Both of the LPG gases, propane ($\ce{C3H8}$) and butane ($\ce{C4H10}$) are heavier than air. Propane is $1.55$ times heavier and butane is $2.08$ times heavier. Is same-temperature air always lighter than the LPG gases? If not, at what temperature(s) would a change(s) take place?

The reason I am asking this is because I recently opened a barbecue gas canister to drain it empty. But I stopped the draining not too long after, when I read on the net that LPG gas was heavier than air and would therefore not raise into the sky. The thought then came to me that maybe there is a difference between summer and winter here. That the gas may diffuse easier during the warmer part of the year.