Sorry, Zeroflag, I didn't noticed the sequencial numbers. So, judging by the numbers, I guess its just one cloud traveling on the upper line, then on the lower, and again on the upper. You can add a sub event on that action to check if the cloud Y is on the upper line. If it is, order the program to changing the Y alongside the X to the lower line Y, on else, changing the Y alongside the X to the upper line.