This week, Hurricane Sandy is bearing down on the East Coast of the US. Most of us are concerned with the experience of a hurricane from below – and for good reason. But putting on our HTE (Here, There & Everywhere) hats, we like to consider the spiral shape of a hurricane when seen from above.
Let’s think of two other spiral-shaped objects on very different scales: water going down a drain and a spiral galaxy. The common thread for all three of these is angular momentum, a physical principle that remains constant with time for a spinning object and applies over all scales.
When water is pulled down towards a drain by gravity, it spins faster near the drain so that angular momentum is conserved (like a skater bringing in their arms to rotate more rapidly). This increase in speed helps generate the spiral structure in the flow. The direction of spin is mainly determined by the shape of the bowl and the initial direction of the water, not by the so-called “Coriolis effect”.
Moving to hurricanes, they are driven by powerful uplift from warm, humid air and the very low pressures that result. As a storm forms and intensifies, winds are pulled inwards and pick up speed because of the conservation of angular momentum. The initial rotation of hurricanes is determined by the Coriolis effect. Because the Earth rotates about an axis, different points on the Earth’s surface will rotate at different speeds. Points on the equator go the fastest and points at the pole are stationary. This difference in speed imparts a rotation to air flowing into regions of low pressure. Because of this Coriolis effect, hurricanes rotate in different directions depending on what hemisphere they are in.
Finally, spiral shapes bring us to galaxies. The formation of a spiral galaxy involves the collapse of an enormous cloud of gas. To conserve angular momentum, a rotating disk results. The source of the familiar spiral pattern is thought to be density waves that pass over the disk. As the density wave passes over a region, the gas is condensed resulting in the formation of bright stars in a spiral pattern.
So the bottom line is because all three objects involve spin, the conservation of angular momentum plays a role in each case—but there are crucial differences in the details of the physics. Can you think of other objects in your daily life, somewhere on the planet, or anywhere across the Universe where spiral shapes might have a connection?