# In an ideal gas, if temperature increases, will pressure or volume increase?

This question came up when I was talking about the atmosphere. Someone had mistakenly claimed that as temperature increases, the density of the atmosphere should increase as well. I reasoned from the ideal gas law that, as temperature ($T$) increases, then so should $V$, leading to lower density. But I realized that pressure could increase instead; the atmosphere is not in a sturdy laboratory container where volume is a constant. Under these circumstances, how can one predict whether pressure or volume (or both) will increase?