This model demonstrates a cubic spline interpolation of 12 monthly values that can be used for a daily time step model for 1 or more years. A cubic spline is constructed using 3rd order polynomials that pass through a set of control points. The 2nd derivative of each polynomial is set to zero at the end points to provide boundary conditions. A tridiagonal system is solved to develop coefficients for the polynomials. This model only requires that the data be entered as a unitless vector of months.