Falling Snowflakes

Snowflakes, cylindrical rocket bodies without tail fins, skydivers and other irregularly shaped bodies tend to fall through the air with their flattest sides downward. This behavior seemingly defies the logic that says an object should orient itself to slip through the air with the least resistance.

The reason for the unexpected orientation of a falling object is the turbulence it creates in the air it falls through. When a snowflake or other body first starts to fall it is moving so slowly that the air passes around it in smooth layers, the so-called laminar flow. As long as the flow is laminar, the air does not act on the snowflake to change its original orientation, even though there is some resistance to the motion because the object pushes some air aside and, because the air is viscous, the object tries to drag the nearest air along with it. The resistance is directly proportional to the fall speed.

Even a snowflake eventually reaches a fall speed that creates turbulence. The air rushing past it (from the snowflake's point of view) curls in behind it in swirling patterns that vastly increase the resistance to the downward motion of the snowflake. The faster the motion and the larger the falling object, the greater becomes the turbulence, and the total resistance now is proportional to the square of the fall speed.

In a real sense, the turbulence increases the area of moving material passing through the air; the area becomes that of the snowflake itself plus the area that the attached turbulent air presents to the surroundings. Even a needle-shaped snow crystal falling pointed end downward, through the action of turbulence, effectively has a resistance many times that which the needle would have if it merely had to push enough air aside to allow its own passage.

The only way the snowflake can minimize the turbulent resistance is to slow down, and the way to slow down if you are a snowflake is to put your biggest face forward so that there is even more air to push against. By putting the biggest area forward, an object minimizes the total resistance to its forward motion through a fluid or a gas, and that is just what our logic says it should do--minimize resistance.

The turbulence not only causes an object to orient until the biggest area is presented to the direction of motion, it also makes the object flutter and move in circular patterns. Falling pieces of cardboard, leaves, snowflakes and even spent rocket motors flutter as they fall. The fluttering results from parcels of the turbulent air breaking away from their attachment to the falling object. Each parcel is in the form of an eddy, a spiraling vortex of motion that seems to take up a life of its own, like a dust devil. When each vortex breaks away, the falling object is given a sideways kick.

Yet another consequence of turbulence is that snowflakes and other bodies tend to catch up with each other. A snowflake caught in the turbulent wake of another tends to move faster and so can collide with the flake ahead and combine with it. This is the same effect that allows a motorcyclist to draft in behind a big truck and get a free, if somewhat dangerous, ride and why geese fly a V-formation; all except the head goose have easier flying. It's also why fish swim in schools, each fish senses the wake of the others ahead and swims accordingly. It probably is the same reason why a group of skydivers are sometimes able to move together to join hands and fall together as a group.