Weather sensor positioning is very much a fine art. Over the 10 years of running a weather station here at Starlight Cascade Observatory & Gardens, we have moved the sensors around a lot. Sometimes because the wires were in the way, got broken, needed to be moved or failed completely. Then we went wireless and moved them around more.
Over time we noticed that the initial placement of some of them artificially affected the data. For instance the temperature sensor was within 1m of the house and the morning sun reflected off the house onto the sensor, raising it quite a lot. After the sensor went into the shade after solar noon, the house would still radiate heat for quite some time.

The current (2012 July) position of the temperature sensor is about 2m off the ground, mounted to a tree surrounded by greenery, in the shade. In the early spring when there are no leaves, the sensor is sometimes directly illuminated by the sun. We will be adding a mini stephenson screen to it so that it will always be in shade.

The wind speed and direction sensor was another that was affected by local trees, houses, buildings. It’s current position is about 3m off the ground out in the veg4 garden, well away from other structures that may interfere with it.

The raingauge is the least affected sensor by placement, as long as nothing is dripping on it. It is located on the observatory.