An environmental scientist is examining the temperature change in a soil profile. At the surface, where depth is $0$, the temperature is $18$${}^{{}^{}\text{o}}\text{C}$. At the depth of $1.5$ $\text{m}$, the temperature is $30$${}^{\text{o}}{\text{C}}^{}$. A temperature gradient exceeding a threshold of $6$${}^{\text{o}}\text{C}/\text{m}$ indicates the potential for rapid soil moisture evaporation. Using the Mean Value Theorem, find the temperature gradient and determine if it exceeds the critical threshold.