drop top temperature

i have a query about dropping the soft top, it is recommended not to drop it if the temperature is under 17 o celsius(Farenheit?) but is this the ambient temperature or the plastic window temperature, obliviously if it is 17 ambient the window might be a good bit hotter, specially in the sun light and the heat generated from the engine, but if it is the window, the ambient temperature could possibly be less, i have had a cracked whitening rear window before after the car spend time in a mechanic, can someone clarify this for me