Past exam of the physics of atmospheres and oceans course of the University of Oxford / 2022 / 1 / b / Solution

Let $Y=-\ln (p/p_{\ast })$. For the temperature-dependent latent heat,Keeping the latent heat of vaporization fixed changes only the upper branch toBoth curves fall from $+\infty$ as $T\to 0^{+}$, pass through $Y=0$ at the triple point, and are convex. At the triple point there is a corner because the latent heat changes from $L_s$ to $L_{\ast }$. Their right-hand slopes there agree. Above the triple point,The variable-latent heat curve lies above the fixed-latent heat curve: its downward slope has smaller magnitude at every $T>T_{\ast }$. It reaches a horizontal tangent at $T_c$, whereas the fixed-latent heat curve is still descending. Both limiting ordinates are finite; this model gives $Y_{\mathrm{variable}}(T_c)\simeq -7.30$ and $Y_{\mathrm{fixed}}(T_c)\simeq -11.40$. These are extrapolations of the stipulated model, not accurate water liquid-gas critical point data. Extending the variable branch beyond $T_c$ would give negative latent heat and is physically inadmissible.