RealAle, you probably don't want to go TOO high above the roof level, but it really depends on how tall other buildings and trees are around your house.
If there are taller buildings or trees than the height your weather station will be at (I'd say a radius of 100 ft. maybe?), then you should be alright.
But if your weather station is going to be taller than all of the buildings and trees around your house, then I'd say it's a bad idea.
I'm not a real expert on this or anything, but I'm just giving you some advice. Hope this bit of info helps.