November 16, 2016
Have you ever noticed long streaks of foam on large bodies of water, especially on very blustery days? I took this photograph on a very windy afternoon (October 22, 2016) on a bridge overlooking the Lafayette River in Norfolk, Virginia. The streaks are caused by Langmuir circulation and are concentrations of scum, bubbles and other debris floating on the surface of the water. In 1938, Irving Langmuir noticed linear patterns of sargassum (floating seaweed) during a cross-Atlantic sea voyage. Once home, he investigated the phenomenon and realized that rotating cells or tubes (horizontally aligned just below the surface) caused the flotsam to congregate on the water's surface in the general direction of the wind. These Langmuir cells arise because of wind-generated shearing forces at the surface.
Imagine two adjacent tubes of water just below the surface and in the direction of the wind. The one on your right circulates in a counter-clockwise fashion, and the other does so clockwise, creating an upwelling/downwelling effect, causing the scum to converge approximately linearly at the surface. Other pairs of cells repeat the overall effect -- the distance between two adjacent streaks, anywhere from 30 ft to 160 ft (9-49 m) is twice the diameter of a single tube. Note that their lengths can range from several meters to several kilometers depending on where they're seen. According to an article in the journal Fluid Dynamics the separation of the bands in the open sea ranges from about 6 ft to 0.6 mi (2 m-1 km) and their lengths are three to ten times greater. Typically, they extend to depths of about 12 to 20 ft (4-6 m).
In gale-force winds the appearance of such streaks on the sea surface can be used to categorize wind speed (Beaufort scale 7-9, i.e. wind speeds of 31 mph to 54 mph or 13.9-24.4 m/s), but of course, they also appear at lower wind speeds. These streaks can last on a scale of minutes to hours.
Photo Details: Camera Maker: OLYMPUS IMAGING CORP.; Camera Model: SP-100EE; Focal Length: 4.3mm (35mm equivalent: 24mm); Aperture: ƒ/2.9; Exposure Time: 0.0016 s (1/640); ISO equiv: 125.