Which of these provides a warning that dangerous weather is approaching?

The observation of dark clouds and shifting winds is a reliable indicator that dangerous weather is approaching. Dark clouds, especially cumulonimbus clouds, often signal thunderstorms, heavy rain, or severe weather conditions. When these clouds build up, they can lead to significant weather events such as high winds, lightning, or even tornadoes. Additionally, shifting winds can indicate changes in atmospheric pressure, suggesting that a storm system is moving into the area.

While clear skies might be associated with good weather, a sudden calmness of the water can sometimes precede a storm, but it is not as direct an indicator of imminent dangerous weather as dark clouds and shifting winds are. A change in temperature can indicate a shift in weather patterns, but it lacks the immediacy and reliability of visual cues like cloud formations and wind patterns. Thus, the combination of dark clouds and shifting winds serves as a critical warning for anyone outdoors to take precautions against approaching severe weather.