Flow tracer

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On May 2, 2001, the Moderate-resolution Imaging Spectroradiometer (MODIS) obtained this spectacular image of the Atlantic Ocean's Gulf Stream. The false colors in the image represent "brightness temperature" observed at the top of the atmosphere. The brightness temperature values represent heat radiation from a combination of the sea surface and overlying moist atmosphere.
Source: NASA.

Fluorescein in the Chicago River on the St. Patrick's Day (added for celebration, rather than tracing).

A flow tracer is any fluid property used to track flow. The concentration of a chemical compound in the fluid can be used as a chemical tracer, and characteristics such as temperature are physical tracers. Tracers may be artificially introduced, like dye tracers, or they may be naturally occurring. Conservative tracers remain constant following fluid parcels, whereas reactive tracers (such as compounds undergoing a mutual chemical reaction) grow or decay with time. Active tracers dynamically alter the flow of the fluid by changing fluid properties which appear in the equation of motion such as density or viscosity, while passive tracers have no influence on flow.

Tracers are used in oceanography to deduce flow patterns in the ocean. Common examples include potential temperature, salinity, and concentration of CFCs, tritium (introduced to the ocean surface by atomic bomb tests), oxygen, and many other compounds.