E The reason why the land is vast and sparsely populated is (a) the air is thin, the climate is cold, the environment is hot and humid, and the weather is dry and hot.

The climate of Qinghai-Tibet Plateau is as follows

(1) Low temperature, large daily temperature difference and small annual temperature difference. In most areas, there is no summer all the year round, with constant frost and snow, and the annual average temperature is mostly below 5℃. The lowest temperature zone is along 35 N, and the annual average temperature is -40℃ ~-8℃, which is the lowest temperature zone in China. However, the average temperature of the whole plateau in winter is not too low, and the average temperature in summer is not high, with little annual variation. Due to the thin air in the plateau, especially in winter, the weather is sunny during the day, the solar radiation is strong, and the ground temperature rises rapidly; At night, the ground is rapidly cooled by long-wave radiation, and the cold air sinks, so the temperature in the lower layer is extremely low, so the daily range is very large.

(2) The air is thin, the air pressure is low and the oxygen content is low. The plateau has high terrain, low atmospheric density, low air pressure and low oxygen content. The annual average pressure, oxygen content and atmospheric density are equivalent to 50%, 60% and 66% of sea level respectively. The boiling point of water is 84 ~ 87℃ in most areas. Low air density intensifies the intensity of air cooling and heating, and increases the daily variation of air temperature. At the same time, the buoyancy and wind pressure of the air are also reduced.

(3) Long sunshine time and strong radiation. The plateau is towering, with early sunrise, late sunset and long sunshine time. The air is thin and clean, with less dust and water vapor content and great atmospheric transparency. There are many sunny days, and the rainy season is mostly sunny days and rainy nights. The energy loss of sunlight passing through the atmosphere is less, which is the region with the largest amount of solar radiation in China. The total annual radiation in most areas is more than twice that in eastern China. Lhasa is also called "Sunlight City".

(4) The dry and wet seasons are distinct, and the dry season is windy. Due to the hot low pressure in summer, wet and warm precipitation weather appears on the plateau, and cold and high pressure in winter forms dry and cold windy weather, and the unique plateau monsoon changes obviously in dry and wet seasons. The prevailing wind system changes significantly with the seasons. In winter, the westerly belt controls the plateau to be a dry season, and in summer, it is affected by the humid southwest and southeast monsoon, and the precipitation is obviously concentrated in summer. Therefore, there is an obvious phenomenon of alternating dry and wet seasons. In addition, the precipitation on the Qinghai-Tibet Plateau is still characterized by thunderstorms, hail and night rain.