Data from population studies dating back from the Framingham study/substudies among others demonstrated quite clearly that a sedate lifestyle and a diet high in calories were risk factors for the development of atherosclerosis. I am not aware of any studies looking specifically at "screen time" which I assume you mean computer screen time but any such activity, whether watching TV, playing video games or what have you that limit physical activity will have the same effect of classically measured low activity lifestyles. I am not aware of a US study on the effects of weather but the Scandinavians may have conducted a few.