Page "Cloud" Paragraph 39

In the modern system of nomenclature, cumulonimbus is something of an anomaly when classified strictly as a cumuliform-category cloud.

The cumuliform designation appears in the prefix rather than the root, which refers instead to the cloud's ability to produce storms and heavy precipitation.

This apparent reversal of prefix and root is a carry-over from the mid 19th century, when nimbus was the root word for all precipitating clouds.

Since nimbus was classified separately from the cumulus, stratus, and cirrus categories during that time, it was usual to refer to it as a nimbiform cloud that comprised its own physical category.

Even with modern classification, a mature cumulonimbus with its flat base, heaped midsection, and feathery top, along with accessory clouds that can appear in a variety of shapes and forms, has a composite structure that has led to a partial revival, initially by NASA, of the old nimbiform designation that uses the more restricted cumulonimbiform category name.