What is often the first indicator of an approaching thunderstorm?

The buildup of dark clouds is a key indicator of an approaching thunderstorm, primarily because these clouds, known as cumulonimbus clouds, are essential for the development of thunderstorms. As a thunderstorm develops, the atmosphere becomes increasingly unstable, and warm, moist air rises, cooling and condensing to form these thick, dark clouds. The dark color indicates a higher concentration of moisture and a greater likelihood of rain, which typically precedes a thunderstorm. Observing this cloud formation not only serves as an early warning sign but also signifies the potential for severe weather conditions like heavy rain, lightning, and strong winds. Familiarity with these cloud patterns can be crucial for boaters, as it enables them to take necessary precautions and seek shelter well in advance of the storm.