Oil Code Thickness and Concentration Values scale

Oil Slick from IStockPhoto



Sheen: Sheen is a very thin layer of oil (less than 0.0002 inches or 0.005 mm) floating on the water surface and is the most common form of oil seen in the later stages of a spill. According to their thickness, sheens vary in color from rainbows, for the thicker layers, to silver/gray for thinner layers, to almost transparent for the thinnest layers.

Metallic: The next distinct oil color, thicker than rainbow, that tends to reflect the color of the sky, but with some element of oil color, often between a light gray and a dull brown. Metallic is a “mirror to the sky.”

Transitional Dark (or True) color: The next distinct oil on water layer thickness after metallic, that tends to reflect a transitional dark or true oil color. At the “Transitional” stage, most of the oil will be just thick enough to look like its natural color (typically a few thousandths of an inch, or few hundredths of a millimeter), and yet thin enough in places to appear somewhat patchy.

Dark (or True) Color: Represents a continuous true oil color (i.e., its natural color), commonly occurring at thicknesses of at least a hundredth of an inch (or, a little over a tenth of a millimeter). Oil thickness at this “Dark” stage (especially in a calm and/or contained state) could range over several orders of magnitude. At sea, however, after reaching an equilibrium condition, most oils would not achieve an average thickness beyond a few millimeters. Heavy fuel oils and highly weathered or emulsified oils (especially on very cold water) could, of course, reach equilibrium states considerably greater than a few millimeters.


Streamers: Narrow bands or lines of oil (sheens, dark or emulsified) with relatively clean water on each side. Streamers may be caused by wind and/or currents, but should not be confused with multiple parallel bands of oil associated with “windrows,” or with “convergence zones or lines” commonly associated with temperature and/or salinity discontinuities.

Convergence Zone: A long narrow band of oil (and possibly other materials) often caused by the convergence of two bodies of water with different temperatures and/or salinities. Unlike “windrows” and “streamers,” commonly associated with wind, convergence zones are normally associated with the interface between differing water masses, or with the effects of tidal and depth changes that cause currents to converge due to density differences or due to large bathymetric changes. Such zones may be several kilometers in length, and consist of dark or emulsified oil and heavy debris surrounded by sheens.

Windrows: Multiple bands or streaks of oil (sheens, dark, or mousse) that line up nearly parallel with the wind. Such streaks (typically including seaweed, foam, and other organic material) are caused by a series of counter rotating vortices in the surface layers that produce alternating convergent and divergent zones. Sometimes referred to as Langmuir vortices (after a researcher in 1938), the resulting “windrows” begin to form with wind speeds of approximately six knots or more.

Patches: An oil configuration or “structure” that reflects a broad range of shapes and dimensions. Numerous “tarballs” could combine to form a “patch”; oil of various colors and consistency could form a patch or single layer 10s of cm to 10s (or even 100s) of meters in diameter; and a large patch of dark or rainbow oil could have patches of emulsion within it. Patches of oily debris, barely able to float with sediment/plants in them, might be called “tarmats,” circular patches at sea might be called “pancakes”; REALLY BIG patches might simply be called “continuous” slicks. But, they are all “patches.”

Tarballs: Discrete, and usually pliable, globules of weathered oil, ranging from mostly oil to highly emulsified with varying amount of debris and/or sediment. Tarballs may vary in size from millimeters to 20- 30 centimeters across. Depending on exactly how “weathered,” or hardened, the outer layer of the tarballs is, sheen may or may not be present.

No Structure: Random eddies or swirls of oil at any one or more thicknesses. This distribution of oil is normally the result of little to no winds and/or currents.


Black oil: A black or very dark brown-colored layer of oil. Depending on the quantity spilled, oil tends to spread out quickly over the water surface to a thickness of about one millimeter. However, from the air it is impossible to tell how thick a black oil layer is. The minimum thicknesses for a continuous black oil layer would commonly be around a hundredth of an inch to about two tenth of a millimeter. Dark (or Black) oils just begin to look their natural color at around a thousandth of an inch (or, a few hundredths of a millimeter). See chart on page 10.

Dispersion: The breaking up of an oil slick into small droplets that are mixed into the water column as a result of sea surface turbulence. For response purposes, dispersed oil is defined as oil droplets that are too small to refloat back to the surface. The physical properties of the oil and the sea state are the main factors that determine how much oil is dispersed. Chemical dispersants can be used to change the chemical properties of the oil and enhance oil dispersion.

Emulsification: The formation of a water-in-oil mixture. The tendency for emulsification to occur varies with different oils and is much more likely to occur under high energy conditions (winds and waves). This mixture is frequently referred to as “mousse.” Emulsification will impact the cleanup by significantly increasing the volume and viscosity of the oil to be collected.

Entrainment: The loss of oil from containment when it is pulled under a boom by a strong current. Entrainment typically occurs from booms deployed perpendicular to currents greater than 3/4 knot.

Recoverable Oil: Oil that is in a thick enough layer on the water to be recovered by conventional techniques and equipment. Only black or dark brown oil, mousse, and heavy Metallic layers are generally considered thick enough to be effectively recovered by skimmers. Thinner films may be recoverable with sorbents and/or concentrated with booms or chemical herders to enhance their recovery.

Slick: Oil spilled on the water that absorbs energy and dampens out the surface waves making the oil appear smoother or “slicker” than the surrounding water. “Slicks” refer to oil layers that are thicker than Rainbow and Silver “sheens”. Natural slicks, from plants or animals, also may occur on the water surface and may be mistaken for oil slicks.

Weathering: A combination of physical and environmental processes such as evaporation, dissolution, dispersion, photo-oxidation, and emulsification that act on oil and change its physical properties and composition.


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