In winter, in the industrialised and metro cities, especially in the morning hours, the visibility is greatly lowered due to the formation of a dark fog, generally resulting in serious road accidents. It also causes suffocation.
$(a)$ What does this indicate and also name this phenomenon?
$(b)$ How is this phenomenon produced? Name two gases which cause acid rain.

(N/A) This indicates high levels of air pollution. This phenomenon is known as $Smog$.
$(b)$ In cold weather, water vapor in the air condenses around suspended particulate matter, such as unburnt carbon particles, hydrocarbons, and other pollutants, to form a thick, dark, or yellowish fog known as $Smog$.
The two gases that primarily cause acid rain are $Sulfur \text{ } dioxide$ $(SO_2)$ and $Nitrogen \text{ } oxides$ $(NO_x)$.