When playing, change the clouds from clear/slight overcast to 
the thunderclouds. Hold one finger on the thundercloud and hold
it there. 
After that, change the clouds back to clear skies while keeping 
your finger where it was. When you have changed the clouds back 
to clear skies, drag your finger to one of the islanders like you
would if the thundercloud was there, and it will make lightning 
without the clouds.