I went outside at about 9 p.m. (dusk) to aid loading my nephew's car. I looked up into a mostly clear sky to the west with some clouds to the north and saw what looked like a sort of semi-transparent cloud streaking at a high rate of speed from south to north for about 6 seconds. It started roughly 20 degrees above the horizon when I noticed it and running nearly parallel to the horizon, ended perhaps 10 degrees higher before suddenly disappearing in clear air. There was no noise. In size, it looked roughly the size of my thumb nail at arms length. My impression watching it was that this cloud-like object had right angles within its body, sort of giving it the look of a semi-manufactured structure. Amazed, my first thought was how could a cloud move that fast. Then, it being July 4th, I thought it might be some kind of firework except that there was no smoke, no trail, no fire, no illumination of any kind. Just a gray cloud moving rapidly without changing shape or dissipating. There was no way it could be smoke left over from some firework detonating. Such smoke doesn't streak across the sky intact. (There were people starting to light fireworks, but their fireworks were unmistakable.) I then thought it might be a plane, but aside from deviating from the usual flight path in my neighborhood, there were no lights and it simply didn't look like a solid object. Moreover, it moved much faster than any plane I've seen around my residence. (Charlotte Douglas Airport is a few miles north of me so there's steady air traffic overhead.) After it disappeared, I looked to see if anyone else was watching it. My nephew hadn't noticed it and what neighbors I could see were engrossed in their festivities. I went inside the house and told the family I had just seen my first UFO, so called because I had no idea what I had just seen. They just looked at me blankly.