Which weather condition poses a significant risk due to potential visibility reduction?

Rain and fog are prominent weather conditions known for significantly reducing visibility. When it rains, droplets in the air can scatter light, making it difficult for pilots and drivers to see long distances. This effect can be exacerbated in heavy rain, where visibility might drop to dangerous levels. Fog, on the other hand, consists of tiny water droplets suspended in air, resulting in a blanket that can impair sight drastically even in relatively short distances. Both of these weather phenomena can pose challenges not just for navigation but also for safety, making it crucial for aircrew to be aware of and ready to manage the risks associated with these conditions.

In contrast, heat waves generally do not directly impede visibility, as they primarily affect temperatures rather than atmospheric clarity. Clear skies offer optimal visibility since there are no obstructions or reducing factors in the atmosphere. Scattered clouds also typically allow significant visibility, as they do not cover the sky completely and can often have no effect on sight. Thus, rain and fog stand out as the conditions that most critically impact visibility.