Temperature is not only a function of solar heating, but also the amount of insulation and ambient temperature.

I recommend using a solar panel and a volt meter. You can calibrate it by measuring the voltage on a very sunny clear day, with the panel perpendicular to the suns rays. Then divide 1000w/m^2 by that voltage; this is your slope coefficent.

Now to measure solar intensity on a cloudy day for instance, simply measure the voltage on your panel and multiply it by the slope coefficent and you will have your irradiation value.