William Marshall reported in his 'Rural Economy of Yorkshire' in 1796 that on a calm day when the air is seen to undulate near the surface of the ground it can appear to rise 'as from hot embers'. The phenomenon is expressed by saying, 'the summer-colt rides'.
This conjures up all sorts of visions of a horse in Summer galloping across the landscape. It makes me think of the work of Hannah Willow -see http://hannahwillow.com/ especially her painting of 'The Uffington Mare'.
I'm inspired to get my sketch book out and play with ideas for 'the summer-colt rides'. It has a magic and a mystery to it.