Your SKY / Tempest leaves the factory with a wind sensor calibration in place. To ensure in-situ accuracy, the sonic wind sensors in SKY / Tempest are designed to self-calibrate over time once installed in the field. The initial in-field process requires collecting a certain amount of data at all four cardinal points. This initial process runs through a check to ensure all four sonic sensors are balanced properly.
For those of you with access to a calibrated wind tunnel, or reliable testing equipment, you may notice slight directional variation out of box. This is normal. Once you have it on the roof for a few weeks and confirm you’ve had some significant wind (12 mph or more) from each direction, put it back in your wind tunnel and see if you don’t get better numbers at all points on the compass.
edit : June 2020 - added Tempest since both use same sensors