My student and I were taking off in poor visibility from our short runway at an RAF airfield and were just rotating when a glider appeared directly overhead and landed right in front of us. The next few seconds were a bit hectic but we missed him. However, our right wing tip passed over his left with us both on the ground, us on our left wheel. The glider pilot tried to blame us for not giving way. ATC disagreed. It would have been helpful if he had bothered to use his radio and announced his presence and intentions instead of breaking unannounced into a military circuit pattern containing five other aircraft and landing on a runway with one landing and one lining up.