I recently encountered a trail which is open to hikers when there is no snow, but closed to hikers when there is snow cover on the ground (it is groomed for cross country skiing).
I think that foot:conditional=no @ snow is meant to convey this, but I actually couldn’t find any documentation that the snow condition means “snow cover on the ground” and not “It is snowing right now”
As the tag is added to the way (and not to the air above it) I’d say
foot:conditional=no @ snow
is reflecting that there is no foot access to this track when it is covered with snow. I have tagged a very steep section of a road being closed to traffic at “snow and ice” in the same way.
In my area, there is a regular speed reduction on highways with an additional sign with a snowflake symbol. According to the law, this means “snow conditions” or “ice formation”.
We also implement this with the conditional “snow”. For us, “snow” means that there must be snow on the road if the sign is present.
In addition, regardless of the traffic sign, a reduction in the maxspeed applies if visibility must be reduced in rain, fog or snowfall. However, we do not use a conditional for this.
On OSM, we do not map the weather, so you can safely go with this tag.
There is a common pattern of maxspeed:conditional=… @ wet for roads that become wet, regardless of the form of precipitation that wet the road. (maxspeed:wet was deprecated in favor of this pattern.)
There isn’t normally a need to regulate speed, street parking, etc. based on the ambient weather, but if we ever had to, something like raining and snowing would easily distinguish these conditions from the ground conditions. However, the approved tagging for 
still uses wet, assuming that a self-driving car isn’t simultaneously measuring moisture on the roadway and in the air to make a distinction. An even more specific sign, 
, has no established tagging yet.
But on the topic of snow, note that some restrictions are based on the amount of accumulated snow. The recommended tags for this sign treats snow as a measurement rather than a Boolean flag. (Depending on the jurisdiction, the “2 Inches” on this sign might not be worth tagging; it might merely be reciting the rule for declaring a local snow emergency.)
I didn’t want to go into that much detail as I didn’t think it was relevant for the moment. But of course we also have both conditions. The two signs are defined in the Austrian Road Traffic Regulations in § 54, snow or ice formation under paragraph f and wet road surface under paragraph g.
According to the definition, a road is considered wet if the entire road surface is covered with water. It can be disregarded if the road is only damp or if puddles can be seen occasionally. However, if a vehicle in front is following a spray plume, this is a clear sign of wetness. Such a sign is put up on stretches of road where aquaplaning can occur in the rain, i.e. the dreaded “floating” of the tires, which makes the car unsteerable. Snow conditions, on the other hand, are described as snow remaining on the road, i.e. when the road remains white.
We have defined that paragraph f is entered with traffic_sign=AT:54f and *:conditional=* @ snow and paragraph g with traffic_sign=AT:54g and *:conditional=* @ wet.
The both signs are:
There is also the occurrence of both additional signs together, then it is traffic_sign=AT:54f;AT:54g and maxspeed:conditional=* @ wet; * @ snow. You can see this here on both sides:
https://www.mapillary.com/app/?pKey=1441277189751396
Here is another sign regulating the maxspeed in the event of fog:
https://www.mapillary.com/app/?pKey=714200419656065
Happy mapping!
Here we have 5 signs
1 The double snow flake indicating there’s risk of snow/black ice on the road.
2 The sub sign that winter tires on vehicle or snow chains/socks are obligatory to have in trunk from Nov.15 to April 15 (i.e. every motorist here has one or the other, or both)
3 A sign with maxweightrating:hgv:conditional=7.5 @ snow for trucks to enter motorways & (certain) bridges.
When it snows, count on it you’ll reach a point on the way where it starts to cover it but in case of tracks being groomed for nordic that will be clear from the start whether there’s a risk of encounter i.e when there’s snow cover.
A 4th sign reflects there’s risk of skidding when wet or snow and a 5th (never seen here) that a snow plough is active in the area.
Speed reduction… ‘veloce moderato’ on Electronic displays i.e. adapt your speed to the conditions given.