Flow tracer

Image of the Gulf Stream obtained by the Moderate-Resolution Imaging Spectroradiometer. The false colors in the image represent "brightness temperature" observed at the top of the atmosphere in the 10.780-11.280 μm band. The brightness temperature values represent heat radiation from a combination of the sea surface and overlying moist atmosphere.
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 the flow velocity (i.e., flow magnitude and direction) and circulation patterns. Tracers can be chemical properties, such as radioactive material, or chemical compounds, physical properties, such as density, temperature, salinity, or dyes, and can be natural or artificially induced. Flow tracers are used in many fields, such as physics, hydrology, limnology, oceanography, environmental studies and atmospheric studies.

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.[1]

  1. ^ Stutter, MI; Deeks, LK; Billet, MF (2005). "Transport of conservative and reactive tracers through a naturally structured upland podzol field lysimeter". Journal of Hydrology. 300 (1–4): 1–19. doi:10.1016/j.jhydrol.2004.04.026.