Question: what does the ef stand for in tornado
[{'answer': 'Enhanced Fujita', 'provenance': [{'wikipedia_id': '3724479', 'title': 'Enhanced Fujita scale', 'start_paragraph_id': 0, 'start_character': 0, 'end_paragraph_id': 0, 'end_character': 15, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '3724479', 'title': 'Enhanced Fujita scale', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 159, 'bleu_score': 0.8134674731911641, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10980769283352470433}, 'section': 'Section::::Abstract.'}]}]