Which type of cloud is least likely to indicate impending rain?

Cirrus clouds are high, thin clouds that often appear wispy or feather-like in appearance. They are typically found at altitudes above 20,000 feet and are composed of ice crystals. These clouds usually indicate fair weather, particularly when they are seen in isolation. While cirrus clouds can sometimes precede storm systems, their presence alone does not signal immediate rainfall.

In contrast, stratus clouds can bring light rain or drizzle but are still relatively benign compared to more intense rain-producing clouds. Cumulonimbus clouds are large and towering, associated with thunderstorms, and Nimbostratus clouds are thick, dark clouds that produce continuous, steady precipitation. Therefore, among the types of clouds mentioned, cirrus clouds are indeed the least likely to indicate impending rain.