Whopity and BEagle got there before I could. Don't complicate matters. This is not a difficult question. Air cools at the DALR until it's saturated; then it forms cloud.
I don't think I've ever heard of this (temp-dew point) x 400. But I might have done because I've forgotten so much stuff you could write half a dozen books with it all if you could find it! But I can't see the use of it really, so think I might forget it again.