Definitely means out in the open air.

'plenn-AYRE', it is Cara. BUT, it does seem a little difficult to pronounce it that way when speaking in English. I imagine that is why people would pronounce it Plain air.

The same type of thing happens with cirque (as in Cirque du Soleil). The CI is supposed to be pronounced like the SI in SIT but it is most often pronounced like the CI in CIRCLE.