As I don’t have a weather station, too, I make use of the weather binding and use the forecast temperature of the day. Of course, this is no very accurate, but as the roof is almost always way too hot… for me this is working like a charm.
If I were you, I’d rather use the temperature sensors (difference of outer and inner ones).
If possible, place the outside ones in direct sunlight.
You can "OR-"combine it with the weather binding, but I wouldn’t use that one exclusively. First, most weather data supplies can be unreliable at times or in some areas (YMMV, of course) and second, it’s not as-local as are your sensor and the sun’s heating effect.
Yup, shading is a complex thing. I myself am using a combination of light and temperature sensors, plus the day’s forecasted maximum (from the weather binding) to start shading even when it’s not hot yet.
Still it ain’t perfect. My wife keeps telling me every day
I started with just two outside temperature sensors, one is always in the sun (on top of the roof), one is in the shade.
It was not sufficient enough to control the shading, so I added a lux meter.
Combination of lux and temp does it more or less …
I use a combination between LUX Sensors and weather binding.
I check the the percentage of clouds and the max outside temperature to determine if shadowing is needed (I want to use the sunlight at cold days for free heating). Because the cloud information isn’t accurate all the time, I use an Fibaro Multisensor with LUX Sensor too. If the brightness exceeds its threshold, I activate the shading.
To determine which side have to be shaded, I use azimuth and the elevation.