person or thing that is warmer than ambient temperature, and how fast that person or thing cools down.