Tracer

tracer

1. A chemical or thermodynamic property of the flow that is conserved during advection.

   
   

   It can be used to track air-parcel movement and to identify the origins of air masses. Examples are absolute humidity, equivalent potential temperature, radioactivity, and CCN composition.

   
   
2. Any substance in the atmosphere that can be used to track the history of an air mass.

   
   

   It can be chemical or radioactive in nature. The main requirement for a tracer is that its lifetime be substantially longer than the transport process under study. An example of an inert chemical tracer is SF₆, which is often released during a field experiment and measured at a later time to assess the extent of dilution of the air mass. Chemicals such as methane (CH₄) and nitrous oxide (N₂O), which are released at the earth's surface and destroyed slowly in the atmosphere, can be used to infer vertical rates of transport. CO released in the boundary layer can be used to trace transport in convection. Radioactive tracers such as ¹⁴C and ⁹⁰Sr have been used to test models of stratospheric circulation. Certain atmospheric gases have also been used as tracers in ocean waters, for example, the chlorofluorocarbons.

   
   

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1. A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), and 2) undergoing specific modifications while in transit away from the source region.
   An air mass is often defined as a widespread body of air that is approximately homogeneous in its horizontal extent, particularly with reference to temperature and moisture distribution; in addition, the vertical temperature and moisture variations are approximately the same over its horizontal extent. The stagnation or long-continued motion of air over a source region permits the vertical temperature and moisture distribution of the air to reach relative equilibrium with the underlying surface.
   See airmass classification.
   
2. In radiation, the ratio of the actual path length taken by the direct solar beam to the analogous path when the sun is overhead from the top of the atmosphere to the surface.
   Extrapolation of surface measurements to zero air mass was the original method for estimating the value of solar irradiance at the top of the atmosphere.
   
3. 
   See optical air mass.

1. A gaseous envelope gravitationally bound to a celestial body (e.g., a planet, its satellite, or a star).

(Rare.) In England, a colloquial term for a sudden gust of wind or the sudden onset of a storm.

Same as: brash

An accumulation of floating ice fragments less than 2 m across, formed by breakage of other ice forms.