# Balloon volume as it rises in constant atmospheric density

I've been considering an hypotetical situation and it's making me a bit confused. If anyone could give me any insight, I would really appreciate it!

It is as follows:
Let's pretend atmospheric density remains constant with altitude. If I fill up a balloon at sea level and start rising it, would the balloon expand, shrink or remain the same?

My first thought it that it will expand. I filled it up at sea level, so it's inside pressure is the same as the outside. If I now start rising it, the outside pressure will decrease (since there will be less air above us), thus making the balloon expand to match it's inside pressure to the outside pressure. But this would mean the density inside the balloon is less than the density outside the balloon, which I find really counter-intuitive.
Furthermore, from what I understand, it's the density inside it that determines it's inside pressure. So it makes it all the more confusing to me that the density inside would be different from the density outside.

The only other possibility I can think of is that the question itself is flawed. The density can never be constant because the air at higher altitudes would always compress the air at lower altitudes, making it more dense.

Any thoughts?

• You write "Let's pretend atmospheric pressure remain constant with altitude." But then you write "If I now start rising it, the outside pressure will decrease (since there will be less air above us)". Those 2 statements are directly contradictory. The second one is physically correct. Which one do you want people to assume in their answers?
– Zaph
Commented Mar 11 at 20:48
• Oh, I meant "Let's pretend atmospheric density remains constant with altitude". Sorry. I've corrected it. Commented Mar 11 at 21:01