1. As a "light rain" in daily spoken language, it means "light rain". The general situation is that raindrops are clearly visible, there is no floating phenomenon, there is no splash on the ground, the water in the depression is slow, the rain in the house is weak, and only the eaves drip. You can walk a short distance in the rain without an umbrella, and you won't get all your clothes wet.

2. As a light rain described from the meteorological point of view, it is stipulated in China's meteorological work that the rainfall within 1 hour is less than or equal to 2.5 mm; Rain with 24-hour rainfall less than10mm. Light rain is the second lowest precipitation intensity grade except micro precipitation in the six-level classification of precipitation grades.

I. Research background:

1. Precipitation is a key physical process connecting weather, climate and water cycle, and it is also the ultimate source of clean water to support ecosystems and human survival. It is extremely important for meteorology and climatology to study the change and forecast of precipitation. In precipitation research, precipitation, precipitation intensity and precipitation frequency are the main research aspects.

2. In recent years, the precipitation in China has been characterized by floods in the south and droughts in the north, and the precipitation intensity has an increasing trend, but the annual precipitation frequency has a consistent decreasing trend, and droughts and floods have increased at the same time. On average, small-scale precipitation mainly depends on the change of rainfall days, and annual precipitation, seasonal precipitation and monthly precipitation all conform to this law. In previous studies, people paid more attention to the extremely strong precipitation that easily caused floods, but paid less attention to the light rain.

3. In most parts of China, especially in the north, the sum of light rain and moderate rain accounts for more than 60% of the annual precipitation. The significant reduction of micro-precipitation events has been reflected in the drought in the north, and the reduction of light rain frequency has become the main reason for the reduction of precipitation frequency.

Third, the formation of rain:

1. After the water on the earth is irradiated by sunlight, it turns into steam and is evaporated into the air. Water vapor condenses into small water droplets when it meets cold air at high altitude. These water droplets are very small, with a diameter of only 0.000 1 ~ 0.0002 mm, and the largest is only 0.002 mm. They are small and light, and are held up in the air by the updraft in the air.

It is these small water droplets that gather into clouds in the air. These small water droplets will become raindrops and fall to the ground, and the volume will increase by about 1 10,000 times. The volume growth of these water droplets to more than 6.5438+0.0000 times mainly depends on two means, one is condensation and sublimation. The second is to rely on the collision and increase of cloud droplets.