When the sun sets behind a building and it's snowing slightly, the rays that get by either side of that building have something in the air to illuminate: the snow. Normally, there's nothing there but air, so it doesn't look like this. The high-rises were already slightly obscured because of the snow between them and my location and the rays that aren't blocked by the buildings are just lighting up that snow.
You may be able to see a few flakes in the center-tree area that's not illuminated.
Click to illarginate (bigger, but not brighter).