What are the white particles floating in the air before or when it snows?

The water vapor in the sky condenses into small ice particles when it meets cold air, which mostly appears before or when it snows. Graupole, also known as snowball or soft hail, is a kind of solid precipitation composed of white opaque ice particles, which has a snow-like structure and a diameter of 2-5 mm. It often bounces back when it hits hard ground and is fragile.

Speaking of graupel, many people think it is hail. The main difference between graupel and hail is that graupel is loose and hail is hard. Hail often appears in summer and autumn with strong convective activities, while graupel often appears before or falls with snow. Hail is a short-term strong convection phenomenon, and graupel is a stable "solid precipitation". Visually, hail is translucent, while graupel is generally opaque.

Extended data:

Generation mechanism:

It is difficult to observe graupel with an optical microscope, because it is difficult to separate extremely cold droplets on the surface of prismatic crystals, and it is also difficult to record the structure of graupel particles, and because of the limitation of the resolution and apparent distance of the instrument. However, it can be clearly seen by low temperature scanning electron microscope (LT-SEM) that cloud droplets can adhere to the crystal surface, and the size can reach 50 microns.

The structures of four basic snow crystals forming frost have been observed, including plane ice crystals, dendritic ice crystals, columnar ice crystals and acicular ice crystals. As the process of Song Dynasty continued, the accumulated cloud droplets blurred the original outline of snow crystals and finally became graupel particles.

The structure of graupel is denser than ordinary snow and particles, which is caused by frost. The weight and low viscosity of the composition make the surface layer unstable on the slope, and the layer of 20 to 30 cm still has the danger of avalanche. Because of the temperature and the characteristics of graupel, graupel became more compact and stable one or two days after the avalanche.

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