Question: what type of air mass is moving in a cold front
[{'answer': 'a cooler mass of air', 'provenance': [{'wikipedia_id': '20279444', 'title': 'Cold front', 'start_paragraph_id': 1, 'start_character': 36, 'end_paragraph_id': 1, 'end_character': 56, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '20279444', 'title': 'Cold front', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 852, 'bleu_score': 0.780882170938434, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9861051542658286223}, 'section': 'Section::::Abstract.'}]}]