作者：朱先祥 陈雪儿 项鸿雁 吴明泽 陈嫱 新华社音视频部制作 "防空警报"是城市防空的重要组成部分，平时用于抗灾救灾和突发事故情况下的灾情预报和紧急报知； 战时用于人民防空，是各级人民政府实施人民防空指挥，组织人员疏散的基本手段；是在城市受到空袭 威胁时鸣响的提醒人们防空袭的警报。 国家规定防空警报信号分为预先警报、空袭警报、解除警报三种，一起了解一下防空警报的相关知识。 ...

Core Points - The article discusses the air raid siren test conducted in Hangzhou on September 18, emphasizing its historical significance and future implications [1] - The test involved synchronized alerts across 13 districts, showcasing the city's preparedness for emergencies [1][2] Group 1: Air Raid Siren Test - The air raid siren test included three types of signals: a preliminary warning lasting 36 seconds, an air raid alert lasting 6 seconds, and a cancellation signal, all conducted in a structured manner [1] - The siren test aimed to raise public awareness about the importance of air raid alerts and emergency preparedness [1] Group 2: Infrastructure and Coordination - Hangzhou's air raid siren coverage has reached 98% in built-up areas, indicating significant improvements in urban defense capabilities [1] - The city’s National Defense Office planned the siren test from early July, conducting over 5,000 checks and maintenance operations on the alert systems to ensure functionality [2] - A comprehensive communication strategy was employed, utilizing various media channels to disseminate alert information to over ten million people [2]