雾是一种常见的天气现象。当低空大气层中的水汽达到饱和状态时，水汽凝结物悬浮在空中形成雾茫茫的天气，使能见度的水平距离小于1公里，气象上把这种天气现象称为雾。雾的轻重程度不同，有轻雾、重雾之分。水平能见度在1.0~10.0公里时称为轻雾，水平能见度低于1.0公里时称为大雾，能见度在500米以下时称为浓雾。根据水汽达到饱和的具体条件不同，通常把雾分为辐射雾、平流雾、蒸汽雾、上坡雾、锋面雾等几种。
【正确答案】：Fog is a common weather phenomenon. When the lower atmosphere is saturated with water vapor, condensed matters of the vapor will suspend in the air and create a foggy weather, making the horizontal distance of visibility less than 1 kilometer. This kind of weather is called fog in meteorology. As fog varies in density, it can be light fog or heavy fog. When the horizontal visibility is from 1.0 to 10.0 km, it is light fog (or mist); when less than 1.0 km, it is thick fog; and when below 0.5 km, it is heavy (or dense) fog. And, according to the different conditions in which water vapor reaches its saturation point, fog is usually categorized into radiation fog, advective fog, steam fog, up-slope, frontal surface fog, and so on.
