Does Balloon Lift Force Increase or Decrease with Altitude?

[the keck]

As a helium balloon increases in altitude, the density of the air inside decreases. However, the density of the air surrounding it (The atmosphere) also decreases
with altitude p=p0*exp(-z/z0), where z0 is the scale height of the atmosphere. So does this mean the lift force acting on the balloon decrease?p=p0#exp(-z/z0)
P=P0#exp(-z/z0) (Pressure decreases with altitude)
PV=nRT

The puzzling thing I find with this problem is this, if both the density of the helium inside the balloon and the air surrounding it decreases with increasing altitude, would this mean that the balloon could rise forever, since these effects cancel each other out i.e. the life force will stay the same as the initial lift force when it left the Earth's surface? (Assuming the atmosphere goes on forever, and the balloon is made of material that is infinitely stretchable i.e. can expand forever and not break)Thanks...I hope the problem is clear enough for you all to understand

Regards,
The Keck

 

[mgb_phys]

In theory yes, actually as the balloon ascends it reaches colder upper layers the helium contract and the balloon descends.
It is possible for balloons to cross intercontnaental distance before all the helium leaks away.