I like halos, and in the mountains you can see them sometimes and in different appearances. This is a typical halo caused by the icy cloud type cirrostratus. The sunlight is reflected by zillions of tiny ice crystals, and as these clouds are stratus clouds (horizontal layering with a uniform base), the layer is quite homogenious. This helps to bring up the full ring of reflection. Additionally, this type of cloud signalizes an approaching warm front bringing rain in the next few hours.
I was lucky the terrain let me change my position to center the halo and the sun to the 2.5 meters high summit cross. The mountains you see below are mainly austrian, on the right side you can see the distant Säntis.
Hope you like my capture!