What would likely cause a gas balloon to start descending when it enters a cold air mass?

When a gas balloon enters a cold air mass, the gas inside the balloon contracts due to the lower temperature. This phenomenon is explained by the ideal gas law, which states that a decrease in temperature, when the number of gas molecules and volume remain constant, leads to a decrease in gas pressure. As the gas contracts, it occupies less volume and the overall buoyancy of the balloon decreases.

This loss of buoyancy results in an inability to maintain lift, causing the balloon to start descending. The processes that affect lift in lighter-than-air vehicles hinge on the temperature of the gas inside the envelope; cooler temperatures lead to reduced lift since the density of the gas increases relative to the surrounding cooler air.

The factors relating to warmer air or an increase in external pressure do not contribute to the descent directly. Rather, it's the contraction of the gas within the envelope that plays the crucial role in this scenario of descending when encountering a cooler air mass.