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There is a large disk of glass sitting outside, it is pretty thick and wide. Via different sensors hooked up to a controller, I can measure the air temperature, glass temperature (on the surface), and air humidity. Since the disk is fairly large, the temperature difference between it and the air could reach a few degrees-- due to thermal inertia and the fact that air temperature can change rapidly after sunset and sunrise.

Is there a formula or some empirical data that will tell me when the conditions are right for the disk to start getting covered in dew?

Would air flow across the glass surface matter? If it does, assume the air is nearly static (except for convection) - there's no wind. Also, humidity such as fog or rain drops falling from the sky can't reach the disk. All I care about is humidity in the air around the disk condensing on the glass surface.

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There's the obvious requirement that the relative humidity at the temperature of the glass surface must be greater than 100%.

However there is a kinetic barrier to nucleation of the water droplets on the glass, and this is extremely sensitive to the condition of the glass surface. If you clean the glas with the traditional chromic acid it will nucleate water droplets very easily and the dew will form almost immediately the humidity exceeds 100%. If you put some grease on the surface then polish it off with a cloth (to leave a very thin film) this will strongly inhibit nucleation and the dew may not form until you get condensation in the air (i.e. a fog).

So exactly when the dew will form is hard to calculate.

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