Question: what is meant by the term nimbus regarding clouds
[{'answer': 'precipitation', 'provenance': [{'wikipedia_id': '166737', 'title': 'Nimbostratus cloud', 'start_paragraph_id': 2, 'start_character': 201, 'end_paragraph_id': 2, 'end_character': 214, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]