#Metview Macro

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# 
#  Copyright 2022 ECMWF. This software is distributed under the terms
#  of the Apache License version 2.0. In applying this license, ECMWF does not
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function mixing_ratio_from_vapour_pressure(e: number, p: number)

    EPSILON = 0.621981
    v = p - e    
    if p - e < 1e-4 then
        return nil
    end if    
    return EPSILON * e / v
    
end mixing_ratio_from_vapour_pressure