What about the following code?

Code:\begin{tabular}{lp{8.5cm}} $d$ & day of the year\\ $H$ & monthly average of daily global radiation on horizontal surface ($\mathrm{W}/\mathrm{m}^2\mathrm{day}$) \end{tabular} \begin{tabular}{lp{8.5cm}} $R_b$ & beam radiation conversion factor\\ $R_r$ & ground reflected radiation conversion factor\\ $R_t$ & total radiation conversion factor\\ \end{tabular}