Complex sentence: So the force pushing the balloon upwards diminishes with altitude and at some particular altitude the upwards force will equal the weight of the balloon: the balloon will be stable in a finite equilibrium altitude range for long periods.
Split into simpler sentences: So the force pushing the balloon upwards diminishes with altitude and at some particular altitude, the upwards force will equal the weight of the balloon. As a result, the balloon will be stable in a finite equilibrium altitude range for long periods.