Final answer:
A stratus cloud that becomes a rain cloud is known as a nimbostratus cloud. These clouds are typically linked to continuous rainfall or snowfall and often lead to an overcast sky signaling a forthcoming storm.
Step-by-step explanation:
A stratus cloud that has become a rain cloud is known as a nimbostratus cloud. These clouds are associated with continuous rain or snow and often cover the entire sky, giving it a dark, gloomy appearance. The change from a stratus cloud to a nimbostratus cloud usually precedes a storm, which can bring significant amounts of precipitation depending on the conditions.
In the provided text, the darkening of the sky and the rumbling roll of thunder indicate the approach of a storm, which suggests that the stratus clouds are transforming into rain-producing nimbostratus clouds. Riders on the sage, as mentioned, would look forward to this as it signifies a change in weather that could bring much-needed rain.