1) Heh, well there are ways. You can trap your opponent with earth, block his vision somehow. That isn't that hard. You have to shoot it into the sky. It's not really strictly specified, as it does heat up the sky and creates the thunderclouds, thus the one round of preparation.Question Time
Will you have to shoot it directly into the sky or could anywhere. And what would be some ways to hide the jutsus plan. I mean just directy shooting the fire into the sky would be showing the opponent what jutsu you are about to use.
How long will they burn after impact
2) Erm, I'm not sure, probably not as long as amaterasu, lol. I believe long enough for you to be clever and use it as a trigger for another technique, when your opponent forgets about it.