Forms when a cooler air mass displaces a warmer air mass and usually indicates drops in temperature, heavy rains, and sometimes hail, thunder, or lightning.

Cold Front

Explanation: When cooler air mass pushes warm air mass it rise into the atmosphere since it is less dense forming clouds. During a cold front there is a relative decrease in the air temperature where the cooler air mass moved.

significant amounts of nitrogen oxides, carbon monoxide, and other pollution

explanation:

passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. in 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.