What is the steepest surface that can hold snow?
What's the minimum "uprightness" for it to not get obscured by snow and stay readable throughout the year?


The answer to this problem is not in the angle at all. No amount of math will give you an angle at which you can reasonably presume your surface will remain clear. Snow can (and does) accumulate on vertical and even negatively sloped surfaces. Instead of thinking about the angle, you should be thinking about the materials and conditions involved.



There are lots of factors that determine whether snow will stick, but a few that stand out most are:


Temperature differentials between a surface and the atmosphere:

This is particularly noticeable on surfaces of enclosures such as vehicles or buildings where the internal temperature causes the surface to be out of sync with the environment. You might notice snot collecting on the vertical glass surface of car side windows. This will be more pronounced in cars that were parked full of warm air when it started snowing. The warm glass would have melted the initial snow flakes. The wet surface will eventually freeze giving  a textured surface for new snow flakes to sit on instead of sliding off.

The same is true for objects with thermal mass that retain heat or cold after a sudden change in air temperature.
Surface properties such as roughness.

If even a few snow flakes find purchase on a surface, more will  find a way to build on that and obscure your surface. This is far more important than angle.
Atmospheric conditions such as humidity and the nature of the snow fakes formed.


To effectively keep a surface clear it should be ① smooth, ② quickly transfer heat to adapt to the current air temperature, and ③ be sheltered from being in contact with falling snow in the first place. Even highway signs with are smooth, vertical and thin metal are only so effective  at staying clear and in some regions will have eaves protecting them from some snowfall.