Write a popular science essay on the causes of rain, snow, clouds, fog, dew, frost, and hail and their relationship with human life.

Rain comes from clouds. The volume of raindrops is 1 million times the volume of cloud droplets. In other words, it takes 1 million cloud droplets to form one raindrop. In moist air, cloud droplets condense due to cooling. For warm clouds with a cloud body temperature higher than 0°C, there are cloud droplets of different sizes in the cloud. Large cloud droplets fall quickly but rise slowly; small cloud droplets fall slowly but rise quickly. Therefore, due to the difference in the relative speed of large and small cloud droplets, large cloud droplets have the opportunity to collide with small cloud droplets. As a result, small cloud droplets merge into large cloud droplets. In this way, large cloud droplets continue to grow, and because of the uneven distribution of updrafts, large cloud droplets can move up and down in the cloud many times. Coupled with the turbulence in the cloud, the chance of large cloud droplets growing increases, so The big cloud droplets get bigger and bigger, until the updraft can no longer support them, and they fall into rain

There is also a more professional opinion, which I think makes more sense:

When you When flying at an altitude of 10,000 meters and seeing a small amount of fog and light clouds at higher altitudes, we often wonder why most of the cloud particles are below the sea of ??clouds. What is so special about these high clouds that they are better than others? Are the clouds floating higher? ?Actually, there are still extremely thin water molecules at an altitude of 20 kilometers. As mentioned before, the water molecules at this height do not evaporate directly from the ground, but after the "second evaporation", the negative hydroxide radicals Water molecules reduced from ions. Because the molecular weight of hydroxide (OH?-) is 17, which is 1 smaller than the molecular weight of water vapor, it floats higher than water vapor. When they are reduced to water (H?2O) at the bottom of the stratosphere, they immediately condense into solid graupel particles at a temperature of -45°C. Their diameter is less than 1 micron, which reflects sunlight and is like a fog barrier, which is particularly dense. At that time, it was like a light cloud.

As a large number of graupel particles fall into the sea of ??clouds, the water mist in the clouds gathers towards the graupel particles and freezes into larger graupel particles. When they gather to a diameter of about 1 mm, the original graupel particles melt into water and fall to the ground as raindrops. In winter, the original graupel particles are not melted and form snowflakes or large graupel particles that fall to the ground. This is the cause of rain and snow. ?On a sunny day, when high-altitude graupel particles pass through cloudless clouds, they melt in mid-air due to the increase in temperature and turn into mist, or fall to the ground and become dew or frost, or on the way down, they are blown away by the sunshine and wind of the next day. Evaporate again. These high-altitude graupel particles are too small, easy to melt, and difficult to "catch" on site, so their existence and role are often ignored by meteorologists. When modern meteorology talks about the cause of rain and snow, it is said that the warm and moist air flow encounters the cold air mass, or the hot and humid air rises and then cools and condenses into clouds. The question is, where do these cold air masses come from during the summer and autumn rainy seasons? Could it be that they come from the Arctic and Antarctic circles to make rain? Since the hot and humid air brings water vapor and heat energy from the ground to high altitudes, it should be hotter at high altitudes, why? What if it cools and condenses into rain and snow? Without first clarifying the reason for the low temperature at the top of the troposphere, this theory of the cause of rain and snow cannot be justified at all. As mentioned before, the second evaporation is the main cause of the cold at high altitudes. A large number of graupel particles fall into the sea of ??clouds and absorb heat and melt, which will make the sea of ??clouds "even worse." When the cloud vapor condenses into raindrops and snow particles under such cold conditions, the specific gravity As it increases, the buoyancy disappears, and of course it will fall downwards, forming rain and snow. The so-called "convective rain", "orographic rain", "frontal rain", "typhoon rain", "artificial rainfall", etc. are just explaining the phenomena accompanying the rainfall process, and do not explain the reasons for the rainfall.

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< p>We all know that clouds are made up of many small water droplets and small ice crystals, and raindrops and snowflakes are formed by the growth of these small water droplets and small ice crystals. So, how is snow formed?

In water clouds, cloud droplets are all small water droplets. They mainly grow into raindrops by continuing to condense and collide with each other and merge.

Ice clouds are made of tiny ice crystals. When these small ice crystals collide with each other, the surface of the ice crystals will heat up and partially melt, and they will stick to each other and freeze again. Repeat this many times, and the ice crystals will increase in size. In addition, there is water vapor in the clouds, so ice crystals can continue to grow by condensation. However, ice clouds are generally very high and not thick. There is not much water vapor there, the growth of sublimation is very slow, and there are not many opportunities to collide with each other, so they cannot grow to a large size and form precipitation. Even if it causes precipitation, it is often evaporated on the way down and rarely falls to the ground.

What is most conducive to the growth of cloud droplets is hybrid cloud. Mixed clouds are composed of small ice crystals and supercooled water droplets. When a group of air has reached saturation for ice crystals, it has not yet reached saturation for water droplets. At this time, the water vapor in the cloud condenses onto the surface of the ice crystals, while the supercooled water droplets are evaporating. At this time, the phenomenon of ice crystals "adsorbing" water vapor from the supercooled water droplets occurs. In this case, the ice crystals grow very quickly. In addition, supercooled water is very unstable. Touch it and it will freeze. Therefore, in mixed clouds, when supercooled water droplets collide with ice crystals, they will freeze and adhere to the surface of the ice crystals, causing them to grow rapidly. When the small ice crystals grow large enough to overcome the resistance and buoyancy of the air, they fall to the ground and become snowflakes.

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 cause the snowflakes to fall to the ground before they have time to completely melt. This is called "wet snow" or "rain and snow falling together". This phenomenon is called "sleet" in meteorology.

Similarly, the size of snow is also classified according to the amount of precipitation. Snow can be divided into three categories: light snow, medium snow and heavy snow. See Table 3 for details.

Table 3. Precipitation of various types of snow Quantity standard

Types

Light snow

Moderate snow

Heavy 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

The formation and types of snow

Author: Dashan Article source: Online collection Clicks: 97 Update time: 2005-1 -16

We all know that clouds are composed of many small water droplets and small ice crystals, and raindrops and snowflakes are formed by the growth and enlargement of these small water droplets and small ice crystals. So, how is snow formed?

In water clouds, cloud droplets are all small water droplets. They mainly grow into raindrops by continuing to condense and collide with each other and merge.

Ice clouds are made of tiny ice crystals. When these small ice crystals collide with each other, the surface of the ice crystals will heat up and partially melt, and they will stick to each other and freeze again. Repeat this many times, and the ice crystals will increase in size. In addition, there is water vapor in the clouds, so ice crystals can continue to grow by condensation. However, ice clouds are generally very high and not thick. There is not much water vapor there, the growth of sublimation is very slow, and there are not many opportunities to collide with each other, so they cannot grow to a large size and form precipitation. Even if it causes precipitation, it is often evaporated on the way down and rarely falls to the ground.

What is most conducive to the growth of cloud droplets is hybrid cloud. Mixed clouds are composed of small ice crystals and supercooled water droplets. When a group of air has reached saturation for ice crystals, it has not yet reached saturation for water droplets. At this time, the water vapor in the cloud condenses onto the surface of the ice crystals, while the supercooled water droplets are evaporating. At this time, the phenomenon of ice crystals "adsorbing" water vapor from the supercooled water droplets occurs. In this case, the ice crystals grow very quickly. In addition, supercooled water is very unstable. Touch it and it will freeze. Therefore, in mixed clouds, when supercooled water droplets collide with ice crystals, they will freeze and adhere to the surface of the ice crystals, causing them to grow rapidly. When the small ice crystals grow large enough to overcome the resistance and buoyancy of the air, they fall to the ground and become snowflakes.

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 cause the snowflakes to fall to the ground before they have time to completely melt. This is called "wet snow" or "rain and snow falling together". This phenomenon is called "sleet" in meteorology.

Similarly, the size of snow is also classified according to the amount of precipitation. Snow can be divided into three categories: light snow, medium snow and heavy snow. See Table 3 for details.

Table 3. Precipitation of various types of snow Quantitative standards

Type light snow, medium snow, heavy snow

24-hour precipitation is less than 2.5, 2.6-5.0 is greater than 5.0

12-hour precipitation is less than 1.0, 1.1-3.0 is greater than 3.0

Shapes of Snowflakes

There are many shapes of snowflakes, and they are very beautiful. If you put the snowflakes under a magnifying glass, you can find that each snowflake is an extremely exquisite pattern. Many artists marveled. But how are the various snowflake shapes formed? Snowflakes are mostly hexagonal, because snowflakes belong to the hexagonal crystal system. The small ice crystals of snowflake "embryos" in the clouds have two main shapes. One is hexagonal, long and thin, called columnar crystals, but sometimes its ends are pointed and looks like a needle, called needle crystals. The other kind is in the shape of hexagonal flakes, just like the flakes cut from a hexagonal pencil, called platelets.

If the surrounding air is less supersaturated, the ice crystals will grow very slowly and evenly on all sides. When it increases and falls, it still maintains its original appearance and is called columnar, needle-shaped and flake-shaped snow crystals respectively.

If the surrounding air is highly supersaturated, the ice crystals will not only increase in size but also change in shape as they grow. The most common one is from flaky to star-shaped.

It turns out that 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 is and the lower the degree of supersaturation. The area close to the surface of the ice crystal has just reached saturation because the excess water vapor has condensed on the ice crystal. In this way, the density of water vapor near the ice crystal is smaller than that farther away from it. Water vapor moves from around the ice crystal to where the ice crystal is. Water vapor molecules first encounter the corners and protrusions of the ice crystal, where they condense and cause the ice crystal to grow. As a result, the corners and protrusions of the ice crystal will first grow rapidly and gradually become branch-shaped. Later, new twigs sprouted from each branch and corner for the same reason. At the same time, in the depressions between the corners and branches.

The air is no longer saturated. Sometimes, there is even a sublimation process here, so that the water vapor is transported to other places. This makes the corners and branches more prominent, slowly forming the familiar star-shaped snowflakes.

What is mentioned above is actually a typical formation process of star-shaped snowflakes. Its corresponding parts, regardless of shape or size, should be the same. This typical star-shaped snowflake can only form in an ideal, calm environment (such as a laboratory). In the atmosphere, it cannot increase step by step as mentioned above, and the shape formed cannot be as typical. This is because the ice crystals are gradually falling and sometimes rotating. Each branch has a different amount of contact with water vapor, and those branches that are exposed to more water vapor grow more. Therefore, the snowflakes we usually see are generally the same but different from each other.

In addition, as the snowflakes fall in the cloud, they will also change from the environment suitable for forming one shape to the environment suitable for forming another shape, so various complex phenomena occur. Snowflake shape. Some are like cufflinks, and some are like thorns. Even though they are all star-shaped snowflakes, there are also ones with three branches, six branches, or even twelve branches, or eighteen branches.

The above is the case of a single snowflake. As the snowflakes fall, individual snowflakes can easily cling to each other and join together to form larger flakes. The merger of snowflakes mostly occurs in the following three situations. (1) When the temperature is below 0°C, snowflakes collide with each other on the way down. The collision generates pressure and heat, causing the colliding parts to melt and stick to each other, and then the melted water freezes immediately. In this way, the two snowflakes merge together. (2) When the temperature is slightly higher than 0°C, the snowflakes are already covered with a layer of water film. If two snowflakes collide at this time, they will stick together through the surface tension of the water. (3) If the branches of the snowflakes are very complicated, the two snowflakes can also be hung together by simple climbing.

Snowflakes descend from the clouds to the ground. The journey is very long. When conditions are suitable, they can climb and merge many times to become very large. When it snows heavily, sometimes there are some large goose-feather-like snow flakes, which are formed by merging many times.

However, sometimes when the snowflakes collide with each other, they do not merge with each other, but are broken. At this time, some deformed snowflakes are produced. For example, when it snows, you sometimes see some individual "star branches", which is the case.

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Clouds are the basis of precipitation and the intermediate link in the water cycle on the earth, and the occurrence and development of clouds are always accompanied by the exchange of energy. The shapes of clouds are ever-changing, and certain cloud shapes are often accompanied by certain weather conditions. Therefore, clouds have certain indicative significance for weather changes.

(1) Cloud formation conditions and classification

In the atmosphere, the important conditions for condensation are the existence of condensation nuclei and the air reaching supersaturation. For cloud formation, supersaturation is primarily caused by adiabatic cooling of air as it rises vertically. The form and scale of the rising motion are different, and the state, height, and thickness of the clouds formed are also different. There are four main ways of upward movement of the atmosphere:

1. Thermal convection

Refers to the convective upward movement caused by uneven heating of the earth's surface and unstable atmospheric stratification. Clouds formed by convective motion are mostly cumulus clouds.

2. Dynamic uplift

Refers to the large-scale upward movement of warm and moist airflow caused by fronts and convergent airflows. The clouds formed by this movement are mainly stratiform clouds.

3. Atmospheric fluctuations

Refer to the wave-like movement of the atmosphere caused by flowing over uneven ground or below the inversion layer. Clouds produced by atmospheric fluctuations are mainly wave clouds.

4. Terrain uplift

Refers to the upward movement caused by the terrain blocking the atmosphere and being forced to lift. The clouds formed by this movement include cumulus clouds, wave clouds and stratiform clouds, which are usually called orographic clouds.

Although the shapes of clouds vary widely, there are always certain rules in their formation. According to the height of cloud formation and combined with its shape, clouds are divided into 4 groups and 10 genera according to the Chinese classification system. The "Cloud Atlas of China" published by our country in 1972 divided clouds into 3 families and 11 genera (Table 3.3, see Chapter 5 of "Practice of Aerology and Climatology" for details).

(2) The formation of various clouds

1. The formation of cumulus clouds

Cumulus clouds are vertically developing clouds, mainly including light cumulus Clouds, Cumulus Congestus and Cumulonimbus. Cumulus clouds are mostly formed in summer afternoons and have an isolated and scattered appearance with a flat cloud base and a convex top.

The formation of cumulus clouds is always related to the convective upward motion in the unstable atmosphere. Whether cumulus clouds can be formed with convection depends not only on the conditions of condensation, but also on the height that convection can reach. Cumulus clouds form if the maximum height that convective rise can reach (the upper limit of convection) is higher than the condensation height, otherwise cumulus clouds will not form. The stronger the convection, the greater the difference between the upper limit of convection and the height of condensation, and the greater the thickness of cumulus clouds. The horizontal range of the convective rising zone is wide, so the horizontal range of cumulus clouds will be larger.

Cumulus leucocumulus, cumulus congestus and cumulonimbus are different stages of the development of cumulus clouds. Cumulus clouds produced by thermal convection within the air mass are the most typical.

In the summer half of the year, the ground is subject to strong solar radiation, and the ground temperature is very high, further heating the air layer near the surface. Due to the inhomogeneity of the earth's surface, the air in some places is heated more intensely, and in other places the air is more humid. As a result, large and small air parcels (heat) that are slightly different from the surrounding temperature, humidity and density are generated in the air layer close to the earth. Bubble). 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 growing and dissipating. Larger air parcels rise to greater heights. When they reach above the condensation height, they form convection cells, which then gradually develop to form isolated, scattered light cumulus clouds with flat bottoms and convex tops. Since the air movement is continuous and compensates for each other, the rising air is cooled and the water vapor condenses into clouds, while the air around the cloud body sinks to replenish it. The sinking air heats up quickly adiabatically and does not form clouds. Therefore, cumulus clouds are scattered, and the blue sky is exposed between the clouds. For a certain area, at the same time, the horizontal distribution of air temperature and humidity is nearly consistent, and the condensation height is basically the same, so the bottom of the cumulus clouds is flat.

If the upper limit of convection is slightly higher than the condensation height, only light cumulus clouds are generally formed. Since the cloud top is generally below the 0°C isotherm height, the cloud body is composed of water droplets. The speed of the updraft in the cloud is not large, generally no more than 5m/s, and the turbulence in the cloud is also weak. At the height where cumulus clouds appear, if there are strong winds and strong turbulence, the cloud body of cumulus clouds will become broken. This kind of cloud is called broken cumulus clouds.

When the upper limit of convection exceeds the condensation height by much, the cloud body is tall and the top is cauliflower-shaped, forming cumulus congestus. The cloud top extends to a height below 0°C, 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 stronger, the cloud top of Cumulus Congestus can extend further upward, and the cloud top can extend to an altitude below -15°C. As a result, the cloud tops freeze into ice crystals, and strand structures appear, forming cumulonimbus clouds. The top of a cumulonimbus cloud, blown by high-altitude winds, expands horizontally into an anvil shape, which is called an anvil cloud. In the direction of the high-altitude wind, the cloud anvil can extend very far, so its extension direction can be used to determine the movement direction of cumulonimbus clouds. The thickness of cumulonimbus clouds is very large, ranging from 5,000 to 8,000m in mid-latitudes and up to more than 10,000m in low latitudes. The speed of ascending and descending airflows in clouds is very high. Updrafts can often reach 20-30m/s. An ascending speed of 60m/s has been observed, and the sinking speed is also 10-15m/s. The turbulence in the clouds is very strong.

Cumulus clouds formed by thermal convection have obvious diurnal variations. Usually, there are mostly light cumulus clouds in the morning. As the convection increases, it gradually develops into cumulus congestus clouds. Convection is strongest in the afternoon and can often develop into cumulonimbus clouds. In the evening, convection weakens and cumulonimbus clouds gradually dissipate, sometimes evolving into pseudo-cirrus clouds, cumulus altocumulus clouds and cumulus stratocumulus clouds. If in the afternoon, there are only light cumulus clouds in the sky, it means that the air is relatively stable, the cumulus clouds can no longer develop and grow, and the weather is good. Therefore, light cumulus clouds are also called sunny cumulus clouds, which are a sign of continuous sunny days. In summer, if cumulus dense clouds appear very early in the morning, it means that the air is already very unstable and may develop into cumulonimbus clouds. Therefore, cumulus dense clouds in the morning are a sign of thunderstorms. Stratocumulus clouds evolve in the evening after cumulus clouds dissipate, indicating that the air stratification is stable. The clouds disperse at night, which is a sign of continuous sunny weather. It can be seen that the diurnal variation characteristics of cumulus clouds formed by thermal convection can help directly judge short-term weather changes.

2. The formation of stratiform clouds

Stratiform clouds are uniform curtain-shaped clouds that often have a large horizontal range, including cirrostratus, cirrus, and altostratus clouds. and nimbostratus clouds.

Stratiform clouds are caused by the large-scale systematic upward movement of air, mainly caused by the upward movement on the front. This kind of systematic upward movement usually has a large horizontal range and an upward speed of only 0.1-1m/s. Because of its long duration, it can make the air rise several kilometers. For example, when warm air moves toward the cold air side, due to the difference in density between the two, the stable warm and humid air slowly slides up along the cold air slope, cools adiabatically, and forms stratiform clouds. The bottom of the cloud is roughly consistent with the inclined surface (also called the front) where warm and cold air meet, and the top of the cloud is approximately horizontal. Cloud thickness varies greatly at different parts of the tilted surface. The front ones are cirrus clouds and cirrostratus clouds, which have the thinnest thickness, generally ranging from a few hundred meters to 2000m. The cloud body is composed of ice crystals. Located in the middle are altostratus clouds, whose thickness is generally 1000-3000m. The top part is mostly composed of ice crystals, and the main part is mostly composed of ice crystals and supercooled water droplets. The last one is the nimbostratus cloud, which is generally 3000-6000m thick. The 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°C.

It can be seen from the above-mentioned systematic stratiform cloud formation that some clouds can serve as signs before precipitation comes. Cirrostratus clouds, for example, usually appear in the front of the layered cloud system, and their appearance is often accompanied by haloes of the sun and moon. Therefore, if you see haloes in the sky, you will know that cirrostratus clouds are moving in, and there will be rain in the future. As the clouds move in, the weather may turn rainy. The farmer's proverb "the sun haloes and rains at midnight, and the moon hazes and winds at noon" refers to this sign.

3. The formation of wavy clouds

Wave clouds are undulating clouds, including cirrocumulus, altocumulus, and stratocumulus clouds. The rising speed in clouds can reach tens of centimeters per second, second only to the rising speed in cumulus clouds.

When there are fluctuations in the air, the air at the peaks of the waves rises and the air at the troughs sinks.

Clouds are formed where the air rises due to adiabatic cooling, but no clouds are formed where the air sinks. If there are layered clouds with uniform thickness before the wave is formed, the clouds will thicken at the wave crest, and the clouds will thin out or even disappear at the wave trough, thus forming parallel cloud strips with small thickness and maintaining a certain distance, in a row. Or rows of wavy clouds.

It is generally believed that there are two main reasons for the formation of fluctuations: First, there are interfaces with different air densities and airflow speeds in the atmosphere, which cause fluctuations on this interface. The second is the fluctuation caused by the air flow over the mountain (called topographic wave or lee wave). When fluctuations occur at the interface where the wind speed is high and the density is low in the upper layer, and the wind speed is low and the density is low in the lower layer, since the wind direction and wind speed at each height often change with time, the direction of the fluctuations also changes, and the newly generated fluctuations are superimposed on the original ones. On top of the fluctuations, clouds form like a checkerboard. Cirrocumulus clouds are formed when the fluctuating air layer is very high, altocumulus clouds are formed when it is high, and stratocumulus clouds are formed when it is low.

The thickness of wavy clouds is not large, generally tens to hundreds of meters, sometimes up to 1000-2000m. When it appears, it often indicates that the air layer is relatively stable and the weather rarely changes. The proverbs "Tired clouds will kill people in the sun" and "There are carp spots in the sky, but there is no need to turn over the grain tomorrow", which means that after the appearance of lucent altocumulus or lucent stratocumulus clouds, the weather will be fine and seldom change. However, systematic wave clouds, such as cirrocumulus clouds, evolve after fluctuations in cirrus clouds or cirrostratus clouds, so they are connected with large stratiform clouds, indicating that wind and rain are coming. "The fish-scale sky, the wind will blow even if it doesn't rain" refers to this kind of omen.

4. The formation of special cloud shapes

In addition to the above-mentioned cloud formations, there are also some special cloud shapes, such as castle-shaped, flocculent, hanging spherical, pod-shaped, etc. , their appearance can often predict weather trends. Therefore, understanding their causes and characteristics will help to use them to judge future weather.

(1) Suspended spherical clouds: refer to clouds hanging down from the cloud base, often appearing at the bottom of cumulonimbus clouds. Sometimes seen at the base of altocumulus, altostratus and nimbostratus clouds.

When there are a large number of water droplets in the cloud, if there is a strong updraft near the cloud base, the falling water droplets will be held up, and a cloud group will be formed that seems to be hanging at the cloud base. This is a hanging spherical cloud. .

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

(2) Castle clouds and flocculent clouds: Castle clouds have a horizontal bottom and small protruding cloud towers juxtaposed on the top, shaped like a distant castle. The formation of this kind of cloud often develops on the basis of wavy clouds. After corrugated clouds are formed under the inversion layer, if the inversion layer is not too thick, when turbulence develops under the inversion layer, strong updrafts will pass through the inversion layer, condensing water vapor, and forming a cloud with an arc top. Clouds, these are castellated clouds. Common castellated clouds include castellated altocumulus clouds and castellated stratocumulus clouds.

The individual flocculent clouds are broken and shaped like cotton wool. They are often formed by strong turbulent mixing in the moist air layer, and are mainly flocculent altocumulus clouds.

Formulated altocumulus clouds or flocculent altocumulus clouds appear in the morning during the summer half of the year, indicating that the air layer at that height is unstable. At noon, once low-level convection develops, the upper and lower unstable air layers combine to produce a strong updraft. , forming cumulonimbus clouds, causing thunderstorms or hail. Convection weakens in the evening, and if fortress-shaped altocumulus clouds appear, it indicates that an unstable system will approach high altitude, and systematic thunderstorms may occur the next day.

(3) Lenticular clouds: Lenticular clouds are thick in the middle and thin at the edges, and the clouds are pod-shaped. Common lenticular clouds are mainly altocumulus lenticularis and stratocumulus lenticularis.

Lentular clouds are formed by the convergence of local updrafts and downdrafts. When the updraft cools the air adiabatically to form a cloud, if it encounters the obstruction of the downdraft, its edge gradually becomes thinner due to the downdraft, thus forming a lenticular cloud. In mountainous areas, lenticular clouds can also be formed when airflow is affected by terrain.

The above introduces the physical process of the formation of cumulus clouds, stratiform clouds, wave clouds and some special clouds. But they are not immutable in isolation. They can develop or dissipate, or transform from this type of cloud to that type of cloud due to changing conditions. For example, in cumulus clouds, light cumulus clouds can develop into cumulus dense clouds, and finally cumulonimbus clouds. When cumulonimbus clouds dissipate, they can evolve into pseudo-cirrus clouds, cumulus altocumulus clouds, and cumulus stratocumulus clouds. For another example, when wave clouds develop, they can evolve into stratiform clouds (obstructing altocumulus clouds can evolve into altostratus clouds, and obscuring stratocumulus clouds can evolve into nimbostratus clouds). When stratiform clouds dissipate, they can also evolve into wave clouds (when nimbostratus clouds dissipate, they can evolve into altocumulus, altocumulus, or stratocumulus clouds). In short, the emergence, development and evolution of cloud are complex and regular.

Reference: "Meteorology and Climatology (Third Edition)" Author: Zhou Shuzhen

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Fog is water vapor condensation composed of small water droplets or ice crystals floating in the air. Fog is generated in the near-surface layers of the atmosphere. Since fog is water vapor condensation, its cause should be found from the conditions that cause water vapor condensation. There are only two reasons why the water vapor in the atmosphere reaches saturation: one is due to evaporation, which increases the water vapor in the atmosphere; the other is due to the cooling of the air itself. Cooling is more important for fog.

When there are condensation nuclei in the air, condensation will occur if the saturated air continues to add water vapor or continues to cool. Fog forms when condensed water droplets reduce 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 areas that are conducive to lower-level cooling of the air, if there is sufficient water vapor, the wind is gentle, the atmospheric stratification is stable, and there are a large number of condensation nuclei, fog is most likely to form. Generally, there are more opportunities for fog to form in industrial areas and urban centers because there are abundant condensation nuclei there.

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Dew is water droplets formed when water vapor condenses when it encounters cold objects. In order to explain the cause of dew, we can simulate the conditions for dew formation in nature, use the evaporation of water to increase the humidity of the air, and use ice to reduce the temperature of objects. In this way, dew will appear on cold objects. Through this experiment, students can not only understand the causes of dew, but also learn the design method of simulation experiments.

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In the early morning of the cold season, grass blades and soil clods are often covered with a layer of frost crystals. They sparkled in the rising sun and melted as the sun rose higher. People often call this phenomenon "frost". Looking at the calendar, there is always the "Frost Descent" solar term in late October every year. We have seen snowfall and rainfall, but no one has seen frost. In fact, frost does not fall from the sky, but forms in the air near the ground.

Frost is a kind of white ice crystal that mostly forms at night. In rare cases, formation can begin before sunset when the sun is shining obliquely. Usually, the frost melts shortly after sunrise. But when the weather is very cold or in a shady place, frost can last all day long.

Frost itself is neither harmful nor beneficial to plants. What people usually call "frost damage" is actually "freezing damage" that occurs when frost is formed.

The formation of frost is not only related to the weather conditions at the time, but also to the properties of the attached objects. When the temperature of the surface of the object is very low, but the temperature of the air near the surface of the object is relatively high, then there is a temperature difference between the air and the surface of the object. If the temperature difference between the surface of the object and the air is mainly caused by radiative cooling of the surface of the object , the air will cool when the warmer air comes into contact with the colder surface, and excess water vapor will precipitate when it reaches water vapor supersaturation. If the temperature is below 0°C, excess water vapor condenses into ice crystals on the surface of the object, which is frost. Frost therefore always forms under weather conditions conducive to radiative cooling of the surface.

In addition, clouds hinder the radiative cooling of ground objects at night. Clouds in the sky are not conducive to the formation of frost. Therefore, frost mostly appears on clear nights, when ground radiative cooling is intense.

In addition, wind also has an impact on the formation of frost. When there is a breeze, the air slowly flows across the surface of cold objects, continuously supplying water vapor, which is conducive to the formation of frost. However, when the wind is strong, the air flows very fast and the time it touches 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 easily mix with each other, which is not conducive to lowering the temperature, which will also hinder the formation of frost. Generally speaking, when the wind speed reaches level 3 or above, frost will not easily form.

Therefore, frost generally forms on clear, breezy or windless nights during the cold season.

The formation of frost is not only related to the above-mentioned weather conditions, but also related to the properties of ground objects. Frost forms on the surface of an object that is cooled by radiation, so the easier it is for the surface to radiate heat and cool quickly, the easier it is for frost to form on it. Similar objects, under the same conditions, if they have the same mass, will have the same amount of heat inside them. If they radiate heat at the same time at night, then at the same time, the object with a larger surface area will dissipate more heat and cool faster, and it will be easier for frost to form on it. That is to say, if an object has a relatively large surface area compared to its mass, frost will easily form on it. Grass blades are very light but have a large surface area, so frost easily forms on the grass blades. In addition, objects with rough surfaces are more conducive to radiation and heat dissipation than objects with smooth surfaces, so frost is more likely to form on objects with rough surfaces, such as clods of soil.

There are two ways for frost to disappear: one is to sublimate into water vapor, and the other is to melt into water. The most common one is that it melts and disappears after sunrise due to rising temperatures. The water melted by frost has certain benefits for crops.

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Hail, like rain and snow, falls from the clouds. However, the hail cloud is a very powerful cumulonimbus cloud, and only a particularly powerful cumulonimbus cloud can produce hail.

Cumulonimbus clouds, like all kinds of clouds, are formed by the rising and condensation of air near the ground.

When the air rises from the ground, the air pressure decreases and the volume expands during the rise. If there is no heat exchange between the rising air and the surroundings, the air temperature will decrease due to the energy consumed by the expansion. This temperature change is called adiabatic cooling. According to calculations, for every 100 meters the air rises in the atmosphere, the temperature will drop by about 1 degree due to adiabatic change. We know that at a certain temperature, there is a limit to the amount of water vapor the air can hold. Reaching this limit is called "saturation". When the temperature decreases, the amount of water vapor that the air can hold will decrease. Therefore, the originally unsaturated air may reach saturation due to adiabatic cooling during its upward movement. After the air reaches saturation, the excess water vapor will adhere to the condensation nuclei floating in the air and form water droplets. When the temperature drops below zero degrees Celsius, excess water vapor condenses into tiny ice crystals. These water droplets and ice crystals gather together and float in the air to form clouds.