Which is a common first indicator of an approaching thunderstorm?

A buildup of dark clouds is indeed a common first indicator of an approaching thunderstorm. This phenomenon occurs as warm, moist air rises and cools, leading to condensation and cloud formation. The dark appearance of the clouds indicates that they are thick and full of water, which can lead to precipitation. As the storm develops, these clouds can intensify, and you may notice the clouds expanding vertically, indicating a more severe weather event is likely.

While high winds, lightning strikes, and a sudden temperature drop can also accompany thunderstorms, they are usually subsequent signs that appear as the storm itself becomes more severe. In contrast, the buildup of dark clouds serves as an early warning sign, allowing boaters and others to prepare for the potentially dangerous conditions that a thunderstorm can bring.