Air within the Bermuda High is comprised mostly of warm, moist "maritime tropical air." Visibility is usually five to 15 nm in this air, obscured slightly because of abundant condensation nuclei and high relative humidity.
Since the 1940s these summertime storms have been known by their misnomer "air mass thunderstorms," suggesting that they just randomly appear anywhere within the warm, humid maritime tropical air. But the situation is actually a lot more complicated than that.
In the relatively flat central United States, continental polar, continental tropical, and maritime tropical air
masses meet easily, which is a factor in creating the baroclinic environments that favor extratropical cyclones.