Several of the posts above have mentioned this water that is "injected" into the atmosphere by jet engines. Unless my jet-engine theory knowledge is seriously flawed, there is no water "injected" by the engine into the atmosphere (except for a few seconds during the Harrier's hovering antics). The water vapour/ice crystals are simply gathered by the hungry engine and pushed out in such a concentration that they are visible.
Additionally, the fundamental argument that cloud cover produces a warmer Earth is utter rubbish. Last Friday it was 32deg C in my garden, but on Saturday we had cloud cover and it only reached 26. Or is it just Cirrus that warms the Earth, and all other clouds keep the heat out?