Debate on whether to deliver takeout in stormy weather

China is a rainy country. Except for some provinces and regions in the northwest, it is almost always heavy rain. China has a monsoon climate. From late spring to midsummer, the cold air in the north fought and retreated. Cold and warm air frequently meet, forming a series of heavy rains. The location of main rain belts in Chinese mainland also moves from south to north with the seasons. From April to June, heavy rains occurred frequently in South China. From June to July, persistent rainstorms often occur in the middle and lower reaches of the Yangtze River, with long duration, wide range and large amount of rainstorms. July-August is the main rainstorm season in northern provinces, and the rainstorm intensity is very strong. From August to 65438+ 10, the rain belt gradually withdrew southward. After summer and autumn, the typhoon rainstorm in the East China Sea and the South China Sea is very active, and the point rainfall of typhoon rainstorm is often very large. Rainstorms and floods in China have the following characteristics:

torrential rain

1) has obvious seasonality and uneven spatial and temporal distribution. As the subtropical high moves northward and southward, the rain belt in China also moves northward and southward in summer, showing obvious seasonal characteristics. Generally speaking, from April to early June, rain belts are mainly distributed in South China. From mid-June to early July, it is the rainy season in the Yangtze River, Huaihe River and Taihu Lake basins. From mid-July to August, the rain belt moves from the north of Jianghuai to North China and Northeast China. In September, the subtropical high moved southward, and then the rain belt moved southward accordingly, which also caused floods in some years, such as autumn flood in Hanjiang River and other places. Rainfall in most parts of China is seasonal. When a typhoon lands in China and goes deep inland, high-intensity storms will also cause heavy rains and floods.

According to statistics, from April to 65438+10, the rainfall in most parts of the country accounts for more than 70% of the annual average rainfall, and from June to August it can account for about 50% of the annual average rainfall. Therefore, heavy rains and floods in China mostly occur in spring, summer and autumn.

(2) The flood peak is high and large, and the main branches and tributaries are prone to floods. China's topography is characterized by low in the southeast and high in the northwest, which is conducive to the intersection of warm and humid airflow in the southeast and cold air in the northwest. The steep slope of the ground and poor vegetation conditions lead to rapid confluence and large flood magnitude. Compared with other countries in the world. For rivers with the same basin area, the magnitude of peak discharge of rainstorm and flood in China is close to the maximum record.

Several major rivers in China have a large area and often encounter floods in the main and tributaries. There are many incoming waters in the interval, and the flood peaks overlap, which is easy to form a rainstorm and flood with high peak.

(3) The flood changes greatly every year. The annual changes of floods and flood peak discharge in seven river basins in China are great, especially in the north than in the south. For example, the flood peak discharge in the south of the Yangtze River is generally 2-3 times that in the dry year, while the flood peak discharge ratio in the wet year and the dry year in the Haihe River Basin can be different by dozens or even hundreds of times.

(4) Stages and repetitions of major floods. According to a large number of flood investigation and research, it is concluded that the floods of major rivers in China have the characteristics of stages and repetitions in time and space. Ivy

From the time point of view, although the distribution of the time series of major floods in a river basin is uneven, from the long-term observation, in many rivers, the frequency of major floods in one period is high, while the incidence in another period is low, and the frequent and low-frequency periods change alternately in stages. In addition, big floods often appear in the upper reaches year after year, which is continuous.

Spatially speaking, the occurrence of rainstorm and flood in China is closely related to local weather and terrain speculation. Similar floods have occurred in recent river basins and areas, and the phenomenon of repeated rainstorms and floods is widespread. For example, the Yangtze River flood in 1998 was as fierce as the 1954 Yangtze River flood.