Very cold temperatures are required for a bison to morph into a primeval winter monster, but on the mornings when this transformation is acute, it is a visual treat. There is a sense of trespassing into an untamed land of ice and fire.

The effect is most pronounced in the geyser fields of Yellowstone National Park where the steam will freeze on whatever is nearby. As informed locals will testify, these days are fewer than they used to be, but on this morning, it was about 25 degrees below zero, which is enough for the magic to unfold.

Of course, finding a bison to work with in these conditions is not a given and we have often failed simply because the lead talent has been nowhere accessible. Geysers are dangerous and quite rightly there are many no-go areas in Yellowstone. To dissolve would be a harrowing death and it has, of course, happened to both bison and cavalier visitors to the National Park.

This portrait is sharp and looks at its best when it is at least life size. The abstract granularity of the bison’s iced fur elevates the photograph. I don’t think I have ever encountered as white a bison as this and I left the park with a sense of mission accomplished.