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The temperature was -5c last night, the cars are coated with frost and ice, except those beneath street and porch lights. Do these lights really heat the surrounding area by +5c?
I can't help feeling that there is some other factor at work; it is unlikely that a 60W portch light could make much difference to the temperature or humidity even one metre away in free moving air. For a 'fair test', all the other variables must be considered / controlled. Distances and actual positions of cars are very relevant to frost / dew formation. Even the angle of the window affects frosting - side windows often stay frost-free whilst the windscreen gets covered. You will have to park your own car beneath your portch light over a number of nights and record whether the light being on or off makes a difference, on a statistical basis.