What is a common first indicator of an approaching thunderstorm?

The presence of threatening clouds, specifically thunderheads, is a common first indicator of an approaching thunderstorm. Thunderheads, or cumulonimbus clouds, are characterized by their towering appearance and can signal the development of severe weather conditions, including thunderstorms. When these types of clouds begin to form, it typically indicates that the atmospheric instability is increasing, which can lead to heavy rain, lightning, and other storm-related phenomena.

While high winds, warm temperatures, and clear skies can all occur in various weather conditions, they do not serve as definitive indicators that a thunderstorm is imminent. High winds may occur during storms but are not the first sign of one developing. Warm temperatures might be present before a storm but do not signal the storm's approach. Clear skies often suggest stable weather conditions, which makes them the least likely indicator of a thunderstorm's arrival. Hence, the appearance of threatening clouds is the most reliable sign that indicates the potential for a thunderstorm.