What shape clouds appear in the sky, and it will snow heavily on the ground?

Generally speaking, thin clouds in the sky symbolize sunny weather, while low and thick clouds indicate rainy and snowy weather.

The formation of snow.

In the mixed cloud, due to the existence of ice water, ice crystals continue to condense and grow into snowflakes. When the temperature under the cloud is below 0℃, snowflakes can always fall to the ground to form snowfall. If the temperature under the cloud is higher than 0℃, sleet may appear. Snowflakes have many shapes, such as star, cylinder and flake, but the basic shape is hexagon.

Snowflakes are mostly hexagonal in shape and varied in patterns because the molecules of ice are mostly hexagonal. Hexagonal flake ice crystals have different saturated vapor pressures because of their different curvatures on the surface, edges and corners, in which the saturated vapor pressure is the largest at the corner and the smallest at the plane.

Under the same actual water vapor pressure, the sublimation growth of ice crystals is different because of the different saturated water vapor pressures of each part. For example, when the actual vapor pressure is only greater than the plane saturated vapor pressure, the vapor only condenses on the surface, forming columnar snowflakes. When the actual vapor pressure is greater than the saturated vapor pressure of the edge, condensation will occur on the edge and surface.

Because the speed of sublimation is also related to curvature, where the curvature is large, sublimation is faster, so the sublimation of ice crystal edge is faster than that of surface, forming more flake snowflakes. When the actual water vapor pressure is greater than the saturated water vapor pressure on the corner, although there is water vapor condensation on the surface, edge and corner, the sharp corner is prominent, the water vapor supply is the most abundant, and the condensation grows fastest, so many branches or star-shaped snowflakes are formed.

Coupled with the constant movement of ice crystals, their temperature and humidity conditions are constantly changing, which makes the growth rates of various parts of ice crystals inconsistent and forms various snowflakes.

principle

Ice cloud is made up of tiny ice crystals. When these small ice crystals collide with each other, the surface of the ice crystals will heat up and melt, and they will stick together and freeze again. Repeat this for many times, and ice crystals will increase. In addition, there is water vapor in the cloud, so ice crystals can continue to grow through condensation.

However, where the ice cloud is generally high but not thick, and there is not much water vapor, the condensation growth is slow, and there are not many opportunities to collide with each other, so it cannot grow to a great extent to form precipitation. Even if it causes precipitation, it often evaporates on the way down and rarely falls to the ground.

The most favorable cloud droplet growth is the mixed cloud. The mixed cloud consists of small ice crystals and supercooled water droplets. When a mass of air is saturated with ice crystals, it is unsaturated with water droplets. At this time, the water vapor in the cloud condenses on the surface of ice crystals, while the supercooled water droplets are evaporating, which produces the phenomenon that ice crystals "adsorb" water vapor from supercooled water droplets.

In this case, ice crystals will grow rapidly. Besides, supercooled water is very unstable. If you touch it, it will freeze. Therefore, when supercooled water droplets collide with ice crystals in mixed clouds, they will freeze and adhere to the surface of ice crystals, making them grow rapidly. When small ice crystals grow up, they overcome the resistance and buoyancy of air and fall to the ground. This is snow.

When the air near the ground is above 0℃, but this layer of air is not thick and the temperature is not very high, the snow falls to the ground before it can completely melt. This is the so-called "wet snow" or "both rain and snow". This phenomenon is called sleet in meteorology.

situation

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 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.

There are condensation nuclei in the air.

Some people have done experiments, if there is no condensation nucleus, the water vapor in the air will only condense into water droplets if it is supersaturated to a relative humidity of more than 500%. But such a large supersaturation phenomenon will not exist in the natural atmosphere. So without condensation nuclei, it is 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. So we sometimes see clouds in the sky, but there is no snow. In this case, people often use artificial snowfall.

Extended data:

In meteorology, clouds can be divided into three families, ten genera and twenty-nine categories.

Among them, the three families refer to low clouds, medium clouds and high clouds. The low clouds are 600-2000m away from the ground, the medium clouds are 2000m-4500m away from the ground and the high clouds are 4500m- 10000m away from the ground. Generally speaking, thin clouds in the sky symbolize sunny weather, while low and thick clouds indicate rainy and snowy weather.

From the appearance, there are four kinds of clouds: accumulation, layer, roll and rain. "Accumulation" refers to clouds piled up like cotton; The "layer" is like a layer of gray sand covering the sky; "Curly" means like a curly cloud; "Rain" refers to clouds that bring rain.

In fact, ten kinds of clouds are all beautiful clouds.

For example, cumulus clouds are "cotton candy" formed on sunny days, which looks like broccoli, and the clouds are bright white.

Stratocumulus is a low fluffy cloud. The color of stratocumulus varies from bright white to blue-gray, and it will appear a little reddish under the sunlight.

Altocumulus clouds are like layers of bread rolls and bread sticks in the sky. They are usually composed of many small water droplets and may also contain ice crystals.

The altocumulus cloud is the kind of cloud we see in Yongchun today, just like the bread rolls and bread sticks in the middle layer of the sky, which are usually composed of many small water droplets and may also contain ice crystals. It is worth mentioning that this "cumulus cloud" has been misunderstood as "earthquake cloud" since ancient times.

References:

Baidu encyclopedia-snow