Xi’an has a temperate climate that is influenced by the East Asian monsoon, classified under the Köppen climate classification as situated on the borderline between a semi-arid climate (BSk) and humid subtropical climate (Cwa). The Wei River valley is characterised by hot, humid summers, cold, dry winters, and dry springs and autumns. Most of the annual precipitation is delivered from July to late October. Snow occasionally falls in winter but rarely settles for long. Dust storms often occur during March and April as the city rapidly warms up. Summer months also experience frequent but short thunderstorms. The monthly 24-hour average temperature ranges from around the freezing mark in January to 27.0 °C (80.6 °F) in July, with an annual mean of 14.08 °C (57.3 °F). With monthly percent possible sunshine ranging from 31 percent in December to 47 percent in August, the city receives 1,536 hours of bright sunshine annually. Extremes since 1951 have ranged from −20.6 °C (−5 °F) on 11 January 1955 to 41.8 °C (107 °F) on 21 June 1998. A highest record of 42.9 °C (109 °F) was registered in another station on 17 June 2006.
Xi’an lies on the Guanzhong Plain in the south-central part of Shaanxi province, on a flood plain created by the eight surrounding rivers and streams. The city has an average elevation of 400 metres (1,312 ft) above sea level and an annual precipitation of 553 mm (21.8 in). The urban area of Xi’an is located at. The Wei River provides potable water to the city.
The city borders the northern foot of the Qin Mountains (Qinling) to the south, and the banks of the Wei River to the north. Hua Shan, one of the five sacred Taoist mountains, is located 100 km (62 mi) away to the east of the city. Not far to the north is the Loess Plateau.
At the beginning of Han dynasty, Prime Minister Zhang Liang advised the emperor Liu Bang to choose Guanzhong as the capital of the Han dynasty: ‘Guanzhong Plain, which is located behind Xiao Pass and Hangu Pass, connects Long (Gansu) and Shu (Sichuan). Land of thousands miles and rich in harvest can be found here, as if this place is belongs to the nation of the heaven.’ (《关中左崤函，右陇蜀，沃野千里，此所谓金城千里，天府之国也》) Since then, Guanzhong is also known as the ‘Nation of the Heaven’.
National Time Service Centre
The Shaanxi Astronomical Observatory was established in 1966. In 1975, according to the Geodetic Origin Report of the People’s Republic of China, ‘in order to avoid bias in the mensuration as much as possible, the Geodetic Origin would be in central mainland China.’ Lintong (临潼), a town near Xi’an was chosen. Since 1986, Chinese Standard Time (CST) was set from NTSC. The NTSC in Lintong is 36 km (22 mi) away from Xi’an.
National Time Service Centre (NTSC), the Chinese Academy of Sciences is an institute which is mainly engaged in the service and research on time and frequency. NTSC takes charge of generating and maintaining the national standard time scale, disseminating the time and frequency signals. The autonomous standard time scales of universal time and atomic time and the dissemination techniques with LF radio and HF radio were established successively during the 1970s and 1980s, which meet all the requirements for different applications on the whole, such as the scientific researches, national economy, etc.