0 C level) it is called a cold cloud › met › msc › fezzik › Synop-Met › Ch06-5-Slides.pdf · 5 Microphysics of Cold Clouds If a cloud extends above the freezing level (0

5 Microphysics of Cold CloudsIf a cloud extends above the freezing level (0◦C level) it iscalled a cold cloud.


Even though the temperature may be below 0◦C, waterdroplets can still exist in clouds, in which case they arereferred to as supercooled droplets.



Cold clouds may also contain ice particles. If a cold cloudcontains both ice particles and supercooled droplets it issaid to be a mixed cloud.



Cold clouds may also contain ice particles. If a cold cloudcontains both ice particles and supercooled droplets it issaid to be a mixed cloud.

In this section we are concerned with the origins and con-centrations of ice particles in clouds, the ways ice particlesgrow, and the formation of precipitation in cold clouds.

Nucleation of Ice ParticlesA supercooled droplet is in an unstable state. For freezing tooccur, enough water molecules must come together withinthe droplet to form an embryo of ice large enough to surviveand grow.

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If a water droplet contains no foreign particles it can freezeonly by homogeneous nucleation.

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Homogeneous nucleation occurs at about −41◦C for dropletsabout 1 µm in diameter, and at about −35◦C for drops 100 µmin diameter.

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Homogeneous nucleation occurs at about −41◦C for dropletsabout 1 µm in diameter, and at about −35◦C for drops 100 µmin diameter.

Hence, in the atmosphere, homogeneous nucleation of freez-ing occurs only in high clouds.

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Heterogeneous NucleationIf a droplet contains a rather special type of particle, calleda freezing nucleus, it may freeze by a process known asheterogeneous nucleation:

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Water molecules in the droplet collect onto the surface ofthe particle to form an ice-like structure that may increasein size and cause the droplet to freeze.

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Since the formation of the ice structure is aided by the freez-ing nucleus, heterogeneous nucleation can occur at muchhigher temperatures than homogeneous nucleation.

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Certain particles in the air also serve as centers upon whichice can form directly from the vapour phase. These particlesare referred to as deposition nuclei.

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Certain particles in the air also serve as centers upon whichice can form directly from the vapour phase. These particlesare referred to as deposition nuclei.

Ice can form by deposition if the air is supersaturated withrespect to ice and the temperature is low enough.

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Ice NucleiParticles with molecular spacings and crystallographic ar-rangements similar to those of ice (which has a hexagonalstructure) tend to be effective as ice nuclei.

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Most effective ice nuclei are virtually insoluble in water.

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Most effective ice nuclei are virtually insoluble in water.

Some inorganic soil particles (mainly clays) can nucleate iceat fairly high temperatures (i.e., above −15◦C), and play animportant role in nucleating ice in clouds.

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Measuring Ice Nuclei ConcentrationSeveral techniques have been used for measuring the con-centrations of particles in the air that are active as ice nucleiat a given temperature.

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A common method is to draw a known volume of air into acontainer and to cool it until a cloud is formed. The numberof ice crystals forming at a particular temperature is thenmeasured.

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In expansion chambers cooling is produced by compressingthe air and then suddenly expanding it.

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In mixing chambers cooling is produced by refrigeration.

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In mixing chambers cooling is produced by refrigeration.

In diffusion chambers temperature, supersaturation andpressure can be controlled independently.

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Concentrations of Ice Particles

Percentage of clouds containing ice particle concentrations greater

than about 1 per liter as a function of cloud top temperature. Blue

curve: Continental cumuliform clouds. Red curve: Clean marine

cumuliform clouds and clean arctic stratiform clouds.

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The probability of ice particles being present in a cloudincreases as the temperature decreases below 0◦C, as illus-trated in the Figure above.

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The results indicate that the probability of ice being presentis 100% for cloud top temperature below about −13◦C.

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At higher temperatures the probability of ice being presentfalls off sharply, but it is greater if the cloud contains drizzleor raindrops.

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At higher temperatures the probability of ice being presentfalls off sharply, but it is greater if the cloud contains drizzleor raindrops.

Clouds with top temperatures between about 0 and −8◦Cgenerally contain copious supercooled droplets. It is inclouds such as these that aircraft are most likely to en-counter severe icing conditions, since supercooled dropletsfreeze when they collide with an aircraft.

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Growth of Ice Particles in CloudsWe consider several methods of growth:

• (a) Growth from the vapour phase.

• (b) Growth by riming; hailstones.

• (c) Growth by aggregation.

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Schematic of ice development in small cumuliform clouds.

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(a) Growth from the vapour phase.In a mixed cloud dominated by supercooled droplets, theair is close to saturated with respect to liquid water and istherefore supersaturated with respect to ice.

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For example, air saturated with respect to liquid water at−10◦C is supersaturated with respect to ice by 10%.

This value is much higher than the supersaturation of cloudyair with respect to liquid water, which rarely exceed 1%.

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Consequently, in mixed clouds dominated by supercooledwater droplets, in which the cloudy air is close to watersaturation, ice particles will grow from the vapour phasemuch more rapidly than droplets.

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Consequently, in mixed clouds dominated by supercooledwater droplets, in which the cloudy air is close to watersaturation, ice particles will grow from the vapour phasemuch more rapidly than droplets.

In fact, if a growing ice particle lowers the vapour pressurein its vicinity below water saturation, adjacent droplets willevaporate (Figure below).

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Laboratory demonstration of the growth of an ice crystalat the expense of surrounding supercooled water drops.

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Cumulus turrets contain-ing relatively large ice par-ticles often have ill de-fined, fuzzy boundaries, asfor the clouts in the back-ground here.Turrets containing onlysmall droplets have well-defined, sharper bound-aries, particularly if thecloud is growing (see cloudin foreground).

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Saturation vapourpressure over water

(Red) and thedifference between

the saturationpressures over waterand over ice (Blue).

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The lower equilibrium vapour pressure over ice than overwater at the same temperature allows ice particles to mi-grate for greater distances than droplets into the non-satur-ated air surrounding a cloud before they evaporate.

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For this reason, ice particles that are large enough to fall outof a cloud can survive great distances before evaporatingcompletely, even if the ambient air is sub-saturated withrespect to ice.

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The trails of ice crystals so produced are called fallstreaksor virga.

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The trails of ice crystals so produced are called fallstreaksor virga.

Ice particles will grow in air that is sub-saturated with re-spect to water, provided that it is supersaturated with re-spect to ice.

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Fallstreaks of ice crystals from cirrus clouds. The characteristic curved

shape of the fallstreaks indicates that the wind speed was increasing

(from left to right) with increasing altitude.15

Shape and Form of Ice particlesThe majority of ice particles in clouds are irregular in shape(sometimes referred to as “junk” ice).

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Laboratory studies show that under appropriate conditionsice crystals that grow from the vapour phase can assume avariety of regular shapes that are either platelike or column-like.

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Laboratory studies show that under appropriate conditionsice crystals that grow from the vapour phase can assume avariety of regular shapes that are either platelike or column-like.

The simplest platelike crystals are plane hexagonal plates,and the simplest columnlike crystals are solid columns thatare hexagonal in cross section.

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Ice crystals grownfrom the vapour phase:(a) hexagonal plates,(b) column,(c) dendrite,(d) sector plate,(e) bullet rosette.

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(b) Growth by riming; hailstones.

In a mixed cloud, ice particles can increase in mass by collid-ing with supercooled droplets which then freeze onto them.

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This process, referred to as growth by riming, leads to theformation of various rimed structures (figure follows).

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This process, referred to as growth by riming, leads to theformation of various rimed structures (figure follows).

When riming proceeds beyond a certain stage it becomesdifficult to discern the original shape of the ice crystal. Therimed particle is then referred to as graupel.

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(a) Lightly rimed needle; (b) rimed column; (c) rimed plate; (d) rimed

stellar; (e) spherical graupel; (f) conical graupel.

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HailstonesHailstones represent an extreme case of the growth of iceparticles by riming. They form in vigorous convective cloudsthat have high liquid water contents.

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The largest hailstone reported in the USA (Nebraska) was13.8 cm in diameter and weighed about 0.7 kg. However,hailstones about 1 cm in diameter are much more common.

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If a thin section is cut from a hailstone and viewed in trans-mitted light, it is often seen to consist of alternate dark andlight layers (Figure follows).

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If a thin section is cut from a hailstone and viewed in trans-mitted light, it is often seen to consist of alternate dark andlight layers (Figure follows).

The dark layers are opaque ice containing numerous smallair bubbles, and the light layers are clear ice. Clear ice ismore likely to form when the hailstone is growing wet.

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Thin section through the center of a hailstone.

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Artificial hailstone (i.e., grown in the laboratory) showing alobe structure. Growth was initially dry but tended towardwet growth as the stone grew.

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Artificial hailstone (i.e., grown in the laboratory) showing alobe structure. Growth was initially dry but tended towardwet growth as the stone grew.

The development of lobes in a hailstone may be due to thefact that small bumps on a hailstone will be areas of en-hanced collection efficiencies for droplets.

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(c) Growth by aggregation.

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The third mechanism by which ice particles grow in cloudsis by colliding and aggregating with one another.

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Ice particles can collide with each other provided their ter-minal fall speeds are different.

For example, graupel particles 1 and 4mm in diameter haveterminal fall speeds of about 1 and 2.5 m s−1 respectively.

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Ice particles can collide with each other provided their ter-minal fall speeds are different.

For example, graupel particles 1 and 4mm in diameter haveterminal fall speeds of about 1 and 2.5 m s−1 respectively.

Consequently, the frequency of collisions of ice particles inclouds is greatly enhanced if some riming has taken place.

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The second factor that influences growth by aggregation iswhether or not two ice particles adhere when they collide.

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The probability of adhesion is determined primarily by:

1. The types of ice particles

2. The temperature

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2. The temperature

The probability of two colliding crystals adhering increaseswith increasing temperature as the ice surfaces become more“sticky”.

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2. The temperature

The probability of two colliding crystals adhering increaseswith increasing temperature as the ice surfaces become more“sticky”.

Some examples of ice particle aggregates are shown below.

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Aggregates of (a) rimed needles; (b) rimed columns; (c) dendrites; (d)

rimed frozen drops.

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Precipitation in Cold CloudsAs early as 1784, Benjamin Franklin suggested that “muchof what is rain, when it arrives at the surface of the Earth,might have been snow, when it began its descent . . . ”.

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This idea was not developed until the early part of the lastcentury when Alfred Lothar Wegener, in 1911, stated thatice particles would grow preferentially by deposition fromthe vapour phase in a mixed cloud.

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Subsequently, Tor Bergeron, in 1933, and Theodor RobertWalter Findeisen, in 1938, developed this idea in a morequantitative manner.

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Subsequently, Tor Bergeron, in 1933, and Theodor RobertWalter Findeisen, in 1938, developed this idea in a morequantitative manner.

Since Findeisen carried out his field studies in northwesternEurope, he was led to believe that all rain originates asice. However, as we have seen , rain can also form in warmclouds by the collision-coalescence mechanism.

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Calculations indicate that the growth of ice crystals by de-position of vapour is not sufficiently fast to produce largeraindrops.

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Unlike growth by deposition, the growth rates of an ice par-ticle by riming and aggregation increase as the ice particlencreases in size.

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A simple calculation shows that a platelike ice crystal, 1 mmin diameter, falling through a cloud with a liquid contentof 0.5 gm−3, could develop into a spherical graupel particleabout 0.5 mm in radius in a few minutes.

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A graupel particle of this size, with a density of 100 kgm−3, has a terminal fall speed of about 1 m s−1 and wouldmelt into a drop about 230µm in radius.

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A graupel particle of this size, with a density of 100 kgm−3, has a terminal fall speed of about 1 m s−1 and wouldmelt into a drop about 230µm in radius.

We conclude from these calculations that the growth of icecrystals, first by deposition from the vapour phase in mixedclouds and then by riming and/or aggregation, can produceprecipitation-sized particles in about 30 minutes.

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Bergeron-Findeisen Process

Saturation vapourpressure over water

(Red) and thedifference between

the saturationpressures over waterand over ice (Blue).

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The vapour pressure over water is greater than over ice(es > esi).

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Consider two interconnected chambers, one with super-cooledwater, the other with ice (Draw Picture).

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When the chambers are isolated form each other, there willbe fewer molecules of water vapour (per unit volume) overthe ice than over the water.

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If the chambers are then connected, there will be a flow ofwater vapour from the region over the water to that overthe ice.

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Now consider an ice crystal in the atmopshere, surroundedby supercooled water droplets.

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Now consider an ice crystal in the atmopshere, surroundedby supercooled water droplets.

If the air is saturated with respect to water (100% RH), itis supersaturated with respect to ice.

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Therefore, water molecules will deposit onto the crystal,thus lowering the relative humidity of the air.

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Consequently, water will evaporate from the droplet.

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Thus, ice crystals grow at the expense of water droplets ina cloud where both are present.

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Thus, ice crystals grow at the expense of water droplets ina cloud where both are present.

This Bergeron-Findeisen Process results in rapid growth ofice crystals and therefore enales precipitation in cold clouds.

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Radar Bright BandThe role of the ice phase in producing precipitation in coldclouds is demonstrated by radar observations.

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Shown here is a radar image, with the radar antenna point-ing vertically upward.

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Shown here is a radar image, with the radar antenna point-ing vertically upward.

The horizontal band (brown) just above a height of 2 km wasproduced by the melting of ice particles. This is referred toas the “bright band”.

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The curved trails of relatively high reflectivity emanatingfrom the bright band are fallstreaks of precipitation, someof which reach the ground.

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The radar reflectivity is high around the melting level be-cause, while melting, ice particles become coated with a filmof water that greatly increases their radar reflectivity.

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The radar reflectivity is high around the melting level be-cause, while melting, ice particles become coated with a filmof water that greatly increases their radar reflectivity.

When the crystals have melted completely they collapse intodroplets, and their terminal fall speeds increase so that theconcentration of particles is reduced. These changes resultin a sharp decrease in radar reflectivity below the meltingband.

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The sharp increase in particle fall speeds produced by melt-ing is illustrated below, showing the spectrum of fall speedsof precipitation particles measured at various heights witha vertically pointing Doppler radar.

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The sharp increase in particle fall speeds produced by melt-ing is illustrated below, showing the spectrum of fall speedsof precipitation particles measured at various heights witha vertically pointing Doppler radar.

At heights above 2.2 km the particles are ice with fall speedscentered around 2 m s−1. At 2.2 km the particles are par-tially melted, and below 2.2 km there are raindrops withfall speeds centered around 7 m s−1.

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Spectra of Doppler fallspeeds for precipitationparticles at ten heightsin the atmosphere. Themelting level is at about2.2 km.

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Classification of Solid PrecipitationThe growth of ice particles by deposition, riming, and ag-gregation leads to a very wide variety of solid precipitationparticles. A relatively simple classification into ten mainclasses is given now.

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1. A plate is a thin, platelike snow crystal the form of whichmore or less resembles a hexagon or, in rare cases, a trian-gle. Generally all edges or alternative edges of the plateare similar in pattern and length.

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1. A plate is a thin, platelike snow crystal the form of whichmore or less resembles a hexagon or, in rare cases, a trian-gle. Generally all edges or alternative edges of the plateare similar in pattern and length.

2. A stellar crystal is a thin, flat snow crystal in the form ofa conventional star. It generally has six arms but stellarcrystals with three or twelve arms occur occasionally. Thearms may lie in a single plane or in closely spaced parallelplanes in which case the arms are interconnected by avery short column.

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3. A column is a relatively short prismatic crystal, eithersolid or hollow, with plane, pyramidal, truncated, or hol-low ends. Pyramids, which may be regarded as a partic-ular case, and combinations of columns are included inthis class.

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4. A needle is a very slender, needlelike snow particle of ap-proximately cylindrical form. This class includes hollowbundles of parallel needles, which are very common, andcombinations of needles arranged in any of a wide varietyof fashions.

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4. A needle is a very slender, needlelike snow particle of ap-proximately cylindrical form. This class includes hollowbundles of parallel needles, which are very common, andcombinations of needles arranged in any of a wide varietyof fashions.

5. A spatial dendrite is a complex snow crystal with fern-like arms which do not lie in a plane or in parallel planesbut extend in many directions from a central nucleus. Itsgeneral form is roughly spherical.

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6. A capped column is a column with plates of hexagonal orstellar form at its ends and, in many cases, with additionalplates at intermediate positions. The plates are arrangednormal to the principal axis of the column. Occasionallyonly one end of the column is capped in this manner.

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7. An irregular crystal is a snow particle made up of a num-ber of small crystals grown together in a random fashion.Generally the component crystals are so small that thecrystalline form of the particle can only be seen with theaid of a magnifying glass or microscope.

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7. An irregular crystal is a snow particle made up of a num-ber of small crystals grown together in a random fashion.Generally the component crystals are so small that thecrystalline form of the particle can only be seen with theaid of a magnifying glass or microscope.

8. Graupel , which includes soft hail, small hail, and snowpellets, is a snow crystal or particle coated with a heavydeposit of rime. It may retain some evidence of the out-line of the original crystal although the most commontype has a form which is approximately spherical.

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9. Ice pellets (frequently called sleet in North America) aretransparent spheroids of ice and are usually fairly small.Some ice pellets do not have a frozen center which in-dicates that, at least in some cases, freezing takes placefrom the surface inwards.

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9. Ice pellets (frequently called sleet in North America) aretransparent spheroids of ice and are usually fairly small.Some ice pellets do not have a frozen center which in-dicates that, at least in some cases, freezing takes placefrom the surface inwards.

10. A hailstone is a grain of ice, generally having a laminarstructure and characterized by its smooth glazed surfaceand its translucent or milky-white center. Hail is usu-ally associated with those atmospheric conditions whichaccompany thunderstorms.

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0 C level) it is called a cold cloud › met › msc › fezzik › Synop-Met › Ch06-5-Slides.pdf · 5 Microphysics of Cold Clouds If a cloud extends above the freezing level (0