A prognostic variable is a variable that a global climate model

Global climate model

A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...

 predicts by integration of a physical equation

Equation

An equation is a mathematical statement that asserts the equality of two expressions. In modern notation, this is written by placing the expressions on either side of an equals sign , for examplex + 3 = 5\,asserts that x+3 is equal to 5...

, typically vorticity, divergence

Divergence

In vector calculus, divergence is a vector operator that measures the magnitude of a vector field's source or sink at a given point, in terms of a signed scalar. More technically, the divergence represents the volume density of the outward flux of a vector field from an infinitesimal volume around...

, temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, surface pressure

Surface pressure

Surface pressure is the atmospheric pressure at a location on Earth's surface. It is directly proportional to the mass of air over that location....

, and water vapour concentration.

The term prognostic is given to some values or variables that are directly predictive by the model, such as temperature, water vapour, salinity, depth in atmospheric or ocean models; 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 which may be a diagnostic variable obtained from the model's prognostic variables, temperature and water vapour.