Relative to $Trec$

  $$\frac{\partial Tcal}{\partial Trec} = \frac{\partial Tcal}{\part... ...rec} + \frac{\partial Tcal}{\partial Tau} \frac{\partial Tau}{\partial Trec}$$ (33)

From Eq. (18)

  $$\frac{\partial Tcal}{\partial T_{emi}} = \frac{Tcal}{T_{emi}-T_{load}}$$ (34)

and from Eq. (20)

  $$\frac{\partial T_{emi}}{\partial Trec} = -1 + \frac{T_{emi}+Trec} {T_{load}+Trec}$$ (35)

thus

  $$\frac{\partial Tcal}{\partial T_{emi}} \frac{\partial T_{emi}}{\partial Trec} = \frac{Tcal}{T_{load}+Trec}$$ (36)

For the second term, from Eq. (18)

  $$\frac{\partial Tcal}{\partial Tau} = Air\_mass * Tcal$$ (37)

and from Eq. (27)

  $$\frac{\partial Tau}{\partial Trec} = \frac{\partial Tau}{\partial T_{emi}} \frac{\partial T_{emi}}{\partial Trec}$$ (38)

  $$\frac{\partial Tau}{\partial Trec} = \frac{1}{Air\_mass * T_{atm} * F_{eff}} \frac{\partial T_{emi}}{\partial Trec}$$ (39)

  $$\frac{\partial Tcal}{\partial Tau} \frac{\partial Tau}{\partial Trec} = \frac{Tcal}{T_{atm} * F_{eff}} \frac{T_{emi}-T_{load}}{T_{load}+Trec}$$ (40)

giving finally

  $$\frac{1}{Tcal} \frac{\partial Tcal}{\partial Trec} = \frac{T_{atm}*F_{eff} - T_{load} + T_{emi}} {F_{eff}*T_{atm}*(T_{load} + Trec)}$$ (41)

The typical numbers mentionned above, with $T_{load} = T_{cab}$, yield

  $$\frac{1}{Tcal} \frac{\partial Tcal}{\partial Trec} = \frac{F_{eff}-1}{F_{eff}} \frac{1}{T_{load}+Trec}$$ (42)

of the order of $3$ $10^{-4}$ per K. Note that this could be higher for higher opacities (hence higher $T_{emi}$).