The app is called Thundergod [iTunes link]. With creative microphone technique, and a good set of headphones, you'd swear you were in a raging storm, or a rain forest, or one of 5 other environments, listening to nature at its most spectacular. You can listen and let your mind drift off, consider the state of the universe, or just fall asleep in a soft rain using the built in sleep timer.
I suspect the techniques used to make these recording are similar to the highly regarded binaural technique, using 2 microphones separated by the approximate distance between your ears. Then when you play back with good headphones, you are instantly transported to a hyper-realistic, almost holographic place that closely duplicates being in a real space with real sounds.
This effect doesn't work through speakers. It can sound nice, but it's not startling or compelling. With headphones the sound comes alive, and you can begin to have an emotional reaction to what you are hearing.
The app also allows for a special equalization curve if you are using earbuds like the ones Apple includes, and it is effective. This app as produced by Holographic Audio Theater, who gave us the free app Naturespace that shows off their recording and producing prowess. It's a good introduction to their technique.
Thundergod is not cheap. It's a U.S. $9.99 app, but of you look at the App Store reviews you'll be surprised how many purchasers thought the app was worth the money.
The sounds are not short repeating loops, but something like a long concert that has a beginning, a middle and a conclusion. I liked the 'Thor' environment the best, because it was the most dramatic and attention-grabbing. It's not for relaxation, but for stimulation. If relaxation is what you want, there are sonic environments included for that as well.
My suggestion is that you try the free intro app, and if you like it, then go ahead and get Thundergod. The effect can't really be described, but must be experienced.