Well I have a commercial wireless rain gauge/temeperature unit that communicates with a base unit that also has a temperature sensor and displays the data from both units.
I've notice that if direct sunlight is hitting either unit that the temperature measurement goes way too high. As long as I keep the units in shade they seem to measure temp accuratly and track well, so some kind of box with insulated or relective covering with an open bottom would probably give better results. Keeping wind off the sensor also would probably help.
Lefty