These events tend to occur on hot and humid summer days and are affiliated with high ozone, PM2.5, and RH levels. Note how the brown cloud appears to envelop the city but quickly thins out at higher elevations. These events tend to occur on calm winter mornings during rush hour traffic. This is an example of a foggy day in Boston. It appears similar to a hazy polluted day, but note how the fog tends to be more grey than the white haze and how it does not thin out near the top of the picture.
Fog is most common in the fall and spring.