When does it snow, when does it hail and why?

The rain was changed by clouds. The volume of raindrops is 6.5438+0 million times that of cloud drops. In other words, it takes 6.5438+0 million cloud drops to form a raindrop. In humid air, cloud droplets condense due to cooling. In warm clouds with cloud body temperature higher than 0℃, there are cloud drops of different sizes in the clouds, and the big cloud drops fall fast and rise slowly; Xiaoyun water drops slowly and rises quickly. As a result, because of the different relative velocities of the big and small cloud droplets, the big cloud droplets have a chance to collide with the small cloud droplets, and as a result, the small cloud droplets merge into the big cloud droplets. In this way, the number of large cloud droplets is increasing, and due to the uneven distribution of updraft, large cloud droplets can move up and down many times in the cloud. Coupled with the turbulence in the cloud, the probability of large cloud droplets increasing increases, so the large cloud droplets become larger and larger until the updraft can't hold down and it rains.

There is a more professional opinion, which I think is more reasonable:

When you fly at an altitude of 65438+ 100000 meters and see a few fog barriers and still higher light clouds, you often have such questions. Why are most cloud particles below the sea surface of the sea of clouds? What's so special about these high clouds that they can float higher than other clouds? ? In fact, there are still very thin water molecules at an altitude of 20 kilometers. As mentioned above, the water molecules at this height are not directly transpiration from the ground, but are reduced by negative hydroxyl ions after "secondary evaporation". Because hydroxyl (oh? The molecular weight of-) is 17, which is lower than steam 1, so it floats higher than steam. When they become water (h? 2O), in the air temperature environment of -45℃, it immediately condenses into solid graupel particles, the diameter of which is below 1 micron, reflecting sunlight like a fog barrier, and it is like a light cloud when it is particularly dense.

Because a large number of graupel particles fall into the sea of clouds, the water mist in the clouds gathers on the graupel particles and freezes into larger graupel particles. When the diameter reaches about 1 mm, the original graupel particles melt into water and fall to the ground in the form of raindrops. In winter, the original graupel particles did not melt, forming snowflakes or large graupel particles falling to the ground, which is the reason for the rain and snow. ? On a clear day, when the high-altitude graupole passes through the cloudless clouds in Wan Li, it melts in the air and becomes mist, or falls to the ground and becomes dew and frost, or is evaporated again by the sunshine and wind the next day on the way down. These high-altitude graupel particles are too small, easy to melt and difficult to "catch" on the spot, so their existence and function are often ignored by meteorologists.

When modern meteorology talks about the causes of rain and snow, it is said that warm and humid air meets cold air mass, or that hot and humid air cools and condenses after rising. The question is, where do these cold air masses come from in the summer and autumn rainy season? Is it raining from the South Arctic Circle? Because hot and humid air brings water vapor and heat energy from the ground to the upper air, the upper air should be hotter. Why does it cool down and condense into rain and snow? This rain and snow theory can't be established until the reason for the low temperature at the top of the troposphere is first found out.

As mentioned above, secondary evaporation is the main cause of high altitude cold. A large number of polonium particles fall into the sea of clouds to absorb heat and melt, which will make the sea of clouds "worse". When cloud vapor condenses into raindrops and snow particles in this cold condition, the specific gravity increases and the buoyancy disappears, and of course it will fall downward to form rain and snow. The current expressions such as "convective rain", "topographic precipitation", "frontal rain", "typhoon rain" and "artificial rainfall" only explain the phenomena accompanying the rainfall process, but do not explain the reasons for the rainfall.

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As we all know, clouds are composed of many small water droplets and small ice crystals, and raindrops and snowflakes are composed of these small water droplets and small ice crystals. So, how is snow formed?

In the water cloud, all water droplets are small water droplets. They grow into raindrops mainly through continuous condensation and collision.

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.

In early spring and late autumn, the air near the ground is above 0℃, but this layer of air is not thick and the temperature is not very high, which will make the snow fall to the ground and melt completely. This is the so-called "wet snow", or "both rain and snow". This phenomenon is called "sleet" in meteorology

Similarly, the size of snow is also classified according to precipitation. Snow can be divided into light snow, medium snow and heavy snow, as shown in Table 3.

Table 3. Precipitation standard of all kinds of snow

kind

Minor Snow

moderate snow

Major Snow

24-hour precipitation

Below 2.5

2.6-5.0

Greater than 5.0

12 hour precipitation

Below 1.0

1. 1-3.0

Greater than 3.0

Formation and types of snow

Author: Dashan article Source: Number of clicks collected online: 97 Updated: 2005- 1- 16

As we all know, clouds are composed of many small water droplets and small ice crystals, and raindrops and snowflakes are composed of these small water droplets and small ice crystals. So, how is snow formed?

In the water cloud, all water droplets are small water droplets. They grow into raindrops mainly through continuous condensation and collision.

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 in the process of falling 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.

In early spring and late autumn, the air near the ground is above 0℃, but this layer of air is not thick and the temperature is not very high, which will make the snow fall to the ground and melt completely. This is the so-called "wet snow", or "both rain and snow". This phenomenon is called "sleet" in meteorology

Similarly, the size of snow is also classified according to precipitation. Snow can be divided into light snow, medium snow and heavy snow, as shown in Table 3.

Table 3. Precipitation standard of all kinds of snow

Types of light snow, heavy snow and heavy snow

24-hour precipitation is below 2.5, and 2.6-5.0 is greater than 5.0.

12 hour precipitation 1.0 or less 1. 1-3.0 is greater than 3.0.

The shape of snowflakes

Snowflakes have many shapes and are very beautiful. If you put them under a magnifying glass, you will find that every snowflake flower is an extremely beautiful pattern, and even many artists are amazed. But how are all kinds of snowflake shapes formed? Snowflakes are mostly hexagonal, because snowflakes belong to hexagonal system. There are two main shapes of small ice crystals in the "embryo" of snowflakes in the cloud. One is hexagonal, slender and called columnar crystal, but sometimes its two ends are pointed and look like needles, called acicular crystal. The other is hexagonal flake, just like a flake cut from a hexagonal pencil, called flake crystal.

If the supersaturation of the surrounding air is low, the ice crystals will grow slowly and evenly on all sides. When it increases and decreases, it still keeps its original appearance, which is called columnar, acicular and flaky snow crystals respectively.

If the surrounding air is highly supersaturated, ice crystals will not only increase in size, but also change in shape. The most common is to change from flake to star.

It turns out that while 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. Close to the surface of the ice crystal, because the excess water vapor has condensed on the ice crystal and just reached saturation. In this way, the density of water vapor near the ice crystals is smaller than that far away from the ice crystals. Water vapor moves from around the ice crystals to where the ice crystals are. Water vapor molecules first meet the corners and protrusions of ice crystals, where they condense and make ice crystals grow. Therefore, the corners and protruding parts of ice crystals will grow rapidly first and gradually branch. Later, for the same reason, new branchlets will grow in every branch and corner. At the same time, in the depression between each corner and fork. The air is no longer saturated. Sometimes, there is even a sublimation process here, which allows water vapor to be transported to other places. This makes the bifurcation of edges and corners more prominent, and gradually forms the familiar star snowflake.

What is mentioned above is actually a typical star-shaped snowflake formation process. Its equivalent parts, regardless of shape or size, should be the same. This typical star snowflake can only be formed in an ideal and calm environment (such as in a laboratory). In the atmosphere, it can't be increased step by step as mentioned above, and the formed shape can't be so typical. This is because ice crystals are gradually falling and sometimes rotating, and the amount of water vapor contacted by each branch is different, while those branches that are exposed to more water vapor are much longer. So the snowflakes we usually see are generally the same, but they are different.

In addition, in the process of falling in the cloud, snowflakes will also fall from an environment suitable for forming one shape to an environment suitable for forming another shape, so you will see various complex snowflake shapes. Some are like cufflinks, others are like thorns. Even if they are all star-shaped snowflakes, there are three, six or even twelve or eighteen.

All of the above are the cases of a single snowflake. When snowflakes fall, each snowflake easily sticks to each other and combines together to become a bigger snowflake. The merging of snowflakes is mainly observed in the following three situations. (1) When the temperature is lower than 0℃, snowflakes collide on the way down slowly. The collision produces pressure and heat, which makes the collision parts melt and stick to each other, and then the melted water immediately freezes again. In this way, two snowflakes merged together. (2) When the temperature is slightly higher than 0℃, the snowflake is covered with a water film. At this time, if two snowflakes collide, they will stick together by the surface tension of water. (3) If the branches of snowflakes are complicated, two snowflakes can be hung together simply by climbing.

Snowflakes fall from the clouds to the ground, and the journey is very long. When the conditions are right, you can climb and merge many times and become very big. When it snows heavily, sometimes some goose feather-like snowflakes appear, which are formed by multiple fusion.

But sometimes snowflakes collide with each other, not combined with each other, but broken, which in turn produces some deformed snowflakes. For example, when it snows, sometimes you will see some single "stars", which is the case.

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Cloud is the foundation of precipitation and the middle link of the earth's water cycle. The occurrence and development of cloud is always accompanied by energy exchange. The shapes of clouds are ever-changing, and specific clouds often appear in specific weather, so clouds have certain indicative significance for weather changes.

(A) the formation conditions and classification of clouds

In the atmosphere, the important conditions of condensation are the existence of condensation nuclei and the supersaturation of air. For the formation of clouds, its supersaturation is mainly caused by adiabatic cooling caused by vertical rise of air. The form and scale of ascending motion are different, and the state, height and thickness of the formed cloud are also different. There are four main ways for the upward movement of the atmosphere:

1. Thermal convection

Refers to the convective upward movement caused by uneven ground heating and unstable atmospheric stratification. Clouds formed by convection are mostly cumulus clouds.

2. Dynamic uplift

Refers to the large-scale upward movement of warm and humid airflow caused by the action of front and convergent airflow. The clouds formed by this movement are mainly layered clouds.

3. Atmospheric fluctuations

Refers to the fluctuation caused by the atmosphere flowing through the uneven ground or below the inversion layer. Clouds produced by atmospheric fluctuations mainly belong to fluctuation clouds.

4. Topographic uplift

Refers to the upward movement caused by the forced upward movement of the atmosphere when it is blocked by the terrain. Clouds formed by this movement include cumulus clouds, wavy clouds and layered clouds, which are usually called topographic clouds.

Although the shapes of clouds vary widely, there are always certain rules for their formation. According to the formation height and shape of clouds, China classifies clouds into 4 families 10 genera. 1972 The Cloud Map of China published by China divides clouds into three families, 1 1 genus (see Chapter 5 of Meteorology and Climatology Practice for table 3.3).

(B) the formation of various clouds

1. Cumulus formation

Cumulonimbus clouds are vertically developing clouds, mainly including light cumulonimbus clouds, thick cumulonimbus clouds and cumulonimbus clouds. Cumulus clouds are mostly formed in the summer afternoon, with isolated and scattered appearance, flat cloud bottom and raised top.

The formation of cumulus clouds is always related to upward convection in unstable atmosphere. Whether cumulus clouds can be formed by convection depends not only on condensation conditions, but also on the height of convection energy. If the maximum height of convection (upper limit of convection) is higher than the condensation height, cumulus clouds will be formed, otherwise cumulus clouds will not be formed. The stronger the convection, the greater the difference between the upper convection limit and the condensation height, and the greater the thickness of cumulus. The horizontal range of convection rising area is wide, and the horizontal range of cumulus is large.

Light cumulonimbus, dense cumulonimbus and cumulonimbus are different stages of cumulonimbus development. Cumulus generated by thermal convection in air mass is the most typical. In summer, the ground is strongly radiated by the sun, and the ground temperature is very high, which further heats the near-surface gas layer. Due to the inhomogeneity of the earth's surface, the air in some places is seriously heated, and the air in some places is humid, so gas blocks (hot bubbles) slightly different from the surrounding temperature, humidity and density are produced in the atmosphere near the ground. The internal temperature of these air blocks is relatively high, and they float with the wind due to the buoyancy of the surrounding air, constantly living and disappearing. The larger air mass rises at a higher height, forms a convective monomer when it reaches the condensation height, and then gradually develops into an isolated, dispersed cumulus with a flat bottom and a convex top. Because the air movement is continuous and compensates each other, the air in the rising part condenses into clouds due to cooling, and the air around the clouds is supplemented by sinking, so the sinking air heats up adiabatic quickly and does not form clouds. As a result, the accumulated clouds dispersed and the blue sky was exposed among the clouds. For a certain area, at the same time, the horizontal distribution of air temperature and humidity is almost the same, and its condensation height is basically the same, so the bottom of cumulus is flat.

If the upper limit of convection is slightly higher than the condensation height, generally only light cumulus clouds are formed. Because the cloud top is generally below the 0℃ isotherm height, and the cloud body is composed of water droplets, the velocity of the ascending airflow in the cloud is not large, generally less than 5m/s, and the turbulence in the cloud is also weak. At the height where light cumulus clouds appear, if there is strong wind and strong turbulence, the clouds of light cumulus clouds will become broken, which is called broken cumulus clouds.

When the upper limit of convection is much higher than the condensation height, the cloud body is tall and the top is cauliflower-like, forming cumulus clouds. Its cloud top extends to a height below 0℃, and the top is composed of supercooled water droplets. The updraft in the cloud is strong, reaching 15-20m/s, and the turbulence in the cloud is also strong.

If the updraft is strong, the cloud top of cumulus can extend upward, and the cloud top can extend to the upper air below-15℃. Therefore, the cloud top freezes into ice crystals, and a filamentous structure appears, forming cumulonimbus clouds. The top of cumulonimbus cloud, blown by high-altitude wind, spreads horizontally into an anvil, which is called anvil cloud. In the direction of high-altitude wind, the cloud anvil can extend very far, so its extension direction can be used as a judgment of the moving direction of cumulonimbus clouds. The thickness of cumulonimbus clouds is very large, which can reach 5000 ~ 8000 meters in mid-latitude areas and 10000 meters in low-latitude areas. The descending airflow rises rapidly in the cloud, and the ascending airflow can often reach 20-30m/s, and the rising speed of 60m/s has been observed, and the sinking speed is also10-15m/s. The turbulence in the cloud is very strong.

Cumulus formed by thermal convection has obvious diurnal variation. It is usually light cumulus clouds in the morning. With the strengthening of convection, it gradually developed into cumulus clouds. Convection is strongest in the afternoon and can often develop into cumulonimbus clouds. At night, convection weakens, cumulonimbus gradually dissipates, and sometimes it can evolve into false clouds, cumulonimbus clouds and cumulonimbus stratocumulus. If there are only faint cumulus clouds in the afternoon, it means that the air is relatively stable, cumulus clouds can no longer develop and grow up, and the weather is better, so the faint cumulus clouds are also called clear cumulus clouds, which is a sign of continuous sunny days. In summer, if cumulonimbus clouds appear in the early morning, it means that the air is unstable and may develop into cumulonimbus clouds. Therefore, the appearance of cumulus clouds in the morning is a sign of thunderstorm. At night, the stratocumulus evolves after the cumulus dissipates, indicating that the air stratification is stable and the clouds will disperse at night, which is a sign of fine weather. It can be seen that the diurnal variation characteristics of cumulus formed by thermal convection are helpful to directly judge short-term weather changes.

2. Formation of stratiform clouds

Stratiform clouds are uniform curtain clouds, often with a large horizontal range, including cirrostratus, Cirrus, Stratosphere and nimbostratus.

Stratiform clouds are caused by the ascending motion of large-scale air systems, mainly by the ascending motion of fronts. This systematic upward movement usually has a large horizontal amplitude, and the upward speed is only 0.1-1m/s. Because of its long duration, the air can rise for several kilometers. For example, when warm air moves to the cold air side, due to the different densities, the stable warm and humid air slowly slides upward along the cold air slope and is adiabatic cooled to form a layered cloud. The bottom of the cloud is roughly the same as the inclined plane (also called front) where cold and warm air meet, and the top of the cloud is approximately horizontal. The thickness of the cloud varies greatly in different parts of the slope. In front are cirrus clouds and cirrostratus, which are the thinnest, usually between a few hundred meters and 2000 meters. The clouds are composed of ice crystals. Located in the middle is a high-level cloud, the thickness is generally 1000-3000 m, the top is mostly composed of ice crystals, and the main part is mostly composed of ice crystals and supercooled water droplets. Finally, the rain layer stone, generally 3000-6000 m thick. Its top is composed of ice crystals, the middle is composed of supercooled water droplets and ice crystals, and the bottom is composed of water droplets because the temperature is higher than 0℃.

It can be seen from the formation of layered clouds in the above system that some clouds can be used as omens before the arrival of precipitation. Cirrostratus, for example, usually appears in front of the layered cloud system, and its appearance is often accompanied by solar and lunar halos. So when you see a halo in the sky, you know that cirrostratus has moved, so will nimbostratus, and it may turn rainy. The agricultural proverb "the sun is dizzy in the middle of the night, the rain is dizzy at noon, and the wind is dizzy at noon" refers to this sign.

3. The formation of wave clouds

Wave clouds are wave cloud, including cirrocumulus, altocumulus and stratocumulus. The rising speed in the cloud can reach tens of centimeters per second, which is second only to the rising speed in the accumulation cloud.

When the air fluctuates, it rises at the peak and sinks at the trough.

Due to adiabatic cooling, clouds will form where the air rises, but not where the air sinks. If there is a layered cloud with uniform thickness before the wave is formed, the cloud will thicken at the peak, thin or even disappear at the trough, thus forming a parallel cloud with a small thickness and a certain distance, showing rows or rows of wave clouds.

It is generally believed that there are two main reasons for fluctuation: first, there are interfaces with different air density and airflow speed in the atmosphere, which causes fluctuation. The second is the fluctuation caused by the airflow passing through the mountain (called topographic wave or lee wave). When fluctuations appear on the interface with low wind speed and high density in the upper layer and low wind speed and high density in the lower layer, because the wind direction and wind speed at each height often change, the direction of fluctuations changes with time, and the newly generated fluctuations are superimposed on the original fluctuations, thus forming a checkerboard cloud block. When the fluctuating gas layer is very high, cirrocumulus is formed, when it is high, it forms high cumulus clouds, and when it is low, it forms stratocumulus clouds.

The thickness of wave clouds is not large, usually tens to hundreds of meters, sometimes reaching 1000-2000m. When it appears, it often means that the gas layer is relatively stable and the weather changes little. Proverbs such as "Wayun will kill people" and "There are carp spots in the sky, so you don't need to turn over the grain tomorrow" mean that after the appearance of altocumulus or stratocumulus, the weather will be fine and there will be little change. However, systematic wave clouds, such as cirrocumulus, evolved after fluctuating on cirrus clouds or cirrostratus, so they are connected with large stratiform clouds, which indicates that there will be wind and rain coming. "Fish-scale sky, no rain but wind" refers to this omen.

4. Formation of special clouds

In addition to the formation of the above clouds, there are some special clouds, such as fort-shaped, flocculent, suspended spherical, pod-shaped, etc., which can often predict the changing trend of the weather. Therefore, understanding their causes and characteristics is helpful to use them to judge the future weather.

(1) Suspended spherical cloud: refers to the cloud mass hanging at the bottom of the cloud, which mostly appears at the bottom of cumulonimbus clouds. Sometimes it can be seen at the bottom of nimbostratus.

When there are a lot of water droplets in the cloud, if there is a strong updraft near the cloud bottom, the falling water droplets will be lifted up to form a cloud mass that seems to be suspended at the cloud bottom, which is a suspended spherical cloud.

The appearance of suspended spherical clouds usually indicates precipitation, because once the updraft weakens, the water droplets that were originally held will fall down and form precipitation.

(2) Fort Cloud and Flocculent Cloud: The bottom of the Fort Cloud is horizontal, and the top is a small cloud tower with bumps side by side, which looks like a castle in the distance. The formation of such clouds is often developed on the basis of wave clouds. When the wave cloud is formed under the inversion layer, if the inversion layer is not too thick, when the turbulence under the inversion layer develops, there will be a strong updraft passing through the inversion layer to condense the water vapor and form a cloud with an arc top, which is the castle cloud. Common fort clouds are fort-shaped altocumulus and fort-shaped stratocumulus.

Flocculent clouds are broken and shaped like cotton balls. They are usually formed by strong turbulent mixing in the humid air layer, mainly flocculent cumulus clouds.

In summer, if there are fort-shaped or flocculent cumulus clouds in the morning, it means that the upper air layer is unstable at this height. At noon, as soon as the low-level convection develops, the upper and lower unstable air layers will combine to produce a strong updraft, forming cumulonimbus clouds, thunderstorms or hail. The weakening of convection in the evening, such as the appearance of fort cumulus, indicates that unstable systems will approach at high altitude, and systematic thunderstorms may occur the next day.

(3) Lenticular clouds: Lenticular clouds are thick in the middle and thin in the edge, and the clouds are pod-shaped. Common lenticular clouds are mainly podiform cumulus and lentiform stratocumulus.

Lenticular clouds are formed by the convergence of local updraft and downdraft. When the updraft cools the air adiabatically to form a cloud, if it is blocked by the downdraft, its edge will gradually become thinner due to the downdraft, thus forming a lenticular cloud. In mountainous areas, the airflow will also form lenticular clouds under the influence of topography.

The physical processes of cumulus clouds, stratiform clouds, wavy clouds and some special clouds are introduced above. But they are not isolated and unchangeable. Due to the change of conditions, they can develop, dissipate, and transform from one cloud to another. For example, in cumulus clouds, light cumulus clouds can develop into thick cumulus clouds and finally form cumulonimbus clouds. Cumulonimbus clouds can evolve into pseudocumulus clouds, cumulonimbus clouds and cumulonimbus stratocumulus clouds after dissipation. For another example, when wave clouds develop, they can evolve into stratocumulus (shadow altocumulus can evolve into stratocumulus and shadow stratocumulus can evolve into nimbostratus). When the stratiform cloud dissipates, it will also evolve into a wave cloud (after the nimbostratus dissipates, it can evolve into a stratocumulus, a altocumulus or a stratocumulus). In short, the emergence, development and evolution of clouds are complex and regular.

References:

Zhou Shuzhen's Meteorology and Climatology (3rd Edition).

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Fog is a water vapor condensate composed of small water droplets or ice crystals floating in the air, which is produced in the near-surface layer of the atmosphere. Since fog is water vapor condensation, we should look for its reason from the conditions that cause water vapor condensation. There are two reasons for water vapor saturation in the atmosphere: first, evaporation increases water vapor in the atmosphere; The other is the cooling of the air itself. Cooling is more important for fog. When there are condensation nuclei in the air, if water vapor continues to increase or melt, condensation will occur in saturated air. Fog is formed when condensed water drops reduce the horizontal visibility to less than 1 km.

In addition, excessive wind speed and strong disturbance are not conducive to the formation of fog.

Therefore, in the area that is conducive to the cooling of the lower air layer, if the water vapor is sufficient, the wind is mild, the atmospheric stratification is stable, and there are a large number of condensation nuclei, it is most likely to produce fog. Generally speaking, there are more opportunities to form fog in industrial areas and urban centers, because there are abundant condensation nuclei there.

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Dew is water droplets condensed when water vapor meets cold objects. In order to explain the cause of dew, we can simulate the conditions of dew formation in nature, increase the humidity of air through water evaporation, and reduce the temperature of objects through ice, so that dew will appear on cold objects. Through this experiment, students can not only understand the reason of condensation, but also learn the design method of simulation experiment.

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In the early morning of cold season, grass leaves and clods are often covered with a layer of frost crystals. They shine in the rising sun and melt when the sun rises. People usually call this phenomenon "frosting". Looking through the calendar, every year from 10, there is always the solar term "first frost". We have seen snow and rain, but no one has seen frost. In fact, frost does not fall from the sky, but is formed in the air near the ground.

Frost is a kind of white ice crystal, which mostly forms at night. In a few cases, it can also begin to form before sunset when the sun is tilted. Usually, the frost will melt soon after sunrise. But when the weather is cold or in a cool place, the frost will last all day.

Frost itself is neither harmful nor harmful to plants. What people usually call "freezing injury" is actually the "freezing injury" when frost forms.

The formation of frost is not only related to the weather conditions at that time, but also related to the nature of attached objects. When the surface temperature of the object is very low, but the air temperature near the surface of the object is relatively high, there is a temperature difference between the air and the surface of the object. If the temperature difference between the surface of an object and the air is mainly caused by the radiation cooling of the surface of the object, when the warmer air contacts the surface of the object, the air will be cooled, and when the water vapor is supersaturated, the excess water vapor will be precipitated. If the temperature is lower than 0℃, the excess water vapor condenses into ice crystals on the surface of the object, which is frost. Therefore, frost is always formed under weather conditions that are conducive to radiation cooling on the surface of objects.

In addition, clouds hinder the radiation cooling of ground objects at night, and clouds in the sky are not conducive to the formation of frost. Therefore, frost often appears on clear nights, that is, when the ground radiation cools down strongly.

In addition, the wind also has an influence on the formation of frost. When there is a breeze, air flows slowly over the surface of supercooled objects, constantly supplying water vapor, which is beneficial to the formation of frost. But when the wind is strong, because the air flows quickly, the time to contact the surface of cold objects is too short. At the same time, when the wind is strong, the air in the upper and lower layers is easy to mix with each other, which is not conducive to cooling down and will also hinder the formation of frost. Generally speaking, when the wind speed reaches level 3 or above, it is not easy to frost.

Therefore, frost usually forms in sunny, breezy or calm nights in cold seasons.

The formation of frost is not only related to the above weather conditions, but also related to the properties of ground objects. Frost is formed on the surface of an object cooled by radiation, so the easier it is to radiate heat and the faster it cools, the easier it is to frost on it. Similar objects, under the same conditions, have the same mass and heat. If they radiate and dissipate heat at the same time at night, objects with large surface area will dissipate more heat, and at the same time, the temperature will drop faster, and it will be easier to frost on them. That is to say, if the surface area of an object is relatively large relative to its mass, it is easy to form frost on it. Grass leaves are very light, but they have a large surface area and are easy to frost on. In addition, the rough surface of an object is more conducive to heat dissipation than the smooth surface, so the rough surface is more prone to frost, such as clods.

Frost disappears in two ways: one is to sublimate into steam, and the other is to melt into water. The most common thing is that it melts and disappears after sunrise due to the rising temperature. Water melted by frost is good for crops.

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Hail falls from the clouds like rain and snow. But the clouds that hail are very strong cumulonimbus clouds, and only very strong cumulonimbus clouds can hail.