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. $\endgroup$ 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.

  • $\begingroup$ Note this looks to even potentially rarely apply for freezing rain (liquid precipitation falling in air above 0C\32F). If the dewpoint is lower, the edges of the precipitation will evaporate on the way down, leading to the water to be at\below freezing (called wetbulbing). (It's also coming from being recently frozen aloft) On exposed elevated surfaces where wind may be lowering their temperature, that water may be cold enough to allow freezing on contact, despite air temperatures of like 33 F \ 1 C. But haven't seen it happen at anywhere near like 40F+ like snow can occur at. $\endgroup$ Feb 18 '21 at 11:01
  • $\begingroup$ Here's one of quite a few examples from the recent historic winter storm of Feb 2021. I'll be honest, perhaps from being Floridian, I figured they must be errors. Precip can FALL frozen, sure... but freezing on contact requires a lot of energy release. But talking to other meteorologists, it's quite real. Happened a lot early in the 1st storm (which unfortunately are harder obs to get now) when humidity was low. I believe I saw 34, maybe even 35F. Of course it's also worth realizing water is often supercooled $\endgroup$ Feb 18 '21 at 11:06

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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