Sky Live - Stargazing Forecast uses a special set of parameters including whether or not anything interesting will be in the sky, cloudiness, brightness of the moon, and more to generate a percentage (i.e. 64%, 95%, etc) to give you an idea of just how worthwhile it might be to pull out the telescope. It also displays moon phases, detailed viewing conditions for anywhere in the world, light pollution levels, weather predictions, and so on. The app has been designed specifically for iOS 7 to include stunning visuals and blur effects, but anyone using iOS 5 and up can still make use of it.
Sky Live is available now, with a $0.99 annual subscription fee.