"he beheld, to his astonishment, a bright halo in the cloud of haze, that rose in a semi-circle over his head like a pale rainbow"

 This phenomenon is known as a glory, an optical effect which occurs when light is scattered backwards through individual water droplets. It can be seen when the sun shines upon cloud or mist that lies beneath the observer and resemble shimmering rainbow haloes around the viewer’s head. As a shepherd, Hogg had witnessed and marvelled at this effect many times and wrote a wonderful account of it entitled ‘Nature’s Magic Lantern’. 

A glory recalls the halo used to denote holiness in religious iconography, such as this image of St John the Evangelist. In the novel, this divine manifestation around George contrasts with the malignant shadow associated with Robert.