Spirals

Swirling one's hand through the soapy waters of the bathtub sets up a visible turbulent spiralling pattern of flow. That spiral pattern is observed frequently in nature, in many different ways. Wherever seen, the spiral pattern signals the existence of shear motion in the medium. By shear, one means motion of a part of the medium past another part.

Beautiful spiral patterns in the clouds are sometimes seen to leeward of mountains obstructing the general flow of air. The accompanying photograph shows a row of spirals in the clouds carried by winds rushing through a pass west of Umnak Island in the Aleutian Islands.

Clear air turbulence encountered when an aircraft passes near a high-speed stream of air is another example of the spiralling flow in a region of shear in air. Spirals are seen in the eddies of rivers, and they are what makes a whistle work. If one holds the mouth just right, the turbulent spiral pattern can be formed in the air blowing past the teeth thereby creating a high-pitched sound. The cloud photograph portrays what happens in a whistler's mouth if one imagines that the Aleutian volcanoes are the whistler's teeth.

Within the last few years, spirals have been discovered in the aurora. A graduate student at the University of Alaska, Thomas J. Hallinan, discovered that auroral rays seen from below were another form of the common spiral pattern. This discovery began an intensive study into the shear motions in auroras and what the spiral patterns might imply about the cause of the aurora. Through Hallinan's work, for which he received the Ph.D. in 1976, it is now known that the spiral patterns in the aurora are signs of very large electric fields and currents in the aurora. That knowledge, in turn, helped lead to the development of a new theory that seems to explain most of the auroras we see.