Final answer:
Rapidly decreasing air pressure on a barometer indicates the approach of a low-pressure system, which is usually associated with stormy or rainy weather.
Step-by-step explanation:
In the context of weather, rapidly decreasing air pressure on a barometer indicates the approach of a low-pressure system. Low-pressure systems are usually associated with unsettled and changing weather conditions, such as clouds, rain, and strong winds.
When the air pressure is low, moist air rises and condenses, forming clouds. This movement of moisture and air within various weather fronts is what triggers most weather events.
Therefore, it is likely that the weather will become stormy or rainy after a period of rapidly decreasing air pressure on a barometer.