Comets are not harbingers of ill fortune and plagues. They are not atmospheric phenomenon. They also contain traces of alcohol, methanol, ammonia, silicates, carbonaceous particles, and other cosmic seasonings. The comets are frozen fossils of the formation of our solar system. Sooty snowballs or fluffy, dirty snowdrifts are phrases used to describe the nucleus.
Comets have a nucleus.
When comets are far from the Sun, all they have is a nucleus. The density is roughly that of water. They are irregularly shaped and have very low mechanical strength. It would not take much to pull one apart. Relatively speaking they are very small, maybe 10 - 20 km or so.