A prognostic variable in

engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...

within the context of

prognostics Prognostics is an engineering discipline focused on predicting the time at which a system or a component will no longer perform its intended function. This lack of performance is most often a failure beyond which the system can no longer be used to ...

, is a measured or estimated variable that is correlated with the health condition of a system, and may be used to predict its residual useful life. An ideal prognostic variable is easily measured or calculated, and provides an exact estimation of how long time the system can continue to operate before maintenance or replacement will be required. Real prognostic variables are usually either known with some uncertainty, may be difficult to measure, and their correlation to the state of health of the system may not be exact. Examples of prognostic variables are the age of a vehicle and its

odometer An odometer or odograph is an instrument used for measuring the distance traveled by a vehicle, such as a bicycle or car. The device may be electronic, mechanical, or a combination of the two (electromechanical). The noun derives from ancient Gr ...

reading: the older a car is, and the longer it has been driven, the more worn it can be expected to be. These prognostic variables are useful, but not ideal, since they do not consider other aspects such as the regularity of the maintenance that was applied on the vehicle, how they were driven, in which weather conditions, etc.; they do, however, have the advantage of being easily measured, understood and verified, whereas an in-depth analysis of the mechanical condition of the vehicle would be expensive to perform, require specific skills to be understood and would be difficult to verify.

In climate science

climate science Climatology (from Greek , ''klima'', "place, zone"; and , '' -logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. This modern field of stud ...

, the term indicates a variable that a

computer model Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be deter ...

predicts by integration of a physical

equation In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in ...

, typically

vorticity In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along wit ...

divergence In vector calculus, divergence is a vector operator that operates on a vector field, producing a scalar field giving the quantity of the vector field's source at each point. More technically, the divergence represents the volume density of the ...

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

, surface pressure, and

water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...

concentration in atmospheric models. The term ''prognostic'' is given to some values or variables that are directly predicted by the model, such as temperature, water vapour, salinity, depth in atmospheric or ocean models; i.e., variables that can be directly obtained as a model outcome. On the other hand, there are some other variables that need to be calculated separately as derived variables, such as

relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...

, which may be a diagnostic variable obtained from the model's prognostic variables, temperature and water vapour.

References

Numerical climate and weather models {{climate-stub