Wrong section (should be under 'Game Discussion"), and by holding down the clouds. It continues charging after you charge up enough for "rain", and if you hold it down long enough it continues to "Flood", which costs 2 mana.
Press and hold your finger on the ground, then flick down to make a little ditch. Then press and hold the cloud until it gets to 'flood' and flick down to pour the rain into the ditch.
Thanks, corsair. I have another question ( and I know it is off-topic but I don't want to start a whole new thread...) Is there another game similar to god finger that involves building cities? Seems like I heard of one, but can't remember what it is...