Has there been any explanation in the books about why people don't open gateways in midair? It seems like a think that would have been convenient on several occasions. For examle, Egwene could have opened a tiny gateway to the middle of the chain across the harbour of Tar Valon, only large enough to get a hand through and turn the chain into cuendillar, and very unlikely to be detected. Or Rand could have opened a gateway somewhere above a large Seanchan force, and zapped them all with lightnings. Not being able to feel Saidin, they would have no idea where it was coming from, and if someone happened to look up, he would present a tiny target, and could just close the gateway and disappear.