smog has thickened the air.
烟雾于天空阴霾。
On the night of December1, 1930, a cloud of smog moved over a valley in Belgium, which caused thousands of people to become ill.
1930年12月1日晚上，一片烟雾飘过比利时的一个山谷，这使数千人病倒了。
smog hung in the sky (over the city).
（城市）上空烟雾弥漫.
