A prognostic variable is a variable that a global climate model predicts by integration of a physical equation, typically vorticity, divergence, temperature, surface pressure, 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.