Smoggy describes air that's thick with smog, a nasty mixture of smoke and fog that makes it hard to see and unhealthy to breathe. On a smoggy day, the sky looks hazy and grayish, and distant buildings seem to blur or disappear. Cities with lots of cars and factories sometimes have smoggy air, especially when the weather traps pollution close to the ground.

Smog acts like both smoke and fog: it hangs in the air like fog but contains harmful chemicals from burning fuel. Los Angeles became famous for smoggy skies in the mid-1900s, which eventually led to laws requiring cleaner cars and factories. Beijing and New Delhi still struggle with smoggy conditions that force people indoors on the worst days.

When you visit a place on a clear day versus a smoggy one, the difference is striking. Clear air lets you see for miles, while smoggy air might prevent you from seeing across a schoolyard. People with asthma find smoggy days especially difficult because the pollution irritates their lungs.