I'd say a sunny day easily, but it depends on what there is to absorb the sunlight. Snow is a great insulator and reflects plenty of light, but if there's Even a few specks of dirt on the snow they will condense and melt the snow faster. Same with anything that absorbs light. If it's a barren Arctic desert with only snow, then the temperature would be more important.
This is based on observation, I'm no scientist until a peer reviews this
Curious about your age and social group. Pretty much everyone I associate with doesn't want to have kids, usually citing the world their children would inherit. These definitely aren't the type of people that support eugenics, in any way. Are you of the belief that we should be increasing the world's population? If so, why?