What is the big white ball on the top of the American pillar?

Radar dome.

Inside the big white ball is a polar-orbiting satellite receiving antenna, which radiates greatly and only emits electromagnetic waves toward the sky. It can also project light on the spherical radome of the Doppler radar to predict the weather.

The top of the column is called a column cap. As the name suggests, it is just like a hat worn on the head. Column caps are usually used for basement roofs in buildings.