Normally, the reason why raindrops in mid-latitudes contain ice crystals during their formation, while in low-latitudes there are only water droplets is

Cloud and precipitation microphysics is a discipline that studies the physical laws of the formation, transformation and aggregation growth of cloud particles (cloud droplets, ice crystals) and precipitation particles (raindrops, snowflakes, graupel particles, hailstones, etc.). It is an important component of cloud and precipitation physics and the theoretical basis for artificial weather modification. P./VmY'

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The cloud droplets formed by condensation of water vapor in the atmosphere are very small, with a radius of about 10 microns, and a concentration of 10,000 to 1 per liter. Millions, the falling speed is about 1 cm/second, which is usually much smaller than the speed of the rising air in the cloud, so the cloud droplets cannot fall out of the cloud base. Even if it leaves the cloud base and descends, it will quickly evaporate and disappear in the unsaturated air. Only when cloud droplets gather and transform into precipitation particles through various microphysical processes can they fall to the ground. GguFo+YeZ

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For cloud formation and rain to occur, a series of complex microphysical processes are required: the moist air rises, expands, and cools, and the water vapor in it reaches saturation. And on some highly hygroscopic cloud condensation nuclei, the condensation nucleation process of initial cloud droplets condenses; supercooled water droplets or water vapor in the cloud freeze or sublime on ice nuclei and freeze naturally below -40°C. The process of ice phase formation into initial ice crystal embryos; the condensation growth process (sublimation) of water vapor further condensing (sublimation) on cloud droplets (ice crystals) under conditions slightly higher than saturation, causing the cloud droplets (ice crystals) to grow up (sublimation) Growth process); the gravitational collision process in which larger cloud droplets in the cloud collide with smaller cloud droplets and grow up during the falling process; when ice crystals and supercooled water droplets exist at the same time, due to the saturated water vapor of the supercooled water droplets The pressure is greater than that of the ice surface, causing the supercooled water droplets to gradually evaporate, and the ice crystals gradually grow due to the condensation of water vapor. The size of precipitation particles is about a hundred times that of cloud droplets, but their concentration is only that of clouds. One millionth of a drop +f[ED4E>'(

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Cloud droplets are usually spherical due to surface tension. Saturation of the surface of spherical pure water droplets Water vapor pressure is higher than the saturated water vapor pressure on the flat surface. Taking a water droplet with a radius of 0.01 micron as an example, the saturated water vapor pressure exceeds 12.5% ​​of that on the flat surface. In pure air without any impurities, the initial cloud droplets can only rely on random water vapor molecules. Generated by collision. The possibility of cloud droplets generated by random collision of molecules becomes smaller as the scale increases. >MPr=W%E

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Tiny initial cloud droplets. , only in an environment with a relative humidity of several hundred percent, it will not evaporate. However, the actual water vapor content of the atmosphere rarely exceeds 1% of the saturation value. Therefore, it is difficult to directly form cloud droplets in pure air without impurities. In fact, there are various condensation nuclei in the atmosphere, which provide conditions for condensation into cloud droplets. Xt#1Qs

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Cloud condensation nuclei can be divided into. Two types: large particles of hydrophilic substances, which are insoluble in water, but can absorb water vapor and form a water film on their surface, equivalent to a larger pure water droplet; aerosol particles containing soluble salts, which can absorb water. Water vapor becomes salt solution droplets, which are hygroscopic nuclei. For example, a saturated aqueous solution of sea salt can condense and grow as long as the relative humidity of the environment is higher than 78%. `

As the amount of condensed water increases, the concentration of solution droplets becomes smaller and smaller, and the required saturated vapor pressure becomes higher. However, as the amount of condensed water increases, the size of the solution droplets also increases, and the required saturated vapor pressure decreases as the size increases. Therefore, solution droplets of different concentrations and different scales require different saturated vapor pressure values. When the ambient water vapor pressure is greater than the corresponding critical value, the solution droplets can continue to grow. As the scale increases, the solution droplets gradually become purer. At this time, the saturated vapor pressure of the solution droplets also decreases. A particle containing one hundred billionth of a gram of salt can become a condensation nucleus and generate cloud droplets as long as the relative humidity of the environment is slightly greater than 100%. e${)w-R/e

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In supercooled water without impurities (ice nuclei), the formation of ice phase (the transformation of water from gaseous or liquid state (solid state) is a process in which water molecules spontaneously aggregate and transform into an ice-like structure. Clusters of water molecules that gather together are constantly forming and disappearing as a result of the fluctuations (pulsations) of the thermal motion of the molecules. The probability of occurrence of molecular clusters increases with decreasing temperature. When the size of the molecular cluster exceeds a certain critical value, it can continue to grow and form an initial ice crystal embryo. ?-Vjha@BO

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Pure water droplets with a diameter of several microns will spontaneously freeze only when the temperature is lower than -40°C; but when there are impurities in supercooled water (Ice nucleation), under the action of the impurity surface force field, molecular clusters are more likely to form ice crystal embryos. The formation of ice crystals in natural clouds mainly depends on the presence of impurities (ice nuclei). At -20°C, there is about one ice nucleus per liter of air, which is only one hundred thousandth of the concentration of cloud condensation nuclei in the same volume. Therefore, the concentration of ice crystals in clouds is generally much smaller than the concentration of water droplets.

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When the air in the cloud rises and expands and cools, the water vapor continues to condense. During the condensation process, the growth rate of the cloud droplet radius and The degree of supersaturation of water vapor in the cloud is directly proportional to the size of the cloud droplets themselves, so under certain water vapor conditions, the condensation growth of cloud droplets becomes slower and slower. The initial cloud droplets generated by one billionth of a gram of table salt start with a radius of 0.75 microns. It takes 0.15 seconds to grow to 1 micron, 30 minutes to grow to 10 microns, and four hours to grow to 30 microns. The above time. ?Mee 6

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Although water vapor can produce large cloud droplets after condensing on a few large hygroscopic cores, if it is to continue It will take longer to grow to a drizzle with a radius of 100 microns, and the life of cumulus itself is only about one hour, so it is impossible to form raindrops under the above circumstances; in stratiform clouds, the airflow rises only a few times. Centimeters per second, when large cloud droplets have no time to grow into raindrops, they will evaporate beyond the cloud base. In short, in the actual atmosphere, water vapor condensation alone cannot produce raindrops. cuQ<3Tn

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When cloud droplets approach each other, they collide and merge to form larger cloud droplets, which is called cloud droplet collision and growth under gravity. Cloud droplets falling in the field have a larger radius and are faster. They can catch up with small cloud droplets and collide with each other. This is called gravitational collision. However, when cloud droplets with different radii approach each other, the small droplets will be moved along with each other. The air flow displaced by the large droplets bypasses the large droplets, so only some of the small droplets can collide with the large droplets on the way down, and only some of the colliding cloud droplets can merge, while the others bounce away. 1 1cWy+8D

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The ratio of collision is called the collision coefficient, and its value is determined by the radius of large and small cloud droplets, which is usually less than 1. The radius The collision coefficient of large cloud droplets smaller than 20 microns to small cloud droplets is very small. When large cloud droplets pass through a cloud body composed of small cloud droplets, the growth rate of its radius during the collision process is related to the collision coefficient, the size of the cloud droplets. The relative speed between them is proportional to the water content of the droplets. The larger the radius of the large cloud droplets, the faster they will grow. \.g\Zib )

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In the actual atmosphere, the collision between cloud droplets is a random process. The chance of some large cloud droplets colliding with small cloud droplets is greater than the average result, so they grow very fast; while other cloud droplets have a greater chance of colliding with small cloud droplets. The collision speed is slower than the average result. Since the concentration of raindrops is only about one thousandth of that of large cloud droplets, only the process of the fastest growing large cloud droplets into raindrops needs to be considered. Based on the random collision and growth theory, the resulting raindrop generation time is much shorter than the continuous growth time, which is closer to the actual situation. 5#QXR+ T

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In addition, the turbulent mixing of airflow and the mutual attraction of cloud droplets under the action of the electric field can also cause cloud droplets to approach each other and collide. It is generally believed that these two mechanisms mainly play a role in the growth of small cloud droplets. If a cloud composed of liquid water has sufficient thickness, sufficient updraft velocity, and liquid water content, the large cloud droplets in it can grow into raindrops during the collision process. This process is called warm cloud precipitation process. Xj*vh m%i

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Raindrops with a radius greater than 3 mm will be severely deformed during the descent, and sometimes break into several small raindrops; when large and small raindrops collide with each other During the merging process, some smaller raindrops are sometimes separated. These situations are collectively referred to as the raindrop breakup process. After this small raindrop repeatedly experiences the process of rising, growing, falling and breaking again in the cloud, a large number of raindrops are quickly formed under certain conditions, which is called the Ransel chain reaction.

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At the same subzero temperature, the saturated water vapor pressure of the ice surface is smaller than that of the water surface, so relative to the saturated ambient water vapor of the water surface In terms of pressure, the water vapor pressure on the ice surface is supersaturated, so in supercooled clouds with temperatures below 0°C, once ice crystals appear, they can rapidly desublimate and grow. YA&g$!

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Based on this principle, Bergeron proposed the generation mechanism of precipitation particles in 1933. He believes that in clouds below 0°C, there are a large number of supercooled water droplets. The emergence of ice crystals destroys the stable state of the phase structure in the cloud; the water vapor pressure in the cloud is between the saturation value of the ice surface and the water surface. While water vapor continues to sublimate on the ice surface, water droplets continue to evaporate; ice crystals can quickly grow into snow crystals through the condensation of water vapor. In this way, water is continuously transferred from a large number of supercooled water droplets to a small number of ice crystals, and finally precipitation particles are formed. This is the ice crystal process, also known as the Bergeron process. nQ GQWg`

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On the one hand, the supercooled water droplets evaporate, and the water vapor transfers to the ice crystals, causing the ice crystals to grow; on the other hand, they collide with the snow crystals and freeze , causing the snow crystals to grow further.

If a large number of supercooled water droplets are frozen due to collision, the snow crystals will be converted into spherical graupel particles. Snow crystals may also adhere to each other to form snow clusters during movement and fall. These solid precipitation particles that do not melt before falling to the ground are solid precipitation such as snow graupel; they will melt when they fall in a warm area with a temperature above 0°C. Into raindrops. 4hw@yTUo

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In many cases, the ice crystal concentration is higher than the ambient ice nucleation concentration. This shows that the ice crystals involved in the ice crystallization process are not only generated from the ice nucleation process, but also When solid precipitation particles such as snow crystals collide and freeze with supercooled water droplets with a diameter greater than 24 microns at around -5°C, or when brittle dendritic ice crystals break, some broken ice particles may be produced. This process of producing secondary ice crystals is called ice crystal growth. [V 8{b{

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In mid-latitudes, stratiform clouds with continuous precipitation over a large area are often relatively deep, and the cloud tops are often above the 0°C layer: therefore clouds The upper part of the body has a lower temperature and a large number of ice nuclei are activated, which is the source of ice crystals. After the ice crystal grows up, it falls to the middle of the cloud, where there are a large number of supercooled water droplets, which can supply water to the ice crystal through the ice crystal process, allowing the ice crystal to continue to grow. Therefore, the upper part of this kind of cloud is generally called a seeding cloud, and the middle part is a supply cloud. The snow crystals and snow masses that grow up in this process fall into the warm clouds above 0°C below and melt into raindrops. On radar screens, bright bands showing this melting process can often be observed. ;@ [ 0x

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The understanding of the rules of formation, growth and transformation of clouds and precipitation particles is mainly based on theoretical research and experiments under controllable conditions obtained from. In fact, the environment of natural clouds and the corresponding microphysical processes are very complex. Coupled with the difficulties in observation, our understanding of them is still very superficial. Therefore, the development direction of cloud and precipitation microphysics is mainly to detect and study the evolution rules of particle groups with natural clouds as the macroscopic background.

Jiangchen 2008-01-26 01:33

"Mixue" is a kind of solid precipitation, which is white opaque oblong small particles, solid, and the maximum diameter does not exceed 1 mm. , does not rebound when it touches the ground, and is prone to appear in weather with zero temperatures, fog and high humidity. Rice snow comes from lower stratus clouds, and sometimes it can form in thicker fog and fall to the ground. Michelle is sometimes called "rice grits" by the masses.

Jiangchen 2008-01-26 01:38

Ice needles usually appear in sunny and cold mountains (such as Huangshan in our province) or plateaus. Due to radiation cooling, ice needles in the air The water vapor condenses into needles suspended in the air. It has similar causes to dew, so it is not included in precipitation in the standards. The specification is listed as weather phenomenon, code report 76, which represents the specific weather conditions at that time, so it needs to be recorded and compiled. 4 QZ?}iz

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Author: Sun Jinyuan

ZUIFENG 2008-01-26 11:28

No Find out if the forecaster has any special experience in predicting precipitation patterns.

Jiangchen 2008-01-26 13:12

Quote:

Quote from ZUIFENG on the 4th floor at 2008-01-26 11:28: V ,EF'-F

I don’t know if the forecaster has any special experience in predicting precipitation patterns.

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At noon, Bengbu became warm due to 700HPA, and the snow turned to ice particles. Hangzhou became cold due to below 850HPA, and the freezing rain turned to ice particles. Can you analyze it? tZ2e!

siemens_wolf 2008-01-26 13:23

Quote:

Quoted by General on the 5th Floor at 2008-01-26 13:12 Published by: ^Jp*B;

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At noon, Bengbu became warm due to 700HPA, and the snow turned into ice particles. Hangzhou became cold due to below 850HPA, and the freezing rain turned into ice particles. Everyone Analyze it, right? *Zd84wRSj

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This is indeed the case in Hangzhou. The snow in the morning has turned into rain and ice particles now. . . ?a/n

The road surface is a mixture of ice, rain and mud

siemens_wolf 2008-01-26 13:43

Update, estimate 850HPA It continues to get colder, and it’s now snowing in Hangzhou. . . .

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There is serious snow on roofs, carports, trees, grass and car roofs

eoj329 2008-01-26 13:49

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Quote from the 5th floor general posted at 2008-01-26 13:12: Ae|P"^kZ

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At noon, Bengbu changed due to 700HPA Warmth, snow turns to ice particles, Hangzhou becomes cold due to 850HPA, freezing rain turns to ice particles, please analyze it, right? %]Cjhs"v

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This is Inversion process!

eoj329 2008-01-26 13:49

The following is a simplified version of precipitation formation for the convenience of novices. shi Hy*(v

The occurrence of precipitation requires the condensation of water vapor. Condensation occurs when the relative humidity f>=100% (that is, the actual water vapor pressure in the atmosphere e>=the saturated water vapor pressure in the atmosphere E). The opposite process of evaporation under saturation. In order for the water vapor in the atmosphere to reach saturation or supersaturation, there must be condensation nuclei (such as suspended particles in the atmosphere). Of course, the increase in water vapor content in the atmosphere and the decrease in atmospheric temperature will also contribute to the water vapor. The saturation process appears. lpEDPvD_Vm

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There are generally four types of cooling in the atmosphere: adiabatic cooling, radiative cooling, advection cooling and mixed cooling g{DFS. [h

Adiabatic cooling forms clouds and rain; radiative cooling, advection cooling and mixed cooling mainly form fog. Lb q_~

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Two conditions must be met for the formation of precipitation: 1. The falling speed of the rain is greater than the rising speed of the airflow; 2. The rain is not completely evaporated when it falls from the clouds to the ground. {,V$ *

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Precipitation generally occurs in larger scale systems, so it is related to the formation of cloud droplets. The formation process includes cloud There are two parts: the condensation growth of droplets and the collision growth of cloud droplets. F In other words, under the same temperature conditions, since the saturated vapor pressure of the ice surface is less than the saturated vapor pressure of the water surface, the water droplets will continue to evaporate and become smaller, while the ice crystals will continue to sublime and grow. %1jdiHTaL

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The collision and growth of cloud droplets is mainly due to the process of cloud droplets of different sizes forming different speeds under the action of gravity and continuously superimposing and increasing in the atmosphere. For cloud droplets that lack ice water at low latitudes, cloud droplet collision is very important.

Zhejiang Cixi Fengmi 2008-01-26 14:06

My personal understanding is that the conditions for the formation of freezing rain are: there is a warm layer in the middle and low altitudes that can melt the snow or ice crystals in the clouds (ie: inversion layer) exists, but the temperature near the ground (including the surface temperature) must be below 0 degrees, so that supercooled water drops can freeze into ice after falling to the ground, thus forming freezing rain (also called rain).

carzon 2008-01-26 15:01

I am not familiar with the local terrain nhT;b,G.Z

But we can roughly see these trend lines . .WO/=# O

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925-850hPa is the coldest, 750-700hPa is the inverse The top temperature. Among them, the dew point temperature inversion in Guiyang 0126_00Z reaches more than 12 degrees. _jDS"

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1012hPa 125m 1.0C 0.6C 97% 295deg n`QO(pZ6+

1000hPa 232m -0.1C -1.4C 91% 325deg q'2` 0MRa

925hPa 852m -3.7C -5.1C 90% 20deg - &NQ \W

*850hPa 1532m 6.2C 3.5C 83% 250deg km}MqBQl

700hPa 3132m 4.2C 1.6C 83% 240deg 4y1 >

500hPa 5800m -10.3C -23.3C 34% 230deg u'=(&><

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1009hPa 185m -1.5C -3.1C 89% 0deg y]<#%F h

1000hPa 252m -2.3C -3.8C 89% 355deg O%prD}x < /p>

955hPa 610m -4.6C -6.1C 89% 45deg A1=_nt)5

925hPa 866m -6.3C -7.8C 89% 25deg b h*^{

884hPa 1219m -3.3C -4.8C 90% 215deg D{Oq\*

850hPa 1528m -0.7C -2.1C 90% 150deg IO*l vy

760hPa 2438m 1.6C 0.1C 89% 260deg Q g$($

*700hPa 3122m 3.4C 1.7C 89% 250deg nW~$ (Qnd

654hPa 3658m 0.6C -1.1C 88% 250deg @ W#fui<<}Y

561hPa 4877m -5.7C -7.6C 86% 255deg L 43`^ ;u

500hPa 5790m -10.5C -12.5C 85% 245deg amMjuyW

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1008hPa 166m -0.3C -2.2C 87% 0deg sw[oQ!f

1000hPa 229m -0.9C -2.7C 88% 10deg 1B6C

925hPa 846m -4.9C -6.2C 91% 35deg -MDO Zz\

*850hPa 1517m 3.4C 1.9C 90% 130deg 5/ tj

700hPa 3114m 2.8C 0.7C 86% 235deg v#iFQVBq

500hPa 5770m -10.3C -34.3C 12% 240deg &S

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850hPa 1533m -7.3C -8.6C 90% 40deg JOJh,8C) 6

794hPa 2066m -5.3C -6.6C 91% 342deg ('T4Db < /p>

771hPa 2299m -0.7C -2.1C 90% 317deg a_Z.J3

*756hPa 2458m 5.8C 4.2C 89% 300deg O|? Z~

700hPa 3084m 1.6C 0.1C 90% 235deg 5woIGO3X

683hPa 3282m 0.0C -1.3C 91% 236deg MhHr*!N"}

650hPa 3677m -3.9C -21.9C 23% 237deg \nC5 ,Rz

640hPa 3799m -4.7C -35.7C 7% 238deg g4zT(,ZY

604hPa 4258m -2.3C -13.3C 43% 239deg =t@:F < /p>

574hPa 4661m -4.5C -14.5C 46% 241deg =ZCH1J5"

500hPa 5730m -13.3C -20.3C 56% 245deg

Chasing the Wind Elf 2008- 01-26 20:30

Quote:

Quoted by Zhejiang Cixi Fengmi on the 10th floor at 2008-01-26 14:06: e 7Yb=/F

My personal understanding is that the conditions for the formation of freezing rain are: there is a warm layer (ie: inversion layer) that can melt snow or ice crystals in the clouds at mid- to low-altitudes, but the temperature near the ground (including the surface temperature) must be below 0 degrees , so that the supercooled water drops can freeze into ice after falling to the ground, thus forming freezing rain (also called rain).

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Haha~correct it.... `l HKQwu

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The crystals formed when supercooled raindrops fall on cold objects on the ground and freeze are called raindrops~~

eoj329 2008-01-26 21:38

Another point is that rainstorms are generally easy to form on the windward side, that is, there is a greater chance of appearing in areas affected by northerly winds

Jiangchen 2008-01-27 02:55

After looking at the critical forms of freezing rain and snowfall in some areas, it seems that the conditions of freezing rain, the thickness, temperature, temperature difference, and duration of the inversion layer and cold layer are a comprehensive and interactive whole. It is difficult to predict without accurate computer calculations. explain. pHI%jHHJ

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Can any hero analyze the following sounding and weather conditions: Y-}hNZn"{

(Youlao K version Please proofread the sounding and weather live data to see if they are correct) ZaEBdBv

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At 20:00 on the 26th in Qu County, Zhejiang, it rained (58633 Qu Xian) (&N$W&

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At 8:00 on the 26th in Hangzhou, freezing rain turned to ice particles (58457 ZSHC Hangzhou) { V =:O

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At 20:00 on the 26th in Hangzhou, ice particles turned to snow (58457 ZSHC Hangzhou) 1W6n[Xg

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Changsha at 20:00 on the 26th, sleet and snow (57679 ZGCS Changsha) ^*owD;]4_

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Freezing rain in Guilin at 20:00 on the 26th (57957 ZGKL Guilin) ​​jy)9EU=

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Jiangchen 2008-01-27 03:04

Freezing rain in Nanchang at 08:00 on the 26th (58606 ZSCN Nanchang) -%QEzu&

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Light rain at 20:00 on the 26th in Nanchang (58606 ZSCN Nanchang) ?2bE=|

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Guiyang has a high altitude, so don’t watch