Why does it snow?

There are two conditions for the formation of snowfall:

One condition is that water vapor is saturated. 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 grow only when the relative humidity (relative humidity refers to the ratio of the actual vapor pressure in air to the saturated vapor pressure of air at the same temperature) is not less 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.

Another condition is that there must be condensation nuclei in the air. If there were no condensation nuclei, it would be difficult for us to see rain and snow on the earth. Condensation nuclei are tiny solid particles suspended in the air. The ideal condensation nuclei are those particles that absorb the most water. Such as sea salt, sulfuric acid, nitrogen and other chemicals.

When these two conditions are met, the water vapor in the air movement can form snowfall the next day.

Extended data:

Maintenance items in snowy days:

One of the precautions: improper way to keep out the cold.

Improper way to keep out the cold. Young people who like fashion must pay attention to increasing or decreasing clothes in time when entering and leaving the environment with large temperature difference between indoor and outdoor. On snowy days, in order to pursue fashion, you can't show your legs or wear thin clothes. At the same time, pay attention to a light diet and don't eat too much cold food at a time to prevent acute gastroenteritis.

Note 2: Prevent respiratory diseases.

Prevent respiratory infection. Winter and spring are the high incidence seasons of respiratory diseases. Children, the elderly and patients with chronic diseases with relatively low resistance should appropriately reduce outdoor activities, pay attention to cold and warmth, and maintain regular ventilation indoors.

Note 3: Prevent stomach bleeding and digestive tract ulcer.

Prevent gastric bleeding and digestive tract ulcer. Cold can easily lead to stomach bleeding and digestive tract ulcers, so pay attention to the warmth of the stomach and diet conditioning. The daily diet should be light, light and easy to digest, so as to eat less and eat more, eat regularly, avoid cold diet, quit smoking and drinking, and also choose some Chinese patent medicines that warm the stomach and strengthen the spleen.

Note 4: Prevent gas poisoning.

Prevent gas poisoning. In the cold winter, when using gas and coal stoves for heating, families must pay attention to maintaining ventilation. Pay attention to the safety of heating equipment, remember to exhaust and ventilate frequently, and beware of waste gas accumulation.

Note 5: Prevent accidental falling.

Prevent accidental falls. Rain and snow make the road slippery. Therefore, the general public is specially reminded to pay attention to travel safety and prevent accidental falls.

Note 6: Prevent respiratory diseases.

When the weather gets cold, the first thing to test is the respiratory system. Usually do not pay attention to exercise, coupled with the closed indoor air does not often circulate with the outside world, it is likely to cause upper respiratory tract infection.

Baidu Encyclopedia-Snow (Natural Phenomenon)

Baidu encyclopedia-snow