The Phidgets light sensor is too sensitive for quantifying solar radiation and would need filters to reduce the light intensity to <1000 lux. The old Phidgets light sensor was insensitive enough, or you could salvage one of the photocells from a night light that turns itself off in the daytime. Alternatively, one could use a solar cell and measure the current through a load resistor which would be proportional to sunlight intensity.
One source of soil moisture sensors I'm testing are made by Vegetronix (http://vegetronix.com/
) and they run off 5 V and produce a voltage in the range of 0-3 V which can be converted to soil moisture percentage by a piecewise linear approximation. They can also be used as fluid level sensors. I used a Phidgets A/D connector to easily connect it up to an 8/8/8 board. I've tested the VH400 soil moisture probe only.
As far as a rain gauge goes, I suppose you could use the Vegetronix 400 in the fluid level measuring mode and have some sort of mechanical actuator dump the contents of the rain gauge when it gets full.
Haven't decided how to best do the wind speed sensor yet as I'm still looking for a simple means of quantifying wind direction. Putting a propeller on an electric motor will give a voltage proportional to the rotation rate and the system can be calibrated by mounting it on the outside of a vehicle and driving at various speeds.
The IR sensor can be used as a cloud sensor as the back-radiation one gets from a cloud is considerably warmer than from clear sky. There hasn't been enough completely dry weather lately where I live for me to risk trying this as I don't think the IR sensor would like rain very much. The IR sensor has an on-board temperature sensor which has a relative accuracy of better than 0.1 C.