Hello, with the weather effects, why do some surfaces not collect snows while others do? I thought it had to do with the slope being too steep, but it's also not collecting on flat horizontal surf...

Hello, with the weather effects, why do some surfaces not collect snows while others do? I thought it had to do with the slope being too steep, but it’s also not collecting on flat horizontal surfaces such as stairs, chimney caps, rock parapets, etc.