In Paris, actually, it is snowing smoothly and the temperature, according to many sources, is around $\small\sf{9^o}$ Celsius.

I had always the feeling that snow is possible when temperature approaches $\small\sf0^o$ or below $\small\sf0^o$ Celsius (apparently, it is a huge prejudice)

How is that possible?

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    $\begingroup$ Because air temperature (generally) decreases with altitude. It's below 0C up where the snowflakes form, and they don't have time to melt before they reach the ground. $\endgroup$ – jamesqf Mar 2 '16 at 18:39
  • $\begingroup$ @jamesqf has put it right. It is easy to see the upper troposphere temperature (or air temperature not surface temperature) that drops to 0 or below during ice formation. $\endgroup$ – Gemechu Fanta Garuma Mar 2 '16 at 21:46

The clouds in which snow originates must of course be below the freezing point 0°C = 32°F.

Two factors allow snow to reach the surface when the near-ground air temperature is above freezing.

  1. The time delay --- snowflakes may not have time to melt before reaching the surface if the surface temperature is not much above freezing. Strong downdraft air motion and larger snowflakes helps at this, bringing snowflakes down faster.
  2. Once snowflakes fall below the clouds, they are in air at less than 100% relative humidity. Sublimation --- evaporation from solid to vapor --- of some snow on the outer edges of a snowflake can then occur. This removes energy from the snowflake overall, so it can help keep the rest of the snowflake a few degrees cooler than the surrounding air.

The highest near-surface (at the official 5 foot height of instrument shelters) air temperature at which I have ever read of snowflakes reaching the ground is 10°C = 50°F.

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As mentioned in the comments, the air temperature is the deciding factor. In the Troposphere, until ca. 15 km, the temperature decreases by about 6 °C per km altitude (more info on the lapse rate here).

As an additional factor, the air temperature on ground levels is generally higher in urban environments, like Paris. Therefore, the natural outer conurbation area could provide lower temperatures, while it is warmer in the city due to the anthopogenic heating (cars/engines, house heating, ...).

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