Changes in Sunlight Duration in Lhasa, the "City of Sunlight"
Lhasa, the capital of Tibet Autonomous Region, is known as the "City of Sunlight . Because of the rich sunlight radiation, thermal, and water resources, Lhasa is one of the best regions for development of agriculture and livestock production. Many scholars have studied climate change in terms of temperature and precipitation on the Qinghai-Tibet Plateau, but few have studied the changes in solar energy resources in this region, which includes Lhasa. Until now, there is limited understanding of how Lhasa' s solar energy resources have changed and the main impact factors. These problems are relevant in the context of global climate change. Sunlight duration, as an index of solar energy resources, is an important climatic factor. In this article, we analyze the climatic features of sunlight duration in Lhasa, using data on monthly sunlight duration, mean cloudiness, surface vapor pressure and precipitation from 1970 ~ 2004. Results show that the average annual duration of sunlight in Lhasa is 2980h. The month of October has the most sunlight, with 281h, and August has the lowest number of sunlight, with 219h. Of the four seasons, autumn has the most sunlight, with 777h, followed by spring (769h), winter (736h) , and then summer (698h). From 1970 to 2004, the annual sunlight duration decreased at an average rate of 58h/10a, and this trend is the most obvious beginning in the late 1990s. Except for a somewhat weaker trend of decreasing sunlight hours in winter, the other three seasons exhibit an obvious decline. The summer months had the most rapid decline, of 27h/10a. Results indicate that the main factors influencing sunlight duration in Lhasa are surface vapor pressure and precipitation. The significant increase in surface vapor pressure and precipitation led to the decrease in sunlight duration over the study period. These two factors are especially prominent from the late 1990s through 2004.
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