These are sometimes known as the hazy, lazy, crazy days of summer. I'll leave the lazy and crazy parts up to you, but the hazy I can explain.

It starts with particles of dust, smoke, salt, and other materials that are always in the air - thousands in every cubic inch - but these are usually too small to be seen. It's the same with ever-present water vapor molecules in the air - there but not seen. But all this changes when the relative humidity starts to increase, say to 60 or 70 percent. That's when some water vapor molecules start to accumulate on the particles of dust, smoke, and salt.

Slowly, these particles grow large enough to be seen. Eventually, a veil of whitish-gray can cover all or part of the sky. And that's what you're seeing when the sky turns hazy.