I confirm what @Iriss says: The slider is there at the beginning. When I try to modify the displayed layers it suddenly disappears. I did not find a way to make it visible again.

I guess @IrIss is talking about the slider that is visible in the attached screen shot that I have taken on my Android phone. I am having the same problem: I cannot find the corresponding functionality in the web-application "windy.com" . Is it hidden somewhere or not available at the moment. (Sometimes the weather situation is better understood by looking at levels above the ground. This slider therefore, seems very useful to me.)