# Air Mass?

1. Nov 25, 2015

### Skim Halo

If E=mc^2, why is hot air lighter than cold air? Hot air has more energy and should thus have greater mass and therefore be heavier as weight = mg

2. Nov 25, 2015

### .Scott

At the same pressure, hot air is less dense - there's less of it per volume.
Aside from that, a number of molecules on a hot gas would be heavier than the same number of the same cold gas. But it would be very hard to measure the difference. For E=mc^2, you have an awful lot of E equating to very little m.

3. Nov 25, 2015

### Skim Halo

So it's the fact that the air's volume is not constrained which allows it to expand it's volume and being less dense it rises above the colder denser air.

I would assume then however that if two volumes of air were contact rained in equal sized separate vessels, the hotter air vessel would be infinitesimally heavier?

4. Nov 25, 2015

### Drakkith

Staff Emeritus
Exactly. Given two containers of gas, identical in all respects except for temperature, the hotter one would have a tiny bit more mass than the cooler one.