Yes but (ugh, sorry for the cliche) you're missing the forest for the trees.
The issue is not that Tolkien didn't know of airplanes or of aerial warfare. But his most indelible memories of World War I was that of ground campaigns, and even though planes were used in WWI and reputations made (such as that of the Red Baron), they were by and large a fringe part of what comes to mind, at least compared to the introduction of mustard gas and machine guns.
The gap between knowing of something and experiencing something can be quite large. If you hired me to write a novel about conquering adversity through sport, I'd probably write something about basketball, a sport I don't really play but watch and keep tabs on on a regular basis. I would never to think to use soccer as a plot point, even though it's apparently the most popular sport in the world and it's not too hard for me to look up stories/information about soccer today. Sometimes you just go with what you know.
And keep in mind there's a decent lag between introduction of technology and the mainstreaming of technology. Before flight became an integral component of war, Tolkien and everyone else had seen and studied war exclusively through the mindset of land and naval warfare.
You sure you replied to the right comment? I was merely disagreeing with the assertion by btilly that airplanes were only used during the day in WW1. :-)
I threw that in because I knew that it wasn't until 1929 that Doolittle demonstrated instrument flying, that allowed safe flight in the dark, fog, etc.
But I guess that even at night, if conditions were right, you could fly a plane. And people did during WW I. Though the descriptions in http://www.century-of-flight.net/Aviation%20history/airplane... of the practical problems in landing when you didn't know how close the field was are somewhat harrowing.
Night fighters were used during WW1 by the UK to defend against Zeppelin and Gotha heavy bomber attacks:
http://en.wikipedia.org/wiki/Night_fighter