code: def _calc_b(w, aod700):
      
  
      b1 = 0.00925*aod700**2 + 0.0148*aod700 - 0.0172
      b0 = -0.7565*aod700**2 + 0.5057*aod700 + 0.4557
  
      b = b1 * np.log(w) + b0
  
      return b
docs: Calculate the b coefficient.