The Beijing Meteorological Center on Sunday afternoon issued a code yellow alert to warn about fog in the capital, saying visibility would drop to less than 500 meters in most parts of the city.

北京气象中心周日下午发布了大雾黄色预警称，绝大部分地区的能见度将不足500米。

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The US is currently at code yellow for an "elevated" level of terrorist risk.

BBC: US may scrap colour-coded terror alerts

Today the United States Government raised the national threat level from an Elevated to High risk of terrorist attack - or from Code Yellow to Code Orange.

BBC: NEWS | Americas | US terror alert: Full text