 Until the end of the 19th century , weather prediction was entirely subjective and based on empirical rules , with only limited understanding of the physical mechanisms behind weather processes . In 1901 Cleveland Abbe , founder of the United States Weather Bureau , proposed that the atmosphere is governed by the same principles of thermodynamics and hydrodynamics that were studied in the previous century . In 1904 , Vilhelm Bjerknes derived a two @-@ step procedure for model @-@ based weather forecasting . First , a diagnostic step is used to process data to generate initial conditions , which are then advanced in time by a prognostic step that solves the initial value problem . He also identified seven variables that defined the state of the atmosphere at a given point : pressure , temperature , density , humidity , and the three components of the flow velocity vector . Bjerknes pointed out that equations based on mass continuity , conservation of momentum , the first and second laws of thermodynamics , and the ideal gas law could be used to estimate the state of the atmosphere in the future through numerical methods . With the exception of the second law of thermodynamics , these equations form the basis of the primitive equations used in present @-@ day weather models . 
