Abstract: The summer downward longwave radiation (L_↓) observed in Naqu, Lhasa, Nyingchi and Ali is analyzed. The averaged L_↓ at 4 sites are 299, 319, 368 and 305 W·m^-2, respectively. L_↓ is lower in the local morning and subsequently increases significantly in the afternoon. The mean diurnal variation at Naqu and Ali is about 30 W·m^-2, while it's 9 and 19 W·m^-2 in Lhasa and Nyingchi respectively. Based on solar shortwave radiation observations, a method to determine the daytime sky cloud-coverage index (cloud fraction, C_F) is presented by the solar radiation comparisons between the empirically calculation for cloud-free situation and the observed. C_F of -5% to 5% is assumed as daytime cloud-free situation to test the suitability of 10 empirical formula of L_↓ on the Plateau. It shows that the empirical formula of Ångström (1915) is most suitable for the Nyingchi where the vapor pressure is high, while the empirical formula of Konzelmann (1994) is most suitable for Naqu, Lhasa and Ali. Errors of the calculated daytime cloud-free L_↓ from the observed at Naqu, Lhasa, Nyingchi and Ali are 2.1%, -0.27%, -0.89% and 0.94%. The cloud-induced L_↓ enhancement effect (measured L_↓ minus the calculated cloud-free values given the surface temperature and humidity) clearly shows that the mean L_↓ enhancement effect at Naqu, Lhasa, Nyingchi and Ali are 30.8, 22.1, 38.8 and 15.6 W·m^-2 with the median values of 24.4, 17.3, 42.7 and 6.8 W·m^-2. With the increase of artificially visual total cloud amount, the increasing trend of L_↓ enhancement is obvious, especially when the cloud amount increase from less than 20% to 70% and above, the corresponding L_↓ enhancement effects rapidly increases from above 20 to more than 50 W·m^-2 at all the 4 sites. Given the same visual cloud mount, the L_↓ enhancement effects induced by the low clouds in Lhasa and Ali are obviously higher than those induced by the total cloud. The effect from cloud coverage and height on the L_↓ enhancement is further confirmed by the aerosol lidar cloud base height at zenith direction and the C_F. The decreasing cloud base height (no available cloud base height data in Lhasa) corresponding to increasing trends of C_F and L_↓ enhancement is detected. The L_↓ enhancement effects are only about 5 W·m^-2 with cloud-free condition, but they may rise to 60 W·m^-2 when C_F is above 90% (the average cloud base height is less than 3.5 km). Given the fixed cloud base height, the L_↓ enhancement obviously increases with the growth of C_F. C_F, significantly more than the zenith cloud base height, which affects the enhancement of L_↓ on the Tibetan Plateau.