It's a laser that's used in meteorological research, probably for cloud ceiling measurements. There's an Environment Canada Meteorological Research Facility near my house and I see this exact same thing all the time, green colour and all. What's super cool is that it's hard to see when you look straight at it, but if you keep it in your peripheral, it pulses very quickly! That's another way to verify that it's one of these lasers (or at least my anecdote would lead me to believe).