名词解释：根据气象专业制定的“寒冷程度等级表”，气温从-40℃以下至9.9℃，由低到高共分为八级。其中一级为“极寒”，代表-40℃以下的天气。你知道怎么用英语表达吗?

The City of Toronto has called an Extreme Cold Weather Alert to ensure vulnerable homeless people are aware of the dangers of cold weather and have safe and warm places to go. This alert is in effect until further notice.

Extreme cold weather can be life-threatening to people who spend much of their time outdoors.

多伦多市已经发布极端严寒天气警报，确保无家可归的人意识到严寒天气的危险，并且有安全温暖的地方可去。这一警报将始终有效，除非有进一步通知警报解除。极端严寒天气可能会对那些在户外活动时间较长人的生命产生威胁。

【讲解】文中的extreme cold weather 即指极寒天气，extreme weather指极端天气，与极端天气相关的词汇有cold spell (春寒期)，cold snap 寒潮，drought 干旱，hurricane 飓风，typhoon 台风，ice rain 冻雨等。全球变暖(global warming)正是引起此次极寒天气的深层次原因,而环境污染(environmental pollution)、碳排放(carbon emission )正是全球变暖的一个重要推手。