Why smog forms on calm, sunny days

Have you ever noticed on a calm and sunny day how haze begins to obscure the horizon?

This is likely the result of smog, which is air pollution that becomes trapped in the lowest levels of the atmosphere.

To understand how air becomes trapped, it helps to understand the structure of the atmosphere on calm, clear days.

When the sun rises on a clear day, its rays swiftly heat the ground, which in turn heats the air directly above it. The surface layer of the atmosphere slowly warms this way, from the bottom up, with heat slowly rising and continually mixing the atmosphere.

This mixing is what typically helps to dilute locally sourced pollution, such as vehicle and factory emissions, into the higher levels of the atmosphere, where stronger winds whisk it away.

However, when high pressure promotes a string of sunny and calm days, there is nowhere for the pollution to go.

This problem is most pronounced in industrious valleys, such as the Salt Lake Basin in Utah (home to Salt Lake City) in the wintertime and the San Bernardino Valley of California. Industry and traffic in these areas are relatively high, and surrounding mountains maximize how much pollution remains trapped in a small area.

This terrain also enhances any temperature inversion - when warm air exists above a layer of cool air, preventing it from rising and mixing. In these scenarios, valleys can remain socked in smog for several days and even weeks at a time.

Even in wide open areas with relatively low population and industry, such as portions of the central and southeastern United States., smog-forming particles can be emitted from agriculture and even forests in warm, calm and sunny weather patterns.

With the potential for such a dramatic and stagnant build-up of pollution, Air Quality Action Days are often announced when a stagnant weather pattern begins to take hold.

It is advised that people living in these conditions avoid exerting themselves outside, especially if they suffer from respiratory or cardiovascular conditions such as asthma or heart disease.

In mountainous areas, traveling to a higher elevation above the smog level can offer relief from the choking valley air. However, this option is sometimes too popular, creating a scenario where a surge of road traffic further contributes to the dangerous smog.

Stagnant conditions and high levels of pollution have combined to form historically fatal smog conditions before. In London and southwestern Pennsylvania, the dangers of poor air quality were fully realized during tragic smog events, leading to the first Clean Air Act in the U.S. in 1956.

In and around many cities in India, dangerous smog continues to be a major health concern and everyday problem.

When windy, cloudy and rainy weather finally returns to these areas, smog is often "washed" out of the atmosphere and a good air quality day ensues.

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