Explanation

A weather balloon is not fully inflated on the ground because the air inside the balloon would expand and may burst as it rises.

The atmosphere is less dense at higher altitudes, so the pressure inside the balloon becomes greater than the pressure outside.

This causes the balloon to expand until it reaches its elastic limit and bursts.

Weather balloons typically burst at altitudes of over 15 miles (24.1 kilometers).

If a balloon were fully inflated on the ground, the gas would need to be vented out and wasted as it expands.