Standard weather-report term. China's meteorological warnings classify fog by thickness:
大雾 (heavy fog, visibility < 500 m) and
浓雾 (nóngwù, dense fog, visibility < 200 m). Often confused with the air-quality term
雾霾 (wùmái, smog) —
大雾 is natural moisture;
雾霾 is pollution-laden.
