Why do you see snowflakes falling at a few degrees below zero?

When the temperature is below zero, it may not only snow, but also rain sometimes, because if water vapor wants to crystallize, snowfall must have two conditions: one is that water vapor is saturated, and the other is that there must be condensation nuclei in the air.

(1) Reasons for water vapor saturation:

The maximum amount of water vapor that air can hold at a certain temperature is called saturated water vapor. The temperature at which air reaches saturation is called dew point. When saturated air is cooled to a temperature below the dew point, the excess water vapor in the air becomes water droplets or ice crystals.

Because the saturated water vapor content of ice surface is lower than that of water surface, the water vapor saturation required for the growth of ice crystals is lower than that of water droplets. That is to say, water droplets can only grow when the relative humidity is not lower than 100%; As for ice crystals, they tend to grow when the relative humidity is less than 100%.

For example, when the temperature is -20℃ and the relative humidity is only 80%, ice crystals can grow. The lower the temperature, the less humidity is needed for the growth of ice crystals. Therefore, in the high-altitude and low-temperature environment, ice crystals are more likely to be produced than water droplets.

(2) condensation nuclei:

Ice crystals are the necessary medium for the formation of snowflakes. It is a substance in the clouds in the sky. It takes some dust as the center and forms an ice-like substance with water vapor at low temperature. As the ice crystals grow, the water vapor near the ice crystals will be consumed. Therefore, the closer to the ice crystal, the thinner the water vapor and the lower the supersaturation. This will form ice flowers, which will turn into snowflakes when they fall to the ground.

Extended data:

The necessary conditions for snow are:

(1) Adequate water: As the saying goes, "A clever woman can't cook without rice". The essence of rain and snow is water, so if it rains and snows, water vapor is essential and needs to be enough.

(2) Updraft: Water vapor alone is not enough. It needs the help of updraft to rise to high altitude, and condenses into small water droplets or small ice crystals to form clouds with the decrease of temperature. When water droplets or small ice crystals are large enough, they will fall from the air and form rain and snow.

(3) Sufficient condensation nuclei: The condensation of water vapor in the air does not happen out of thin air, but needs the help of condensation nuclei in the air, which can be dust, particles or inorganic salts. Water vapor will condense into small water droplets or small ice crystals on the surface of condensation nuclei, and will continue to grow until it exceeds the gravity limit and the supporting force of updraft falls to the ground (the falling process may continue to grow).

(4) The temperature near the ground is low enough so that snowflakes or ice crystals will not melt into rain when they fall to the ground. Generally speaking, it is possible to snow when the near-surface temperature is below 4℃, and it is basically certain to snow below 0℃.