It seems to be generally held that warm air is passing polewards from the equator continuously at the high levels.

Here is one heading towards John O'Groat's, grubbing its way like a mole around the firths, cutting spiral gains into the rock-ribbed hills, bridging the deep and dark gorges, and holding on steadily north-poleward with a brave faith and faculty of patience that moves mountains, or as much of them as blocks its course.