What is a common initial sign of an approaching thunderstorm while on the water?

The buildup of dark clouds is often the first visible indication that a thunderstorm is approaching. These clouds, specifically cumulonimbus clouds, are a sign of strong updrafts and moisture in the atmosphere, which are crucial for thunderstorm formation. When you notice darkening skies and the presence of towering clouds, it suggests that unstable air is rising and cooling, which can lead to thunderstorm activity.

While high winds, heavy rain, and lightning strikes are certainly associated with thunderstorms, they typically occur after the dark clouds have formed and the storm is closer. Therefore, recognizing the early sign of dark clouds allows boaters to prepare and take necessary precautions before conditions worsen.