When Alexandra Day created "Carl's Christmas," she was living in San Diego, and it does NOT snow in San Diego, and since she is a realistic painter, there was a problem.
Fortunately, she has sister who lives in Hamilton, Ontario, where there is plenty of snow. Her sister is also an artist and understood the problem and what was needed and, luckily, it was winter and they had just had a wonderfully all-covering snow. She got busy with her camera (cell phones with cameras being non existent in 1990) and packs of photos arrived in sunny Southern California. Alexandra was even able to use her sister's fireplace and front door, looking out into a snowy evening.
By the time "Carl's Snowy Afternoon" came around, Alexandra was living in Seattle, so snowy landscapes were not a problem, and it turned out that Carl loved cavorting in the snow, so he was very cooperative about posing in the snow.