The standard initiation key for rain
is Shift-F9. I have trouble changing the key combinations myself, it usually takes a long time for me to change it.
I'm not sure, but I think if you double-click under Key, where it says 'Disabled' until it says 'New Accelerator', you can set a key combination to initiate the ripple effect. I personally use Shift-F10, because it's right next to F9