What changes have taken place in the climate in the western region?

The middle and upper reaches of the Yellow River are rich in light energy resources and the climate is mild, which is beneficial to forest plants to produce photosynthetic products and improve their quality. However, less precipitation, strong wind, great change and great intensity limit the utilization of light and heat resources. Generally speaking, the middle and upper reaches of the Yellow River are characterized by warm Leng Xia in winter.

1. Solar radiation and sunshine area The solar radiation is strong and the sunshine hours are long. The annual total radiation is 50× 108 Joule/m2 in the southeast and 63× 108 Joule/m2 in the northwest. The annual sunshine hours are generally 2000 ~ 3 100 hours. This provides a rich energy base for photosynthesis of green plants and is a prerequisite for high yield of plants and improvement of fruit quality.

2. The temperature in the middle and upper reaches of the Yellow River changes obviously in cold and hot seasons, and the annual range diurnal range is obvious, and the growing period is short. The annual average temperature in the area varies between 3.6 ~ 14.3℃, and the temperature decreases with the increase of topography and latitude. From south to north, the average temperature drops by 65438 0.33℃ with each latitude increase. From east to west, the longitude difference is 65438 0, and the average temperature drops by 0.73℃. The annual average temperature of 8℃ isoline passes through Yuanping, Lishi, Shenmu, Yulin, Zhidan, Huachi, Huanxian, Pingliang and Jingyuan. South of this line is warm temperate zone, and north is temperate zone. The annual average temperature in warm temperate zone is 8.0 ~ 14.5℃, the average temperature in June is 1℃, the average temperature in July is above 22℃, and the annual average accumulated temperature is ≥10℃ and 3400 ~ 4500℃. All kinds of temperate fruit trees can grow normally. In temperate zone, the annual average temperature is 4 ~ 8℃, the monthly average temperature is-10℃, the July average temperature is mostly above 20℃, and the annual average accumulated temperature ≥ 10℃ is 2000 ~ 3400℃. Most fruit trees in temperate regions need to take certain cold protection measures before planting.

3. Precipitation and evaporation The average annual precipitation in this area is 200 ~ 700 mm, and it is 300 ~ 600 mm in most areas, which also shows a decreasing trend from southeast to northwest. The area from Baoji to Yijun in Guanzhong is rainy, with an average annual precipitation of more than 600 mm, and the average annual precipitation in southeastern Shanxi is about 550 mm, which is the area with more annual precipitation in the middle and upper reaches of the Yellow River. The annual precipitation from southeast Yulin, Shaanxi Province to Lanzhou, Gansu Province is reduced to 400 ~ 600 mm, and less than 400 mm to the north of this line. The annual precipitation in the north of Yanchi-Tongxin-Jingyuan-Gaolan line in Ningxia is only 200 ~ 300mm, which is the area with the least precipitation in the middle and upper reaches of the Yellow River. Due to the influence of special topographical conditions, a more serious drought and less rainfall zone has been formed in Longzhong area of Gansu Province than in other areas of the same latitude.

The precipitation process in the middle and upper reaches of the Yellow River began at the end of March or early April, and strengthened after late June. The precipitation in July, August and September accounts for more than 60% of the total annual precipitation, but the precipitation in spring is less, accounting for only 15% of the annual precipitation, accounting for 55% ~ 65% in summer, 20% in autumn and 3% ~ 5% in winter. The precipitation of a rainstorm in summer and autumn can generally account for 10% ~ 20% or even 40% ~ 50% of the whole year. The number of days of precipitation is less than 100 in most places, with only 50-70 days in the lower reaches of Wei Fen. Moreover, the continuous days without precipitation in various places are long. For example, it doesn't rain for an average of 52 days in Xinxian County, Shanxi Province, including 100 days.

The annual evaporation in the middle and upper reaches of the Yellow River is mostly 1400 ~ 2000mm, and its distribution characteristics are small in the south and small in the north, less in the east and more in the west. Generally, it is the largest in late spring and early summer and the smallest in winter.

There are many northerly winds in the middle and upper reaches of the Yellow River in winter and southerly winds in summer. The monsoon direction in spring and autumn is messy, and the annual average wind speed is 2 ~ 4m/s, which is generally the largest in spring, followed by winter, and smaller in summer and autumn. Except for the southern plains and basins, the number of days with annual gale ≥8 (17.2 ~ 20.7m/s) is about 10 ~ 15 days, especially in the northwest. The number of gale days above magnitude 8 in Hequ, Jingrou, Huining, Gansu and Suide, Shaanxi all exceeded 100. Sandstorms (strong winds blow up dust on the ground, the sky is yellow or reddish yellow, and the visibility drops below 1000m) mostly occur in the northwest. There are many sandstorm days in Jingtai, Gansu, Tongxin and Yanchi in Ningxia, which generally last about 20 days. Sandstorms are most common in spring (March to May) and early summer (June), and least in autumn.