Which cloud type generally indicates impending rainfall within 24 hours?

The correct answer is Cirrostratus. This type of cloud is characterized by a thin, veil-like appearance that often covers the sky. Cirrostratus clouds are typically formed at high altitudes and are associated with the approach of a warm front. They can indicate that precipitation is on the way, usually within 24 hours. As these clouds move in, they can create a halo effect around the sun or moon, which is a visual sign that moisture is present in the atmosphere.

Cumulonimbus clouds are tower-like and can indicate severe weather, including thunderstorms and heavy rainfall, but they are more specific to short-term weather events rather than a general indication of rainfall within the next day. Cirrus clouds are high, thin clouds that usually suggest fair weather. Meanwhile, stratus clouds often cover the sky in a uniform layer and may produce light drizzle, but they do not always indicate impending rainfall as clearly as cirrostratus.