To improve the forecast and early warning ability for the low visibility along the expressway, the hourly observation data of 10 traffic meteorological stations along Shaanxi section of the Lian-Huo expressway and the hourly reanalysis data of the European Center for Medium-Range Weather Forecasts are employed to analyze the distribution characteristics of low visibility and to explore the relationship of low visibility with other meteorological factors. The results show that along Shaanxi section of the Lian-Huo expressway, January has the most low visibility, while February has the least. During a day, the low visibility mostly happens from 00:00 to 10:00, and the low visibility of 0-50 m mainly occurs from 05:00 to 06:00. The low visibility duration is short with the majority of within 2 hours and the longest of 17 hours. The low visibility weathers occur frequently from Xingping to Changxing and Chencang section, where attention should be paid in daily traffic meteorological service. Accordingly to the analysis of the relationship between low visibility and other meteorological factors, the low visibility generally occurs under the conditions of air temperature from 0 to 5 ℃, relative humidity above 90%, wind speed less than 1.0 m·s^-1 and northeast to east winds. The low visibility weather in summer and winter is mostly associated with precipitation, which usually occurs during or after the precipitation and is accompanied by weather systems always. Compared with the low visibility caused by radiation cooling, the low visibility associated with precipitation exhibits longer duration and the wider range. The low visibility weathers occur at different relative humilities in different seasons, which in winter, summer and autumn are high, while in spring are relatively low.

The sunflower growing region in northern China is mainly located in arid and semi-arid areas, and the yield is governed by wet and dry conditions. The non-homogeneous variation of water resources between different regions in the context of climate change has increased uncertainty of sunflower production. Based on daily meteorological data from 296 stations in the sunflower growing region in northern China, the wetness index and standardized precipitation evapotranspiration index (SPEI) calculated on the basis of precipitation and crop evapotranspiration are used to analyze the spatial and temporal characteristics of dry and wet conditions and the influence of major meteorological factors on crop evapotranspiration in the sunflower growing season from 1961 to 2020, and the causes of changes in dry and wet conditions are explored by using sensitivity and contribution rate method. The results show that the frequency of drought in sunflower growing season in northern China generally presented a spatial distribution decreasing from west to east, among which the frequency of drought was higher in northern Xinjiang, northern Ningxia and western Inner Mongolia. In the last 60 years, both precipitation and evapotranspiration in sunflower growing season decreased. The SPEI had an abrupt change in 1980, and compared with the period of 1961-1980, the frequency of slight, medium and severe drought during 1981-2020 generally decreased by 5.63%, 4.41%, 2.49%. Obvious differences between different regions in terms of dry and wet conditions were found, with the climate in Chifeng of Inner Mongolia, southern Liaoning and the North China plain showing a warm and dry trend, and the climate in western Inner Mongolia and Xinjiang becoming wet. The change in temperature and relative humidity increased crop evapotranspiration, but the change in sunshine hours and wind speed decreased crop evapotranspiration in the last 60 years in sunflower growing season. The contribution rate of wind speed and sunshine hours at 55.39% stations to crop evapotranspiration is greater than that of temperature, resulting in a significant decrease of crop evapotranspiration.

Considering the different disaster-pregnant environment, population density and economic condition, the drought disaster chain models in southwest and south China were constructed respectively by using history data and record of references. Here, the disaster-pregnant environment includes climate background, underlying surface, landforms, soil and river network. And on this basis that the transmission characteristics of disaster on separate disaster chains were analyzed. The results show that the drought disaster chain structure is similar in southwest and south China, but their transfer process of disaster on the two chain is different. There are obvious regional characteristics of disaster transfer on their chain. The incipient drought can cause crop drought in southwest China, while moderate drought can cause crop drought in south China. In southwest China, the moderate drought can lead to some problems such as people and livestock drinking water difficult and livestock forage shortage, but these corresponding problems can be generated by the severe drought in south China. Being the difference in disaster-pregnant environment, the rock desertification phenomenon appears in southwest China under the severe drought condition, but the probability of rock desertification is small in south China except for north Guangxi. In the same region, as far as the various disaster-affected bodies are concerned, the threshold of drought degree transmitting is different. For example, shipping problems usually result from the moderate drought, and forest fire and pest disaster are often led by the severe drought, while soil degradations appear in the extreme drought.

Based on the radar dense data of convectional cloud on June 12，2004 in Yongdcng county, choosing two similar convectional clouds as seeding and non一seeding cloud，the remarkable changes of radar echo parameters of the seeding cloud compared with the non一seeding cloud were discussed in this paper, and the rain enhancement effect was evaluated primarily with ground observational rainfall data.  "hhc results show that many aspects of the seeding cloud，such as precipitation，life characteristic and vertical characteristic parameters change of radar echo, had remarkable differences compared with the non一seeding cloud，and about }0 min after seeding() .6 mm precipitation produced，while the non一seeding cloud experienced a short period from development to dissipation and didri t produce any precipitation in the ground，which can conclude this rainfall cnhanccment experiment reached some prospective effect.