The upper portion of a cumulonimbus cloud that becomes flat and spread-out, sometimes for hundreds of miles downstream from the parent cloud. It may look smooth or fibrous, but in shape, it resembles a blacksmith's anvil. It indicates the mature or decaying stage of a thunderstorm.
ANVIL
Meaning of ANVIL in English
Weather and meteorology English vocabulary. Английский словарь погоды и метеорологии . 2012