I know nothing about Android, but in a Java
program I would do a thing like this:
1) have a Timer object fire every so much milliseconds
2) then call an 'updatePanel/Frame/whatever() method
3) in this method, you update the Y-values of every snowflake you have. Maybe even some
random disturbance of the X values.
4) do the repaint()
But, I don't know if Android works in a similar fashion.