Which is a common first indicator of an approaching thunderstorm?

A buildup of dark clouds is commonly recognized as an early indicator of an approaching thunderstorm. This phenomenon occurs because cumulonimbus clouds, which are characterized by their towering presence and dark grey color, form as warm, moist air rises and cools. As these storms develop, they can lead to heavy rain, lightning, and strong winds. Observing dark clouds can alert boaters to the potential danger, prompting them to take necessary precautions, such as returning to shore or seeking shelter.

The presence of a decrease in wind speed, a glassy sheen on the water surface, or the development of a fog bank, while potentially related atmospheric conditions, are not typically reliable indicators of an imminent thunderstorm. Instead, they may signify different weather patterns that do not directly correlate with the approach of a storm. Thus, recognizing the buildup of dark clouds is crucial for safety awareness when out on the water.