Which way is the air blowing?

Something didn’t seem quite right on a recent January afternoon.

The wind — from the northwest — appeared to be blowing in opposite directions on either side of the Washington Monument.

Five minutes, 10 minutes, 15 minutes ticked by. The wind didn’t change. And neither did the flags. Almost all of them fluttered in the “correct” direction of the wind, except for several, which were stubbornly defiant.

Naturally, I had to investigate — and so, like a transfixed tourist, I orbited the monument step by step several times to mentally map the wind field. (I would have climbed each flagpole with my pocket anemometer, but that might have raised some questions with authorities.)

It turns out that there wasn’t any magic trick going on at the Washington Monument that day. Instead, it was likely a classic example of Von Karmen vortex shedding. Allow me to explain.

The physics

In fluid dynamics, we can refer to a background flow as the ambient motion of any fluid moving across an area. In the case of this particular day, it was a stiff wind out of the northwest. The wind wasn’t overly gusty, but rather a more constant, regulated flow.

When a background flow hits an isolated object, it’s forced to divert around the object — splitting just like the current around a stone in a river. But depending on the speed of the flow and the width of the interceding object, the flow in the object’s lee (downwind) can take many different forms.

Once the background flow reaches a certain speed, turbulence can result — the object “shedding” small eddies of alternating orientation. They’re invisible, but you’ve probably experienced them before.

Where you’ve experienced vortex shedding

You know that whistling you hear coming from power lines on a windy day? These are due to tiny Von Karmen vortices.

How about when you’re driving behind a tractor trailer on the highway? If you’re a certain distance behind the semi, your may suddenly feel your vehicle vibrate or shake. Vortex shedding is one of many types of turbulence that dominates flow in the wake of the truck.

At the Washington Monument

As it turns out, the Washington Monument probably proved an ideal source of Von Karmen vortices. With information on the wind speed and the width of the structure, it would even be possible to calculate the radius of any such invisible vortices through a figure known as the Strouhal Number.

If you’re looking to experience this phenomenon yourself at the Washington Monument, try it on a day when the wind has a strong west or east component. That allows the wind to run down the Mall unimpeded, allowing a more uniform flow.

The introduction of invisible vortices in the wake of the monument will only occur when the wind speed is sufficiently high. Below that threshold, the flow is more smooth or “laminar,” and eddies do not result.

Elsewhere in meteorology

Once in a while, Von Karmen vortex shedding occurs on a grand scale — large enough to be captured by weather satellites in space. It’s a common behavior downwind of Jeju, a South Korean island in the East China Sea. It’s most prominent when the wind is north-northwesterly.

[ Mesmerizing von Karman vortices swirl near South KoreaOpens in new window ]

It’s also a frequent appearance downwind of the Canary Islands, islands in the Sea of Okhotsk, and even on occasion off the coast of California.

[ We saw an awesome vortex on satellite—then a pilot actually got a photo of itOpens in new window ]

Hat tip to my friend Camilo Castellano-Sanchez who first noticed the erratic behavior of the flags, motivating me to investigate further.