Explanation:
This is because as the weather balloon rises into the atmosphere the air in the balloon will begin to fill the balloon until the balloon is overstretched. At a particular altitude, the balloon will burst.
The air in the balloon expands because the higher the altitude it does, the less the ambient atmospheric pressure is encountered. This is because the density of air decreases with altitude. The air inside therefore gains more pressure than the ambient air pressure causing it to expands.
Filing the balloon wiht too much air at the surface may cause the balloon to burst even before it reaches the required altitude.
