What is a primary effect of isobar convergence on local weather?

Isobar convergence refers to the scenario where lines of equal atmospheric pressure (isobars) come together, typically indicating that air is moving into a region and may lead to rising air. When air converges at the surface, it has nowhere to go but up, which can lead to upward vertical motion in the atmosphere. This process is crucial for cloud formation and the development of precipitation, especially in conditions that are conducive to storms.

As the air rises, it cools and can lead to the condensation of water vapor, forming clouds and potentially resulting in thunderstorms. Therefore, the primary effect of isobar convergence on local weather is indeed the formation of thunderstorms, particularly in the presence of moist air and an unstable atmosphere.

The other options represent different atmospheric phenomena that are either not directly related to isobar convergence or are effects one would typically observe in more stable conditions. For example, an increase in air pressure usually signals a descent of air rather than convergence, while a decrease in humidity and stabilization of atmospheric conditions suggest drier and more stable air masses, which would oppose the typical outcomes of convergence.