Which of the following is usually the first common visible sign that a thunderstorm is approaching?

Introduction / Context:
Weather observation skills are important for safety and general awareness. Thunderstorms are severe weather events involving thunder, lightning, strong winds, and heavy rain. This question asks which sign is typically seen first when a thunderstorm is approaching, testing your understanding of common atmospheric observations.

Given Data / Assumptions:
- The focus is on a common first indicator that can be observed by an ordinary person.- The event in question is a thunderstorm, not simply a hot day or a windy day.- Options include different weather features such as heat, wind, clouds, and lightning.

Concept / Approach:
Thunderstorms form in unstable air where warm, moist air rises rapidly. Before the storm arrives, you normally see large cumulonimbus clouds building up vertically and becoming dark at the base. These dark, towering clouds appear before heavy rain and lightning become obvious. Heat may contribute to storm formation, but it is not itself a clear visual sign. Lightning and thunder usually come after the storm cloud has developed substantially.

Step-by-Step Solution:
1. Identify typical features of a thunderstorm: dark towering clouds, gusty winds, thunder, lightning, and heavy rain.2. Consider which feature is visible earliest to someone watching the sky.3. A buildup of dark cumulonimbus clouds is often seen on the horizon well before the storm arrives.4. Lightning flashes and strong gusts usually come later, once the storm is nearer.5. Therefore, a buildup of dark clouds is the most reliable first visible indicator among the given options.

Verification / Alternative check:
Weather guides and safety manuals often warn people to watch for dark thunderclouds building up and moving closer as a sign of an approaching storm. Outdoor sports events are frequently halted when organisers see threatening dark clouds, even before any thunder or lightning is observed. This practice confirms that the cloud buildup is treated as the early warning sign.

Why Other Options Are Wrong:
- Excessive heat: Hot weather may occur earlier in the day, but heat alone does not reliably indicate a thunderstorm and may also occur without storms.- Heavy wind without clouds: Gusty wind can appear, but without storm clouds it could be due to other weather patterns and is not a consistent early sign.- Lightning flashes: Lightning typically begins after the storm cloud has already developed and is close, so it is not the first common indicator.

Common Pitfalls:
Many people focus on spectacular events like lightning or thunder and forget that the sky gives earlier subtle signs. To interpret weather correctly, always watch the cloud patterns first. Recognising dark, towering thunderclouds early can provide extra time to seek shelter before dangerous lightning and strong winds arrive.

Final Answer:
A buildup of dark clouds is usually the first common visible sign of an approaching thunderstorm.