Definition
Smog is a word made by blending smoke and fog. It is a kind of air pollution in which exhaust from factories and cars mixes with water vapor and sunlight and turns the air thick, hazy, and dirty.
Smoke meets fog and becomes a giant gray curtain
Imagine simmering a stew in a closed room on a freezing winter day. With no vent, the room fills with smoke and steam that have nowhere to go. The same thing happens in the sky over a city. Coal smoke from factory chimneys and home heating boilers tangles with cold winter fog, and the whole city gets trapped in a murky gloom.
Harmful sulfur in the smoke dissolves into the tiny water droplets in the fog. Plain droplets turn into acidic, toxic droplets that sting the lungs. If there is no wind, this poisonous fog does not blow away. It lingers over the streets for a long time.
This is classic London-type smog, which caused many deaths in history. Pollution made by people met natural fog and turned it into a deadly toxic cloud.
Sunlight makes a bright but stinging haze
To be more precise, the smog we deal with today comes not only from coal smoke but also from strong sunshine on clear days. This is modern smog, and it shows up mostly at midday in the middle of summer rather than in winter.
Exhaust gases and oily residues from cars float in the air. When intense summer sunlight (ultraviolet rays) hits them, molecules in the air split apart and recombine into brand-new substances. Large amounts of harmful chemicals, such as ozone, which irritates your eyes and throat, are produced this way.
This brown haze, created by sunlight-driven chemical reactions on a clear day, is called photochemical smog. It is why your eyes sting and breathing feels tight even when the sky looks sunny. The cause is an invisible chemical reaction.
When the air lid closes, the city becomes a sealed room
Normally, warm air near the ground rises and cool air from high up sinks, so the air mixes naturally. Thanks to this convection, smoke and exhaust near the ground can rise high and disperse.
But sometimes the air above is actually warmer. For example, after a clear, windless night, the ground cools off quickly. Heavy, cold air settles near the surface, and a layer of warm air covers it like a blanket. When the temperatures flip like this and the mixing of air up and down weakens a lot, it is called a temperature inversion.
The warm layer acts like a giant pot lid over the city, so pollutants have no way out. A lid formed by the ground cooling overnight usually opens within a few hours once the morning sun warms the ground. But when a high-pressure system stays put and the air above sinks and warms, a lid forms that does not lift even during the day. Then exhaust keeps piling up, and smog can weigh down the whole city for days.
Common misconceptions
Smog is just thick natural fog caused by cloudy weather.
Natural fog is mostly tiny water droplets. Smog is a dangerous mix of pollutants, where man-made harmful substances like smoke and car exhaust react chemically and blend together.
Where you meet it
On a sunny midsummer afternoon, the sky over a downtown area turns a faint, hazy brown.
On a winter day when the wind dies down and fog rolls in, smoke can't disperse, and your throat and eyes sting when you breathe.
In one sentence
Smog is dangerous air pollution in which smoke and exhaust combine with fog or sunlight and get trapped in the air.