One of our planned projects is a home weather station. We have looked at various temperature, humidity, and barometric pressure sensors. I have been considering how to make a self emptying rain sensor, and wind speed and direction sensor (important for siting wind turbines), as well as a solar insolation detector. Well, a lot of the hard work has already been done by La Crosse Technology with their weather stations, and the Arduino can be setup to eavesdrop on their wireless signal. See the following article:
Home weather stations typically consist of two major parts: the sensors that sit outside and measure temperature, wind speed and direction, humidity, rainfall, and barometric pressure; and the display unit that lives inside in a convenient place so you can read the external temperature while sitting around the fire warming your feet and deciding whether it’s too cold to go fishing. Generally the external sensors connect together with cables with one sensor also acting as a transmitter to send updates wirelessly to the display unit.