Add the UIViewAnimationOptionCurveLinear
to your animation options, like this:
[UIView animateWithDuration:speed
delay:delay
options:UIViewAnimationOptionAllowUserInteraction|UIViewAnimationOptionCurveLinear
animations:^(void){
[UIView setAnimationRepeatCount:5];
cloud.frame = (CGRectMake(cloud.frame.origin.x,
cloud.frame.origin.y+900, cloud.frame.size.width, cloud.frame.size.height));
}completion:nil];