Translucent "sky jellyfish" UFO moving much faster than clouds Novato CA 8/19/16

I was driving on the US 101 North with two other people in the car (not witnesses) when all of a sudden in the distance around the same height as the low clouds in the sky (30 min before sunset) I spotted a translucent UFO moving quite fast in the sky. Because I was driving, I did not take out my phone in an attempt to capture an image but I tried pointing it out to the other two passengers in the car. By that time, the UFO had gone from a background of sky to a background of mountains and it was lost in the changing scenery. I followed it with my eyes for approximately 45 seconds because I had to look away and when I looked back I had lost it. I tried explaining it as a sky jellyfish to my friends. It was the bell-shape of a jellyfish sans tentacles. It was also translucent like a jellyfish. You could see the mountains behind it but it was clearly not a cloud or anything man-made. I would compare it to looking through a bubble floating in the sky. The bottom was slightly wavy, not a straight line but the top was a clear curve. I had first noticed it when I was scanning the horizon for traffic. It was moving at a steady pace across the sky in an eastward direction and I looked around for airplanes when I saw the UFO. I saw 1, and the airplane was minuscule in comparison and clearly solid. This UFO was also 3-D, but like I said it was translucent. I was speechless when I saw it. I have seen lights in the sky in the past and technically I couldn't identify them, but I could imagine they were a satellite or fireball. This was like nothing I have ever seen before and I'm clearly at a loss of words to describe it. I wish I had asked my passengers to try to take a photo or see it with their own eyes but it was lost in the mountains by the time I could articulate my thoughts. I can imagine that it would be hard to see if you hadn't spotted it and followed it. It was nothing with a solid color or bright light that attracted attention. It was trying to blend in with the scenery but had some sort of electromagnetic field around it that made it visible to the naked eye. MUFON CMS# 78555