What appears to be snow, as mentioned by a commenter below, is actually white stone or crushed shell, used to keep the buildings cooler in summer by reflecting the sun's heat up.

The clarity of these old-time shots is astonishing.

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With all those chimneys belching black smoke, it's a wonder the rooftop snow looks so white.

[Um, no snow. Sometimes I wonder about you people ... - Dave]

Sometimes I wonder about YOU, Dave.

The wintry weather is rare in south Louisiana, though the state's northern parishes see it about once a year. New Orleans' last snowfall, in 2004, was a dusting that came nine months before Hurricane Katrina struck. The record snowfall for the city is about 5 inches, recorded Dec. 30, 1963.

It sure looks like snow, melted in some places, and may have been the rare event that triggered the photos. (Look at the rooftop in the center foreground).

However, I've been to Mardi Gras in NOLA (in Feb, I think) when it SEEMED cold enough to snow. But shells make perfect sense the white would reflect the heat. I still stand by my comment that it's a wonder the rooftops are still so white with the smoke.

What a great job stitching those four images together. When I embigulate and then scroll back and forth, left and right, I feel the whole image wrap around me. I can’t believe it’s flat. Exhilarating.