Letting Air Out of Tires II

In a recent post I calculated how cold air coming out of bike tires should feel.  However, at the end of the post, I did note that there are competing explanations for why the air cools.  There's the approach I took, which is adiabatic cooling, but there's also something called the Joule-Thomson effect.  The Joule-Thomson effect has the interesting property that helium being let out of a bike tire would actually be warmer, which suggests an immediate way to test which effect is dominant.  We pressurize a bike tire with helium, and see if the valve gets cold or hot.  This is exactly what I did.

Letting Air Out of Tires

Have you ever noticed how when you let air out of a bike tire (or, I suppose, a car tire) it feels rather cold?  Today we're going to explore why that is, and just how cold it is.  Many people consider the air escaping from a tire as a classic example of an adiabatic process.  What is an adiabatic process?  It is a process that happens so quickly there is no time for heat flow to occur.  For our air in the bike tire this means we're letting it out of the tire so quickly that no energy can move into it from the surrounding air.