Snow is a form of precipitation. Meteorology, rain, snow, hail and other water vapor condensation falling from the sky to the ground are called "precipitation". Snow is precipitation composed of a large number of white opaque ice crystals (snow crystals) and their polymers (snow masses).
Water is the foundation of all kinds of life on the earth, and its changes and movements have created our world today. On the earth, water is constantly circulating. The water in the ocean and on the ground is heated and evaporated into the sky. The water vapor moves to other places with the wind. When they encounter cold air, they form precipitation and return to the earth's surface.
There are two kinds of precipitation: one is liquid precipitation, which is rain; The other is solid precipitation, which is snow or hail.
"Snow" is the product of cold water vapor, representing cold and precipitation. Snowfall mostly comes from nimbostratus and high-level clouds, and the precipitation intensity changes slowly; Cumulonimbus clouds snow in cold days and have the characteristics of showers, which are called snow showers.
Water vapor in the atmosphere condenses directly or water droplets condense directly, which is snow. It can also be said that it snows when the temperature in the cloud is too low and small water droplets form ice crystals and fall to the ground or snow. Ice absorbs heat when it melts, so the ground temperature will be lower than when it snows. Snowfall consists of a large number of snow crystals of different sizes, and generally there are many smaller ones.
In order to describe the particle size distribution characteristics of falling snow crystal population, snow crystal spectrum or solution spectrum after snow crystal melting is often used.
Snow crystals are mainly increased by condensation in clouds. First of all, ice crystals are produced by ice nucleation in cold clouds. They grow into snow crystals through condensation (ice crystal process), and then they can grow by colliding with supercooled water droplets. The shape of snow crystal will change when it meets a large number of supercooled water droplets. Snow crystals have various shapes, which are related to the temperature and humidity of their growing environment.
Snowfall, like all precipitation, is measured by the equivalent water layer thickness, and the unit is mm. In practice, it is sometimes measured by the accumulated depth of snowfall on the flat land, which is called snow depth.