I have mounted my Sky at the top of an old tree trunk (no branches), using a screw mount. Due to the nature of the site (and the joys of getting a ladder up against the tree) its at a slight angle (approx 5-10%).
Would this have a notable impact on the rain / wind readings?
I can of course fix it but thought i would ask before climbing back up,
Andy
I suspect not a great deal (i.e not significant)
I agree. It will skew your windspeeds and sunlight readings a bit, but it should not be significant.
Somewhat related… I have my SKY temporarily mounted on a piece of 1" schedule 40 gray PVC conduit. When the sun comes up in the morning, it heats up one side of the conduit and that side expands. This expansion on one side leads to a bending of the conduit away from the sun. Kind of like an “anti-sunflower”. 
Makes me want to use a stiffer pole when I do end up mounting it up on the roof!