The report of a reader saying it snowed in Rome was an incorrect translation. It was Hailstones. The same thing took place in Calgary. Although strange, they are not unknown even in summer. Hailstones are large, layered ice particles, often spherical in shape. They are produced by thunderstorms having strong, tilted updrafts. Hailstorms form within an unusually unstable air mass. In the case of Calgary, it could have been set off by the wildfires combining with Volcanic particles.