'The X Factor' boss hired five planes to emit a computer-controlled and eco-friendly vapour that spelled out a message from a height of 10,000ft in the sky.
The message over the city's Encore hotel read: ''Cheryl - Congratulations you're 30 next year! ha ha ha Love Simon x.''
The writing could be seen from a 15-mile radius.
The music mogul has been waiting since October to get revenge on Cheryl after she reminded him of his age.
She sent a plane to fly around his house in Los Angeles 52 times with a banner attached saying: ''Simon Cowell is 52 today! Ha ha ha! Love Cheryl xoxo.''
She explained at the time: ''He didn't want anyone to know it was his birthday. A perfect opportunity for me.''