Q. J. R. Meteorol. Soc. (2002), 128, pp. 1461–1483
Nucleation effects on the habit of vapour grown ice crystals from
¡18 to ¡42 ± C
By M. BAILEY and J. HALLETT¤
Desert Research Institute, USA
(Received 28 October 2001; revised 1 February 2002)
S UMMARY
Ice crystals have been nucleated and grown by a new method where supersaturation is controlled by
combining static diffusion chamber and expansion chamber techniques. Crystals were nucleated and grown
on 50 ¹m diameter glass Ž laments at pressures of 500 to 300 hPa, typical of clouds with temperatures from
¡18 to ¡42 ± C, respectively. Crystals were also nucleated on particles of Ž nely powdered silver iodide and
kaolinite adhering to glass Ž laments, in order to investigate effects on habit due to different nucleating materials.
Critical supersaturations for nucleation were measured for each case and were in good agreement with previous
nucleation studies. Crystals were grown at ice supersaturations which ranged from less than 1% to just above
water saturation. For ice growth below a critical supersaturation, nucleation was achieved by applying a brief
adiabatic expansion to the static diffusion chamber with a Ž lament in place and the chamber evacuated to the
target pressure for the temperature of interest. Nucleation occurred at a critical supersaturation, followed by
growth at a lower value. A review of past laboratory experiments and recent in situ observations reveals good
agreement between laboratory and Ž eld observations when the effects of nucleating methods and growth times
are taken into account. Contrary to studies which report the habit of ice as ‘prism’-like or columnar below
approximately ¡22 ± C, the habit is found to be plate-like for temperatures ranging from ¡18 ± C to ¡40 ± C at
ice supersaturations relevant to the atmosphere. Additionally, the plate-like-habit distribution is dominated by
polycrystalline forms, which is consistent with aspects of past laboratory studies and recent observations obtained
with improved aircraft instrumentation. The habits and habit distributions obtained from glass and kaolinite most
closely resemble those observed in the atmosphere, while the results from silver iodide show large differences.
Below ¡40 ± C, the habit becomes columnar except at low supersaturation, and is still dominated by polycrystals
which appear as rosettes for ice supersaturations greater than about 20%. ‘Bullet rosettes’ were observed to
nucleate and grow on clean glass threads below ¡40 ± C but not on kaolinite which predominantly grew single
columns and irregular polycrystals with columnar components; silver iodide produced single columns with few
polycrystals. This behaviour indicates that rosettes result from ice nuclei which share few or no crystallographic
similarities with ice, and is consistent with that expected from homogeneous nucleation. Critical supersaturation
measurements for nucleation were extended down to ¡70 ± C, and revealed that below ¡40 ± C ice crystals can
nucleate on glass at ice supersaturations of 25%, and at values of approximately 15% on kaolinite or silver iodide.
K EYWORDS: Bullet rosettes
Cirrus Critical supersaturation
1.
Polycrystals
I NTRODUCTION
The crystal habit and habit distribution in ice clouds with temperatures below
¡20 ± C exhibit great variability. While the variations are due in part to environmental
properties such as temperature, relative humidity and fall velocity, they may also be due
to the type of ice nuclei (IN) present and the nucleation conditions under which the ice
crystals form. Differences in both nucleation and growth conditions can result in clouds
with quite different optical and radiative properties. In order to characterize the role of
ice crystals and clouds in global circulation and radiative transfer models, it is necessary
to understand the factors which affect crystal size, habit, and habit distribution.
Several cloud chamber studies addressing the effects on crystal habit from different nucleation methods have been performed (Schaefer 1949; Yamashita 1971; Kumai
1982; Gonda 1983). In some of these experiments nucleation was performed at low temperatures, well below the growth temperature under investigation, through the use of dry
ice or liquid nitrogen. Silver iodide aerosol, Ž ne clay particles and localized adiabatic
expansion were also used to initiate nucleation. Crystal concentrations much greater
than those found in the atmosphere were obtained, and estimates of supersaturation
¤
Corresponding author: Desert Research Institute, 2215 Raggio Parkway, Reno NV 89506, USA.
c Copyright Royal Meteorological Society, 2002.
°
1461
1462
M. BAILEY and J. HALLETT
were often questionable due to vapour competition resulting from high particle concentrations. Many of these studies additionally involved very short growth times which only
yielded small crystals. In those studies using silver iodide, which shares nearly the same
crystallographic symmetry and lattice dimensions as ice-I h , pronounced differences in
habit were observed when compared with other nucleating materials.
Diffusion chamber growth studies utilizing Ž ne glass Ž laments as substrates have
also been performed to investigate the effects of temperature and supersaturation on
ice crystal habit (Kobayashi 1957,1961; Hallett and Mason 1958). Naturally occurring
IN typically consist of clays and silicates which are insoluble, share varying crystallographic similarities with ice, and generally exhibit moderate nucleability between ¡10
and ¡20 ± C. Most glasses are a mixture of silica and metal oxides common in aerosol
from crustal materials. While chemical composition has an effect on nucleability, topographic surface features on IN also play a role. Experimental evidence for this comes
from photographs taken during the course of several epitaxial-growth studies, which
reveal that ice crystals appear preferentially at cleavage and growth steps and in cracks
and cavities (Hallett 1961). Glass Ž laments typically have diameters much greater than
naturally occurring nuclei, however they have surface cracks and ridges with sharp edges
and features that are comparable in size to natural nuclei, hence they are likely to provide
similar physical conditions for nucleation sites at the molecular or ice embryo scale.
Contact between an ice embryo and a glass Ž lament might affect growth or habit when
it is smaller than the Ž lament; however, this is also the case for growth on an IN in
the atmosphere. Clay particles are mostly conŽ ned to the ‘larger’ size range of IN, and
studies have shown that they must have a size comparable to or signiŽ cantly larger than
that of the critical ice embryo which forms on their surface. From these considerations,
it is reasonable to conclude that amorphous substrates such as narrow glass Ž laments
can be usefully employed for exploring nucleation and crystal growth when surface
structures or particles on the Ž lament approach the size of typical IN.
Over three decades ago, Magono and Lee (1966) formulated a habit classiŽ cation
scheme which included the results from the previously mentioned habit diagrams in
addition to many of the more complex shapes observed in the atmosphere. At that time,
they recognized an ambiguity that arose from the use of the term ‘prism’ versus column,
prisms referring to thick refracting crystals with rectangular faces of substantial height
which are parallel to the vertical c axis, as opposed to thin hexagonal plates with thin
or short prism faces. The shapes of crystals have been described in the literature by, in
the case of columns, the length (L) to diameter (d D 2a) ratio, L=d, and in the case of
plates, the total height (h) to diameter ratio, h=d. Crystals with 0:5 · h=d < 0:8 have at
times been described as prisms and columns rather than thick plates (Kobayashi 1961;
Hobbs 1974; Wang and Fukuta 1985), and prism has also been used to describe mixtures
of thick plates and short columns. Magono and Lee (1966) adopted the convention that
‘. . . a column is called a thick plate when its length is shorter than its diameter, in the
usual sense’. Pruppacher and Klett (1997) have used this convention and it is used here
also. A plate-like crystal is one with h=d < 1, and plates are either ‘thin’ or ‘thick’
the distinction being somewhat arbitrary. Columns are prism faced hexagonal crystals
with L=d > 1, crystals with L D d or h D d being ‘equiaxed’. There is evidence that the
usage of the term prism to describe crystal habits which are, in fact, plate-like has led
to confusion in the comparison of laboratory and in situ observations, and Magono and
Lee (1966) do not include prism in their classiŽ cation scheme.
While many previous experiments have provided important insight concerning the
characteristics of ice crystal growth under speciŽ c conditions, the results from some
have, justiŽ ably, been met with criticism as to their relevance to crystal growth in
VAPOUR GROWN ICE CRYSTALS
1463
the atmosphere, especially when compared with in situ observations. The conventional
wisdom obtained from nearly 45 years of laboratory studies and summarized in a
number of habit diagrams (Kobayashi 1961; Magono and Lee 1966; Mason 1971) indicates that prism or columnar growth dominates for temperatures approximately below
¡22 ± C. While prisms were observed below ¡22 ± C in some of these studies, plates
and plate-like polycrystals were also observed, often with similar or greater frequencies. In situ crystal populations with pronounced prism-face growth are observed at
times between ¡22 and ¡40 ± C, but they are not representative of the typical habit
distribution. Tape (1994) observed halo-producing crystals in the Antarctic, and photographed several examples of pristine plates and columns which produced exceptional
halos. However, he noted that such habit observations are not the norm, and that poorly
formed and irregular crystals are most common, leading him to express reservations
about the applicability of the habit diagrams outside the laboratory. The rarity of halo
phenomena in the atmosphere appears to indicate that pristine halo-producing crystals
occur with very low frequency, which generally contradicts the ‘prism’ description of
the habit for temperatures between ¡22 and ¡40 ± C.
Laboratory experiments which simulate atmospheric crystal growth are designed to
reproduce natural conditions and minimize laboratory artefacts, and it is a challenge to
the experimentalist to identify and interpret which results are relevant to the atmosphere.
A comprehensive laboratory study of ice crystal growth, which includes cirrus temperatures and pressures, has been performed by the authors (Bailey and Hallett 2000) and
addresses growth rates and habit in addition to nucleation. The study employed a static
diffusion chamber operated between ¡20 and ¡70 ± C with pressures ranging from 600
down to 150 hPa and ice supersaturations from 0.5 up to 200%. Crystals were grown
on 50–70 ¹m diameter glass Ž laments drawn from molten soda-lime glass. In order to
address and improve the interpretation of the results, the effect on crystal habit and habit
distribution was explored using clean glass Ž laments and Ž laments sparsely covered
with Ž ne particles of silver iodide or kaolinite for temperatures from ¡18 to ¡42 ± C,
ice supersaturations from 0.5% to just above water saturation, and pressures from 500
to 300 hPa, typical of those found in the atmosphere for this temperature range. Critical
supersaturations for nucleation of these different materials were measured for temperatures ranging from ¡18 to ¡70 ± C and are compared with previous studies. The results
from this study in comparison with other experiments and recent in situ observations
imply that the traditional habit diagrams are generally not applicable to the atmosphere
for temperatures lower than ¡20 ± C, and are derived from observations in uenced by
the effects of silver iodide, artiŽ cially cold nucleation methods, short growth times and
the misinterpretation of habit designation.
2.
E XPERIMENTAL DESCRIPTION
A schematic of the thermal diffusion chamber used in this study is shown in
Fig. 1(a). The chamber consists of two stainless steel plates separated by a short thick
acrylic cylinder which yields an inner chamber diameter of approximately 30 cm and
a plate separation of 2.8 cm for a width to height ratio in excess of 10:1, substantially
larger than the minimum ratio suggested by Elliott (1971). The top plate has a stainless
steel wick which can be saturated with water and then frozen. Two double-paned
windows set in recessed ports in the chamber wall allows illumination and observation.
A small  ow of dry air between the panes keeps them ice free so that crystal growth can
be recorded with a time-lapse camera and microscope system. The temperatures of the
1464
M. BAILEY and J. HALLETT
filament
housing
(a)
video
camera
window
top plate
ice
microscope
Tt
Tb < Tt
(b)
glass
filament
2.8 cm
bottom plate
crank
thermocouple
acrylic
30 cm
(c)
Figure 1. (a) The static diffusion chamber with a width to height ratio greater than 10:1. A crank-shaped
thermocouple can be rotated to measure temperatures at any height in the chamber under evacuated conditions.
(b) A graph of the supersaturation with respect to ice in the diffusion chamber as a function of height for a midlevel temperature of ¡30 ± C and a supersaturation of 25%; the top and bottom plate temperatures are given in
the upper right-hand corner. Temperatures (left of the curve) and supersaturation values (right of the curve) are
displayed at 2 mm intervals. (c) A plot of the temperature as a function of height in the chamber as measured with
the crank thermocouple. The circles are the measured values and the dotted line is the predicted linear temperature
proŽ le. The uncertainty in the measured temperature values is approximately equal to the size of the symbol.
top and bottom plates are independently controlled by refrigeration units. The cylinder
walls are 5 cm thick and the entire chamber is surrounded by 10 cm of foam insulation.
A linear temperature and vapour-pressure proŽ le between the top and bottom plates,
together with the exponentially varying equilibrium vapour pressure over ice surfaces
in the chamber, creates the nonlinear supersaturation as a function of height shown
in Fig. 1(b). In principle, a crystal grown at a particular supersaturation under static
conditions is approximately equivalent to a crystal falling at terminal velocity at a
lower supersaturation—the ventilation effect, yielding a higher effective supersaturation
equivalent to the static value (Keller and Hallett 1982). Ventilation effects are negligible
for small crystals with small fall velocities but increase with increasing crystal size. Katz
and Mirabel (1975) have shown that assuming a linear vapour pressure as opposed to a
linear vapour-density proŽ le yields a better representation of the ambient supersaturation
in a thermal diffusion chamber. Supersaturations were calculated from a Ž fth-order
polynomial for the logarithm of the saturation vapour pressure over ice (Washburn
1928), which yielded values that differed from those given in the Smithsonian Tables
by less than 0.3% for temperatures ranging from 0 to ¡100 ± C.
VAPOUR GROWN ICE CRYSTALS
1465
Temperature measurements under normal operating conditions with arrays of thermocouples and a rotatable crank-shaped thermocouple indicate that a linear temperature
gradient between the top and bottom plate is obtained for the central 15 cm of chamber
diameter, within the precision of the thermocouples (0.1 degC) and the accuracy of their
positions, resulting in a total uncertainty of §0.3 degC (Fig. 1(c)). This temperature
uncertainty results in a comparably small uncertainty in the calculated supersaturation
as shown in Fig. 1(b). With sufŽ cient insulation, temperature increases of only a few
tenths of a degree are observed near the walls and windows, but are not observed in the
central region of the chamber. This conŽ rms the need to have a large width-to-height
ratio in order to avoid thermal edge effects and convective instabilities.
Ice crystals were nucleated and grown on glass Ž laments drawn from the molten
ends of soda-lime glass rods heated by a propane torch. The straight Ž laments prepared
in this manner were typically 10 to 15 cm long with tips tapering to diameters of 50 to
70 ¹m. Smaller diameter Ž laments could be drawn, but they had a tendency to bend or
curl up, making them difŽ cult to insert and manipulate in the chamber. Straight Ž laments
were suspended from a support enclosed in a housing on top of the chamber which
allowed the Ž laments to be rotated as well as raised and lowered for viewing crystals
at different heights and angles. Filaments were thoroughly cleaned with ethanol and
lint-free tissues, and then brie y warmed over a hot lamp to remove residual ethanol
and pre-absorbed moisture before inserting them into the chamber. Additional Ž laments
cleaned in this manner were drawn through a creased piece of chemical weighing paper
containing Ž nely powdered kaolinite or gel-grown silver iodide. The detectable particles
left adhering to the Ž lament were 5 to 10 ¹m in size and were visible as tiny dark
spots on the Ž lament. The relatively low magniŽ cation of the microscope/video camera
system did not allow detection of individual particles with sizes smaller than about
5 ¹m; however, their presence was clearly indicated by the distinct differences between
the observed critical supersaturation and the density of nucleated sites on bare glass
Ž laments in comparison with coated glass threads which had all the resolvable particles
removed.
3.
N UCLEATION METHOD
Ice crystals were grown over a range of temperatures at chamber pressures approximately equivalent to the Standard Atmosphere for any particular temperature. Pressures
ranged from 500 hPa at ¡18 ± C to 150 hPa at ¡70 ± C. While performing the cirrus
simulation, it was discovered that the rate of evacuation of the chamber and its effect on
nucleation conditions had a signiŽ cant effect on the habit and habit distribution for crystals growing at nearly all ice supersaturations. The nucleation conditions were primarily
affected by the increase in supersaturation, which depended on the rate at which the pressure decreased, creating an effective supersaturation higher than that predicted from the
temperature difference between the two ice coated plates. This variation of nucleation
conditions was found to in uence both the habit and frequency of polycrystalline ice.
Rapid evacuation of the chamber, roughly equivalent to a large adiabatic expansion
of the chamber volume, left the glass Ž lament densely covered with nucleated sites, a
condition to be avoided since the high density of crystals disturbs the expected supersaturation, due to vapour competition between neighbouring crystals. At low supersaturations and low evacuation rates nucleation could be totally suppressed, indicating that
the effective supersaturation was still below some critical value for nucleation on the
glass substrate or coating particles. Evacuation rates could be controlled for simulations
ranging from the relatively rapid change in pressure for crystals growing in the strong
1466
M. BAILEY and J. HALLETT
updraughts of convective cells with vertical speeds greater than 10 m s ¡1 , down to the
more sedate change encountered in cirrus with vertical speeds of approximately 1 m s ¡1 .
However, neither of these rates is adiabatic since they involve evacuation times on the
order of several or tens of minutes, much longer than the time constants for the diffusion
of heat or vapour in the chamber which is on the order of a few seconds. Since adiabatic
approximations could not be applied, a quasi-static method was devised for determining the conditions under which a minimum density of nucleated sites was produced,
signalling the onset of nucleation at the critical supersaturation. The critical supersaturations obtained with this method for silver iodide and kaolinite for temperatures above
¡25 ± C are in basic agreement with earlier measurements by more traditional methods
(Mason and van den Heuvel 1959; Roberts and Hallett 1968). However, the critical
supersaturations measured for temperatures below ¡30 ± C for all materials used in this
study show unusual behaviour previously unreported.
To determine the critical supersaturation at a particular temperature, a coated or
uncoated Ž lament was inserted into the chamber with both the top and bottom plates
set to the target temperature, yielding an apparent ice supersaturation of zero. This
environment was actually slightly subsaturated, apparently due to window radiation,
since it was found that a supersaturation of approximately ¾ D 0:003 (0.3%) was
necessary to maintain a 50 ¹m diameter crystal for several hours at ¡30 ± C with no
apparent growth or sublimation. Filaments were allowed to sit in this environment
for an hour or more in order to make sure no nucleation had occurred from preabsorbed water. Once the absence of any premature nucleation was conŽ rmed, the
temperatures of the top and bottom plate were increased and decreased, respectively, at
a rate of approximately 0.3 degC min ¡1 . The corresponding supersaturation increased
at a similarly slow rate until one or two crystals were observed to nucleate, typically
along the central few millimetres of the glass Ž lament where the supersaturation is a
maximum.
The observed critical supersaturations are shown in Fig. 2. For bare glass Ž laments,
the critical supersaturation at which one or two crystals Ž rst appear are indicated by
£, while C represents the supersaturations where roughly a doubling of nucleated sites
occurred. These data include measurements below ¡40 ± C (Bailey and Hallett 2000).
The results show that an ice supersaturation close to water saturation is necessary for the
initial nucleation of sites on bare glass Ž laments for temperatures down to about ¡38 ± C.
At ¡40 ± C and below, a fairly constant critical supersaturation of about 25% is required.
Above ¡38 ± C, additional nucleation on clean glass Ž laments increased fairly rapidly
above the critical supersaturation which was near water saturation. Below ¡38 ± C, glass
exhibited poor nucleability at the critical supersaturation and even somewhat above
this value. Apparently only the most easily activated sites were involved under these
conditions.
The silver iodide (¦) and kaolinite ( ) measurements on Fig. 2 represent only the
initial appearance of nucleated sites, with somewhat less than 0.1% of the particles on
the Ž lament being nucleated. A small increase of the supersaturation above the critical
value led to substantially more additional nucleation. Initial nucleation on silver iodide
and kaolinite coated Ž laments usually occurred in a few places simultaneously near
the middle of the chamber. This involved a temperature span of about 1–2 degC and a
variation of supersaturation of 1–2% (see Fig. 1). The error bars shown for these data
represent the range of values observed for similar temperatures and supersaturations,
the error in any one value being much smaller. Nucleation on clean glass Ž laments
was much more site speciŽ c with fewer sites initially activated, indicative of the poorer
nucleability of glass in comparison with silver iodide and kaolinite particles.
1467
VAPOUR GROWN ICE CRYSTALS
glass
AgI
kaolinite
glass
w
at
er
sa
tu
ra
tio
AgI
n
kao
kao
AgI
Figure 2. A plot of the critical supersaturation required for nucleation on bare glass Ž laments (£; C) and glass
Ž laments coated with silver iodide (4) and kaolinite ( ). For bare glass threads, £ represents supersaturation measurements at which one or two crystals were observed to nucleate and C represents supersaturation measurements
where approximately a doubling of the number of nucleated crystals occurred. The linear Ž ts from approximately
¡25 to ¡12 ± C in the lower right are from the nucleation studies of Roberts and Hallett (kaolinite; 1968) and
Mason and van den Heuvel (silver iodide; 1959). The upper and lower Ž tted results from Roberts and Hallett for
kaolinite (long dashes) represent results from initially activated nuclei and pre-activated nuclei, respectively.
The results from this study appear to converge smoothly with earlier nucleation
results at temperatures higher than approximately ¡25 ± C. In the study of Roberts and
Hallett (1968) kaolinite dust was allowed to fall on a glass slide where the nucleation
behaviour was observed with a microscope and the behaviour of individual particles was
followed. Additionally, they measured the critical supersaturations for initially activated
kaolinite particles (upper dashed line) and for pre-activated particles (lower dashed line).
The pre-activated results nearly coincide with the results for silver iodide activation
from Mason and van den Heuvel (1959; solid horizontal line) which extend along the
line of water saturation above ¡12 ± C. Mason and van den Heuvel injected silver iodide
smoke into a diffusion chamber and measured the nucleation of particles in free fall; the
results coincide with the silver iodide epitaxial results of Bryant et al. (1959). The error
bars shown for these datasets represent the span of observed values in those studies.
Unlike this investigation, these earlier studies were conducted at laboratory or room
pressure; however, this does not appear to have had any noticeable effect on nucleation
for temperatures above ¡25 ± C.
Above ¡38 ± C, glass clearly has a much poorer nucleability compared with kaolinite and silver iodide, and below this temperature these materials nucleate at moderate ice
supersaturations well below water saturation. The most striking result from this study
is the fairly abrupt change in critical supersaturation observed with glass and silver
iodide, and the less pronounced but still prominent change observed with kaolinite. The
abrupt change in nucleability for glass at approximately ¡38 ± C coincides with a readily
1468
M. BAILEY and J. HALLETT
observable change in crystal habit observed in the cirrus study (Bailey and Hallett 2000)
that occurs between ¡38 and ¡40 ± C, speciŽ cally a shift from predominantly plate-like
behaviour to columnar growth that occurs for ice supersaturations in excess of about
15%. It is intriguing that this transition also occurs near the temperature where theory
and observations indicate homogeneous nucleation of supercooled pure water drops in
the atmosphere.
The changes in nucleability for glass and silver iodide are relatively sharp in comparison with the change for kaolinite which shows a smaller change at a slightly lower
temperature. Glass and silver iodide are very insoluble, and the observed nucleability
for temperatures higher than ¡38 ± C may indicate that microscopic supercooled drops
initially form on their surfaces before freezing. Evidence for this possibility comes from
the observation of clean glass Ž laments during evacuation of the chamber. Cracks on
the Ž laments, normally barely visible with diffuse backlighting, were observed to darken
slightly when the chamber was evacuated at a rate such that the effective supersaturation
was near the critical value. If the evacuation was interrupted before nucleation occurred,
the cracks faded back to their original appearance, probably indicating the evaporation
of micron sized supercooled drops that formed along or in the cracks. This process is
clearly observed at ¡15 ± C where, with the chamber ice supersaturation set below water
saturation, 50 ¹m supercooled drops form during evacuation of the chamber, and subsequently evaporate without nucleation once evacuation has ceased. Kaolinite contains
varying amounts of soluble materials which may form solutions on kaolinite particles
prior to nucleation, this might explain why its peak is shifted to a slightly depressed
temperature in comparison with both glass and silver iodide. Once again, evidence for
this comes from the observation of a slight darkening of kaolinite particles that could
be detected just before crystal growth was observed at temperatures down to ¡40 ± C,
possibly indicating the formation of weak solution drops prior to freezing.
A high degree of cleanliness was maintained during operation of the chamber.
After each experiment the chamber was brought up to atmospheric pressure through
an absolute Ž lter which permitted a low  ow of air into the chamber. The formation
of resolvable nucleated crystals during this ‘airing’ of the chamber was not observed,
except at temperatures lower than ¡60 ± C and ice supersaturations in excess of approximately 50%, indicating the effectiveness of the Ž lter in removing particles from the
room air. Exposure of the chamber to the laboratory air was always minimized, and a
thorough cleaning of all inside surfaces was performed periodically.
In initial experiments, glass Ž laments were stored in a covered box prior to being
placed in the chamber and were only wiped with a tissue to remove any visible lint or
dust. These Ž laments more readily nucleated than those that were subsequently cleaned
with ethanol and warmed just before insertion. In fact, glass Ž laments, properly cleaned
and then placed in the chamber at supersaturations below the critical supersaturation,
were often observed to experience no nucleation after several hours. If a substance
capable of affecting the observed critical supersaturations was present in the laboratory,
it would have been likely to have affected all the nucleation measurements, and the
results for glass, kaolinite, and silver iodide would be expected to be more similar
due to the presence of a common contaminant. It is worth noting that the high desert
environment of Reno usually has low humidity and Ž ne, wind blown dust consisting
mostly of decomposed granite and clays, most of which is removed from the conditioned
laboratory air.
Ice crystals in the atmosphere sometimes nucleate at the temperature, pressure, and
supersaturation at which they grow, and at other times they nucleate and grow under
somewhat different conditions. Because of the existence of critical supersaturations,
VAPOUR GROWN ICE CRYSTALS
1469
growth experiments were performed in three different ways. If the chosen supersaturation for a particular growth study was below the critical supersaturation for that
temperature, the chamber was slowly evacuated to a pressure just above the target
pressure. At that point, the evacuation was sharply increased for 1–2 seconds and then
quickly terminated, the pressure dropping by 10–15 hPa and effecting a quasi-adiabatic
expansion of the chamber volume which raised the supersaturation by an amount
sufŽ cient to meet or just exceed the critical value. The appearance of at most a few
crystals by this method indicated that a critical supersaturation was reached where only
the most active nuclei, in the case of silver iodide and kaolinite particles, or only the most
active sites on the uncoated glass Ž laments, nucleated. If the chosen supersaturation for
growth was at or near the critical supersaturation, the chamber pressure could be slowly
reduced over a period of 10 minutes or so, such that only a few crystals nucleated. Above
the critical supersaturation, the thread could be withheld in the Ž lament housing until
evacuation was completed at a temperature approximately equal to that of the top plate.
Lowering the thread then caused both nucleation and growth to occur above the critical
supersaturation as the Ž lament further cooled to chamber temperatures. Even this Ž nal
method led to the nucleation of crystals with a density of a few crystals per millimetre
of Ž lament length, or a typical initial separation of a few hundred micrometres.
4.
C RYSTAL HABIT AND HAB IT DISTRIBUTION
Crystals with sizes greater than approximately 50 ¹m have been examined over a
broad range of temperatures, pressures, and ice supersaturations. The well established
habit transitions between 0 and ¡18 ± C as a function of temperature and supersaturation have consistently been reproduced. An advantage from using this set-up is that a
spectrum of temperatures and ice supersaturations is obtained allowing the simultaneous
growth of crystals under different conditions, transitions between habit regimes being
readily observable. The habit distributions of crystals observed in this study for temperatures below ¡18 ± C are shown in the bar graphs of Fig. 3. The results for silver iodide,
kaolinite and glass have been arranged in columns according to temperature, and the
habits have been categorized as polycrystals (cross-hatched bars), plates (white bars),
and columns (black bars). Ice supersaturations are characterized by low (1–5%), intermediate (12%) and high (22–47%), to just above water saturation. The bars represent
averages from several datasets, and have values with uncertainties ranging from about
2 § 1% to 90 § 10%. One percent black bars have been used to indicate little or no
observance of columns. Examples of crystals can be found in Fig. 4. The images shown
include crystals grown approximately within §2 degC of the stated temperatures which
also includes an associated small change in supersaturation as can be seen in Fig. 1(b).
(a) ¡20 ± C, ¾ice D 1–5%
At ¡20 ± C and an ice supersaturation of 1%, the habit for all three nucleating
materials was very similar and dominated by plates and polycrystals; columns appeared
with low frequency. Plates were generally thick with clear planar faces. Polycrystals
were typically compact and irregular with incomplete plate-like crystals growing from
a common boundary or a central hub in radiating assemblages, or they were irregularly
faceted like nuggets of quartz. Glass produced polycrystals more often than thick plates,
and columns were rarely observed to grow on glass at any supersaturation at ¡20 ± C.
At supersaturations of 2% or greater, plates grown on glass and kaolinite developed
skeletal features and steps, and were often asymmetric or irregularly faceted. Silver
iodide exhibited a small peak frequency for short columns at a supersaturation of 2%.
M. BAILEY and J. HALLETT
Percent Observed
1470
Ice Supersaturation
Figure 3. The habit distribution of ice crystals grown on clean glass Ž laments, and Ž laments sparsely coated
with kaolinite and silver iodide particles. The habit is categorized according to polycrystals (cross-hatched bars),
plates (white bars) and columns (black bars). Ice supersaturations equivalent to water saturation are indicated by
w.s. at the bottom of the Ž gure.
Columns typically had ratios L=d ¼ 1:4, and occasionally up to L=d ¼ 2, though about
15–20% of crystals were equiaxed. At supersaturation of 2–5%, the habits from glass
and kaolinite were very similar and dominated by polycrystals. In contrast, silver
iodide produced thick regular plates and short columns with high frequency up to a
supersaturation of about 5%. In comparison with glass and kaolinite, the crystals grown
on silver iodide were somewhat more symmetric, and irregular crystals occurred less
frequently. For kaolinite and silver iodide and supersaturations less than or equal to
about 5%, polycrystals were typically compact and irregular, while glass often produced
radiating assemblages of plates.
(b) ¡20 ± C, ¾ice D 12% to water saturation
At supersaturations in excess of 12%, polycrystals dominated the habit for all
three nucleating materials, single plates appearing with modest frequency. Polycrystals
typically consisted of radiating assemblages of skeletal plates or irregular plates with
well-developed step features. Plate-like features became progressively thinner as supersaturation increased, occasional crossed plates and spearhead crystals appearing around
12%. While the habit distribution of kaolinite was quite similar to glass, plates and platelike polycrystals grown on glass were consistently thinner and had considerably more
skeletal features than those grown on kaolinite. At water saturation (22% with respect to
ice), crossed plates with intersecting planes and very thin (h=d ¼ 0:05) large plates, both
VAPOUR GROWN ICE CRYSTALS
1471
skeletal and sector-like, were prevalent. Irregular assemblages of plates, large versions
of types G5 and G6 in the Magono and Lee classiŽ cation (1966), were also observed.
Another common type of ‘crossed plate’ polycrystal often observed at moderate
to high supersaturation was an assemblage of overlapping plates or planes where all
the c axes were parallel, the plates growing to similar size concurrently from common
boundaries but not radiating from a centre. Several examples of these can be found
in Fig. 4 at ¡20 and ¡30 ± C, and they will be referred to as ‘parallel crossed plates’
as opposed to the intersecting type where side planes grow from a speciŽ c grain
boundary (Furukawa and Kobayashi 1978). These were similar to class G5 crystals,
described as ‘assemblages of minute plates’ by Magono and Lee, but that classiŽ cation
includes assemblages where the c axes of the component plates are not parallel. At water
saturation and above, the parallel plates or side planes grew very large and extremely
thin (h=d < 0:05). Parallel crossed plates occurred with about the same frequency as
the intersecting type of crossed plates, both occurring in conjunction with irregular
assemblages of plates, classes G5 and G6, and crystals of the side plane type, S1 and
S2.
The main conclusions drawn from the results at ¡20 ± C is that the habit is predominantly plate-like, when considering both single plates or plate-like polycrystals which
dominate the habit distribution. Silver iodide appears to have an orienting effect in that it
can lead to the appearance of columns and pristine plates at low supersaturation, while
both kaolinite and glass tend to grow irregular plates and polycrystals. The crystals
grown on silver iodide also exhibited fewer irregularities at all supersaturations than did
those grown on kaolinite and glass.
(c) ¡30 ± C, ¾ice D 1–5%
At supersaturations of 1–2%, the habit observed at ¡30 ± C for all three materials
was nearly the same as at ¡20 ± C, columns again appearing with low but slightly
increased frequency and plates appearing to be thicker than those at ¡20 ± C for similar
crystal sizes. At supersaturations of 5%, polycrystals were much more prevalent with
glass and kaolinite than with silver iodide and were relatively thick and compact. One
unusual type of crystal peculiar to silver iodide which was observed on rare occasions in
this study was the ‘long prism’ type identiŽ ed by Shimizu (1963). These columns, with
20 · L=d · 200, were observed at the ground by Shimizu in the Arctic and by Tape
(1994) in the Antarctic at temperatures from ¡30 to ¡60 ± C. They were observed in this
study between ¡30 and ¡40 ± C but only when silver iodide was used for nucleation,
and they grew only at ice supersaturations less than 2%. An example of one of these
columns, grown at a temperature of ¡31 ± C and an ice supersaturation of only 0.5%
(¾ D 0:005), is shown in Fig. 4 (¡30 ± C, A, ¾ D 1%).
(d) ¡30 ± C, ¾ice D 12% to water saturation
For supersaturations ranging from 12–25%, silver iodide yielded a habit which
was roughly an equal mix of thick plates, columns and faceted polycrystals; columns
occurred with greater frequency than that observed at ¡20 ± C. Thick plates and columns
tended to be fairly symmetric and columns were often observed to have length to
diameter ratios of 1:5 · L=d · 2:5. Polycrystals were compact and often contained
both plate-like and short columnar structures. In comparison to this, the habits for
glass and kaolinite were dominated by plate-like polycrystals with few simple plates
and even fewer columns. These polycrystals were typically crossed-plates and irregular
assemblages of plates.
Figure 4. Examples of the crystal habits for ice grown on clean glass Ž laments (G), Ž laments with kaolinite (K) and silver iodide particles (A). Temperatures ranged from
¡18 to ¡40 ± C, and ice supersaturations ranged from 1% to a few percent above water saturation for the corresponding temperature. The glass Ž laments shown are 50–70 ¹m
in diameter and crystals have dimensions of approximately 300 ¹m. Crystals grown at the same supersaturation have been grown for approximately the same amount of time
ranging from tens of minutes to a few hours. (w.s. indicates ice supersaturations equivalent to water saturation.)
1472
M. BAILEY and J. HALLETT
1473
=
Figure 4.
Continued.
VAPOUR GROWN ICE CRYSTALS
1474
M. BAILEY and J. HALLETT
At water saturation (33% supersaturation with respect to ice), the habit for silver
iodide consisted of skeletal plates with ‘hopper’ structures, plates with steps, and
occasionally short robust sheaths with L=d ¼ 2. For kaolinite and glass, the polycrystals
consisted of complex assemblages of skeletal plates, crossed plates, spearheads, side
planes (bottom left of Fig. 4), and irregular crystals. The orienting effect of silver iodide
more strongly affected the habit and distribution at ¡30 ± C than it did at ¡20 ± C, leading
to the appearance of more columns and fewer irregular crystals in general.
(e) ¡40 ± C, ¾ice D 1–5%
The low-supersaturation habits at ¡40 ± C for glass, kaolinite and silver iodide were
very similar to those seen at ¡20 and ¡30 ± C, glass and kaolinite exhibiting a small
increase in the frequency of columns compared with ¡30 ± C. Silver iodide produced
clear symmetric plates in abundance at 1% supersaturation. Polycrystals consisted of
thick plate-like features with an increasing degree of short columnar structures. Silver
iodide produced plates with a frequency similar to glass and kaolinite, however the
plates were generally more regularly faceted and symmetric.
( f ) ¡40 ± C, ¾ice D 12% to water saturation
It is under these conditions that the greatest differences between glass, kaolinite and
silver iodide were observed. At 12% supersaturation, the habits for glass and kaolinite
were nearly the same, with plate-like polycrystals dominating the habit distribution,
plates and columns occurring with lower frequency. An increased frequency of columnar
growth was observed with silver iodide, and many of the polycrystals grown on silver
iodide contained columnar structures. Polycrystals grown on kaolinite also showed some
columnar structures but with frequencies intermediate to silver iodide and glass, glass
showing relatively few such features.
When glass Ž laments were used, the habit was still dominated by plate-like polycrystals down to temperatures between ¡38 and ¡40 ± C, even up to water saturation
(47% supersaturation with respect to ice at ¡40 ± C). Columns, though small in number,
appeared with increasing frequency in this region. However, a fairly sharp transition
to predominantly columnar behaviour occurred just below ¡40 ± C. Above ¡38 ± C, the
infrequent columns were typically short with 1:4 · L=d · 2, even near water saturation.
Between ¡38 and ¡40 ± C, and above a saturation of about 15%, columns increased in
frequency and aspect ratio with 2 · L=d · 3. Below ¡40 ± C and at supersaturations
above 20% or so, ‘bullet rosettes’ and columns with 3 · L=d · 6 dominated the habit.
Silver iodide and kaolinite both grew columns in greater frequency than glass at
¡40 ± C at a supersaturation of 25%, and this trend increased as water saturation was
approached. The most striking difference, however, was the fact that bullet rosettes
grew with high frequency on bare glass and were rarely observed with silver iodide
and kaolinite as shown in Fig. 5. This is consistent with the polycrystalline character
expected from the homogeneous freezing of supercooled droplets at low temperature
(Hallett 1964). The closest form to a rosette observed on silver iodide and kaolinite
were ‘stepped’ columns (two columns growing simultaneously side by side) or columns
with a single branch and a ‘T’ shape. The rosettes grown on glass at ¡40 ± C typically
had three to Ž ve ‘bullets’, and at times six.
Polycrystals grown on silver iodide typically consisted of short columns and thick
plates growing from a common boundary, or from a well-faceted polycrystalline hub.
Polycrystals grown on kaolinite and glass (in this case non-rosette polycrystals) typically
contained both long columnar bullets and plate-like polycrystals, also growing from a
complex polycrystalline hub.
VAPOUR GROWN ICE CRYSTALS
1475
= 400 m
Figure 5.
Examples of columnar forms observed at ¡41 ± C and a supersaturation of approximately 35%.
(g) Summary of habit observations
These results reveal distinct differences between the crystal habits obtained with
silver iodide in comparison with those from kaolinite and glass which are quite similar.
Silver iodide encouraged the growth of columns at ¡20 ± C and low supersaturation,
whereas kaolinite and glass exhibited little or no columnar growth. At all temperatures,
silver iodide consistently led to a greater occurrence of regularly faceted columns and
plates. Plates and short columns grown on kaolinite and glass were more likely to be
somewhat irregular or asymmetric, and polycrystals grew with much higher frequency
on these two materials. This clearly demonstrates an orientating effect from the use
of silver iodide, and may explain why past studies using silver iodide smoke reported
frequent columns and plates and few polycrystals, especially when considering crystals
smaller than about 30 ¹m in size that resulted from very short growth times. Kaolinite
had a similar in uence on crystal habit, though this effect was not nearly as strong as
with silver iodide and is consistent with the microstructure of clays which contain small
crystals of various shapes with ice-related crystallographic orientations. If one includes
thick plates along with short columns, then prisms are indeed the dominant habit for
crystals grown on silver iodide below approximately ¡22 ± C, but this is not the case for
kaolinite or glass.
While crystals smaller than about 50 ¹m could not be clearly resolved with the
long-working-distance microscope and zoom camera used in this study, those crystals
that became polycrystals or developed secondary features (steps, etc.) could usually be
identiŽ ed as such by the time they were approximately 50 ¹m in size. This was easily
veriŽ ed with time-lapse photography which could be viewed in forward or reverse,
allowing detailed analysis of the growth of the smallest resolvable features. Digital
conversion and analysis revealed additional details not visible in standard images. This
method revealed that single crystals were never observed to grow to an appreciable
size and then change to a polycrystalline growth mode; however, they were observed to
develop steps, layers, and skeletal features as they increased in size.
The crystals shown in Fig. 4 are representative of the most common forms observed
at the stated temperatures and supersaturations, and are generally restricted to crystals
with sizes no larger than 300 ¹m. Near water saturation and above, many polycrystals
achieve forms of such complexity that they are indistinguishable from aggregates
observed in the atmosphere, especially when allowed to grow to lengths beyond 300 ¹m.
1476
M. BAILEY and J. HALLETT
Some examples can be seen at the bottom of Fig. 4; they typically appear to be
aggregates of plates, crossed plates, side plane forms, scrolls, spearheads, and even
columns or sheaths at temperatures near ¡40 ± C. Aggregation is an important process
affecting the optical and microphysical properties of ice clouds, and the results from this
study indicate that some crystals identiŽ ed as aggregates from in situ observations are
complex polycrystals growing from a single growth centre, in spite of their appearance.
5.
C OMPARISON OF RESULTS WITH PREVIOUS EXPERIMENTS
Many laboratory ice-growth experiments have been performed over the past several
decades and have involved a number of different techniques. These have ranged from
diffusion chamber studies (Kobayashi 1957, 1961; Hallett and Mason 1958; Rottner
and Vali 1974), cold box/supercooled cloud experiments (Schaefer 1949; Nakaya 1954;
Magono and Sakai 1970; Kumai 1982; Yamashita et al. 1984), cold stage/microscope
approaches (Gonda 1983), epitaxial studies (Kobayashi 1965; Anderson and Hallett
1976; Cho and Hallett 1984), fall tower experiments (Yamashita 1971) and, most
recently, substrate-free experiments involving electrodynamic trapping and suspension
of crystals in a diffusion chamber (Bacon et al. 2000). In most of these studies,
nucleation was forced by exposing a supercooled cloud to silver iodide smoke or
to a cold surface chilled with volatiles like liquid nitrogen or dry ice. Controlled
supersaturations ranged upwards from less than 1% with respect to ice to well above
water saturation. At other times, as with supercooled clouds, the supersaturation was
unknown but less than water saturation. Crystals grown by these methods had varying
sizes depending on the growth times which ranged from several seconds to hours. The
results from a number of these studies are summarized in Table 1 with a comparison of
those obtained in this study.
Several conclusions can be drawn from this comparison. Between ¡18 and ¡40 ± C,
the habit is dominated by plates and plate-like polycrystals, columns generally appear
with low frequency. Epitaxial studies in this temperature range conŽ rm this behaviour.
Some of the habit distributions are dominated by single crystals which are characterized
by small crystal size, typically less than 50 ¹m, and short growth times on the order
of a hundred seconds. These were observed in studies where the supersaturation was
small or undetermined, and most often in the cases where silver iodide was used for
nucleation. A predominance of single crystals was observed in the present study but
only at ice supersaturations less than about 5%. In those cases with undetermined
supersaturation, a supercooled fog or nebulized cloud of droplets was rapidly nucleated
and ice crystals formed with very high concentration. While the supersaturation was
initially near water saturation, the high crystal concentration following nucleation was
likely to have decreased the effective ice supersaturation to very low values due to
vapour competition, subsequently reducing growth rates.
Columns appeared most often in those studies using silver iodide, conŽ rming
the orientating effects of silver iodide as observed in this investigation. The extreme
example of this effect is the epitaxial study by Kobayashi (1965) where crystals were
grown on lead iodide crystals at temperature of ¡40 ± C and below. The habit was
exclusively prism or column-like for all supersaturations as would be expected on a
highly oriented substrate below ¡40 ± C, the columnar habit regime.
The study by Kumai (1982) best exempliŽ es these conclusions. That study involved
very short growth times and an unknown supersaturation, and resulted in the greatest
frequency of solid columns between ¡25 and ¡35 ± C, the number increasing with
decreasing temperature. This also agrees with the present results taking into account
1477
VAPOUR GROWN ICE CRYSTALS
TABLE 1.
C OMPA RISON WITH PREVI OUS LABORATORY RESULTS
T (± C) Investigator(s)
Method
Yamashita et al.
(1984)
¡20 Yamashita et al.
(1984)
¡20 Cho and Hallett
(1984)
¡24 Anderson and
Hallett (1976)
¡23 to Yamashita
¡26 (1971)
AgI
sc fog
dry ice
bentonite
epitaxial
covelite
epitaxial
AgI
LN, fall
tower
¡20 to Rottner and
¡24 Vali (1974)
diff. chmb
Ž lament
(hair)
¡22 to Magono and
¡27 Sakai (1970)
¡25 Kumai
(1982)
¡35 Kumai
(1982)
¡23 to Kobayashi
¡32 (1961)
convect.
cold box
liq prop sc
fog
liq prop sc
fog
diff. chmb
Ž laments
1–120%
electro.
trap
¡20
¡20 to Bacon et al.
¡40 (2000)
Gonda
(1983)
¡40 to Kobayashi
¡90 (1965)
¡40
AgI
sc fog
eptaxial
PbI
¾ice
tgrow
(d)
Habit description
Comparative
results
45–60% single crystals, ¾ D 5–12%, AgI
plate-like polycrystals
<ws
90–95% polycrystals
¾ D 5–12%, kao
plate-like polycrystals
and glass
low
thin and thick plates
¾ < 5%, kao
and glass
1–15%
thin and thick plates
¾ < 5%, kao
and glass
<ws
10% hexagonal crystals, ¾ D 12–22% kao
55% plate-like polys,
and glass
35% irregular
1%–ws
hrs
some solid cols at
¾ D 1% to ws
(d < 1 mm) low ¾ , thick plates,
kao and glass
skeletal plates,
plate-like polycrystals
. ws
NA
plate-like polycrystals
¾ near or above
(d < 1 mm) scrolls, crossed plates
water sat.
<ws
¼10 s
60% plates, 20% short
¾ < 2%, AgI,
d < 50 ¹m cols, 20% polycrystals
kao or glass
<ws
¼10 s
30% plates, 50% short
¾ < 5%, AgI, kao
d < 50 ¹m cols, 20% polycrystals
or glass
1–118%
min/hrs
‘prisms’ D thick plates ¾ < ws, AgI, kao
(d < 200 ¹m) and short columns
and glass
with c=a ¼ 1:4
up to
min/hrs
polycrystals and plates ¾ < ws, kao
ws
·500 ¹m
from frozen drops
and glass
and frost particles,
few short columns
2–3%
min d <
55% single crystals
¾ < 5%, AgI, kao
50 ¹m
45% polycrystals
or glass
low–high
min/hrs
‘prisms’ D short and
¾ > 12%, AgI
· 500 ¹m long columns
and kao; glass
for T < ¡40
<ws
<200 s
<100 ¹m
<200 s
<100 ¹m
min/hrs
<100 ¹m
min/hrs
<100 ¹m
<15 min
(d < 200 ¹m)
The last column lists results from this study which are most similar to the indicated investigation. ¾ D ice
super-saturation; tgrow D growth time; sc D supercooled; LN D liquid nitrogen; diff. chmb D diffusion chamber;
convect. D convective; liq prop D liquid propane; electro. trap D electrodynamic trap; a D radius; c D vertical
axis; d D dimension; NA D data not available; ws D water saturation.
that the crystals grown by Kumai were nucleated with liquid propane at a temperature
of approximately ¡42 ± C, a predominantly columnar growth temperature regime. Due
again to large crystal concentrations, the supersaturation was likely to have been only a
few percent which is consistent with the high frequency of plates observed in this study
at low supersaturation. Unlike temperatures between ¡4 and ¡15 ± C where rapid habit
changes can be observed, crystals which nucleate at a columnar growth temperature
near ¡40 ± C and then grow in the plate regime between ¡20 and ¡40 ± C will retain
their shape since the growth rate is slow at these temperatures.
6.
C OMPARISON OF RESULTS WITH IN SITU OBSERVATIONS
In situ observations have typically involved gathering crystals at the ground, or sampling clouds with airborne replicators and two-dimensional collecting (2D-C) probes.
Correlating collection at the ground with growth conditions aloft is problematical and
1478
M. BAILEY and J. HALLETT
TABLE 2.
T (± C)
C OMPA RISON WITH IN SITU OBSE RVATIONS
Investigator(s) and Imaging
location
method
Kikuchi and
Kajikawa (1979)
Arctic
¡20 to Kikuchi (1970)
¡35
Antarctic
<¡20
replicator
microscope
and replicator
<¡25
Sato and Kikuchi
(1989) Arctic
microscope
<¡35
Kajikawa et al.
(1980) Arctic
microscope
¡25 to
¡40
¡30 to
¡45
Shimizu (1963)
microscope
Antarctic
Kajikawa and
2D-C probe
HeymsŽ eld (1989)
continental cirrus
¡20 to Korolev et al.
¡45
(1999)
Arctic
SPEC Inc.
CPI probe
¡20 to Korolev et al.
¡45
(2000)
stratiform: Arctic,
North Atlantic,
Great Lakes
2D-C probe
Habit description
This study
polycrystals;
seagull, spearhead
side plane
plate-like polycrystals, side
planes, skeletal plates,
scrolls
polycrystals; spearhead,
gohei twins, seagull
glass and kaolinite,
T < ¡25,
¾ near or > ws
glass and kaolinite,
near ws, T < ¡25
plates, short columns,
plate-like polycrystals, side
planes, crossed plates,
bullet rosettes,
plate/column aggregates
thick plates, columns long
‘prisms’ c=a > 10
mostly polycrystals;
plates, short columns,
side planes, crossed
plates, bullet rosettes,
plate/column aggregates
mostly polycrystals; few
plates and short columns,
plate-like polycrystals, side
planes,
bullet rosettes,
plate/column aggregates
mostly polycrystals;
irregular plate-like polycrystals,
side planes,
spatial branches
(spearheads?)
glass and kaolinite
T < ¡25,
¾ near or > ws
glass, kao, ¡30 < T < ¡45,
¾ ¼ 12%, up to ws,
short columns T > ¡40,
bullet rosettes T < ¡40,
crossed plates near ws
AgI, T < ¡30,
¾ < 2%
glass, kao, ¡30 < T < ¡45,
¾ ¼ 12%, up to ws,
polycrystals dominate; short
columns T > ¡40,
bullet rosettes T < ¡40,
crossed plates near ws
glass, kao, ¡20 < T < ¡45
¾ ¼ 12%, up to ws,
polycrystals dominate;
short columns T > ¡40,
bullet rosettes T < ¡40,
crossed plates near ws
glass, ¡20 < T < ¡45,
¾ ¼ 12%, up to ws,
polycrystals dominate the
habit
The last column lists results from this study which are most similar to the indicated investigation. The Ž rst Ž ve
entries involve crystals gathered at the ground and the last three were obtained from aircraft. Crystals ranged in
size from 100 ¹m to over 1 mm. ws is the point where ice supersaturations are equivalent to water saturations.
See Table 1 for abbreviations.
requires accurate sounding measurements. The collection efŽ ciency of airborne probes
is different for different crystal sizes and the correlation of results from different studies
has at times been difŽ cult. A review of several in situ studies is presented in Table 2
where they are compared with the habits observed herein. These studies were performed
in the Arctic (Kikuchi and Kajikawa 1979; Kajikawa et al. 1980; Sato and Kikuchi
1989; Korolev et al. 1999, 2000), the Antarctic (Shimizu 1963; Kikuchi 1970), and in
continental or tropical cirrus and stratiform settings (Kajikawa and HeymsŽ eld 1989;
Korolev et al. 2000). Because of imager resolution, these crystals were generally larger
than 100 ¹m.
The most notable feature is the predominance of polycrystalline forms, which typically consisted of irregular plate-like crystals, side plane types, assemblages of plates,
scrolls, gohei twins, and other types. Pristine plates and columns were observed with
low frequency for temperatures higher than ¡40 ± C, and columns were generally short,
increasing in aspect ratio with decreasing temperature. Large single plates typically had
pronounced skeletal features. Bullet rosettes were at times observed at temperatures as
VAPOUR GROWN ICE CRYSTALS
1479
high as ¡35 to ¡40 ± C, however, based on the results in Fig. 5, these crystals were likely
to have fallen into the collection region from colder altitudes above.
Korolev et al. (1999) employed a Cloud Particle Imager (CPI) with a resolution of
2.3 ¹m developed by Lawson (1997) in a study of the crystal habits in Arctic clouds
for temperatures ranging from 0 to ¡40 ± C . They found that polycrystals regularly
comprised 80 to 90% of the observed habit distributions. A subsequent study employing
a 2D-C optical probe with 25 ¹m resolution was also used by Korolev et al. (2000) to
study ice particle habits in stratiform clouds for temperatures ranging from 0 to ¡45 ± C
with similar results. These results are clearly consistent with the habit distributions in
Fig. 3 for crystals grown on glass Ž laments with ice supersaturations of 5% or greater,
and they are also in agreement with the results for kaolinite for this temperature range.
Additionally, the in situ habits for crystals with sizes in excess of approximately 70 ¹m
are also in substantial agreement with the habits observed here and in the cirrus study
performed by the authors. As in the present study, Korolev et al. (2000) conclude
that “an ice particle keeps an irregular shape during its growth starting from at least
40–50 ¹m in size”. These observations imply that the habit of a crystal is established at
an early stage, and that defects leading to polycrystalline forms continue to propagate
with the growing crystal, both in the case of substrate and substrate-free growth.
Habit distributions consisting primarily of small pristine crystals are observed at
times. The comparison of this study with previous experiments suggests that such a
distribution might result from the following. A parcel of sufŽ ciently moist air reaches
the critical supersaturation for the IN present, which could range from only 15% or so
for clay-like nuclei, up to water saturation for IN with very poor nucleability. Nucleation
just occurs and the initial growth proceeds under conditions of rapidly diminishing
ice supersaturation, cutting off the crystals from any rapid growth. If the environment
remains only weakly supersaturated with respect to ice, such distributions would persist
for relatively long periods of time with the habit basically unchanged and only a small
increase in crystal size.
7.
I MPLICATIONS FOR THE ATMOSPHERE
The largest frequency of pristine crystals observed in this study and others generally
occurred at very low supersaturation, mostly less than 2% with respect to ice. Atmospheric crystal populations with dimension less than about 50 ¹m sometimes exhibit
an even higher frequency of pristine crystals than that seen in the laboratory; however,
populations of large pristine crystals with dimensions of 100–200 ¹m are rare. In his
observations of Antarctic halos, Tape (1994) concludes that pristine crystals with dimensions of at least 40 ¹m, and more likely in excess of 100 ¹m, are generally required to
produce halos. He also notes that, while even spatial polycrystals can at times produce
halos, the main requirement for any halo-producing crystal is that it has nearly perfect
planar faces. The results from this study and in situ observations reveal that most crystals
which grow at temperatures from ¡20 to ¡40 ± C, to sizes greater than 100 ¹m, and at
supersaturations in excess of a few percent, typically have poorly developed facets and
are polycrystalline, characteristics which increase with increasing supersaturation and
crystal size. Since crystals smaller than about 10–20 ¹m do not create perceptible halos
because of diffraction spread, this indicates that halo crystals generally fall within a
narrow range of sizes with dimensions less than about 200 ¹m yet larger than about
20 ¹m.
1480
M. BAILEY and J. HALLETT
The low-level inversions experienced in polar regions can provide the low supersaturation and stable growth conditions which could suppress the development of polycrystalline forms, however these conditions are rare. It may be that 100–200 ¹m sized
halo-producing crystals observed in polar regions nucleate at the critical supersaturation
for the particular temperature and nuclei present, but subsequently experience conditions of rapidly diminishing supersaturation. For temperatures from ¡20 to ¡40 ± C,
the nucleation results from this and other studies indicate that clays and crustal minerals can act as IN at ice supersaturations as low as 12%, however the persistence of
such supersaturation conditions for any considerable length of time would lead to a
habit dominated by polycrystals. Relatively long periods of stable temperature and low
supersaturation are evidently required to produce large well-faceted crystals (Hallett
et al. 2001). This could result from growth in the ascending region of a very long period,
slow moving, gravity wave where the updraught is close to the crystal fall speed. Such
situations could also occur near the tropopause for low latitudes. Alternatively, a special
type of IN may be necessary to form halo-producing crystals under low supersaturation conditions, possibly pollutants or tiny micrometre sized crystals which are lofted
by turbulence to encounter stable conditions as just described. In their electrodynamic
trapping experiment at temperatures from ¡20 to ¡40 ± C, Bacon et al. (2000) observed
that crystals grown from tiny frost particles scraped from the walls of their chamber
always developed into faceted crystals, while those grown from homogeneously frozen
drops always developed into polycrystals. From these observations it is clear that the formation of pristine crystals occurs over a narrow range of conditions that are not typical
of most of the atmosphere. Measurements of growth rates are necessary to address the
time constraints on these conditions.
The results from this and other studies indicate that for sustained supersaturations
in the atmosphere of only a few percent at temperatures from ¡20 to ¡40 ± C, the habit
of ice crystals with sizes in excess of 50 ¹m will be dominated by polycrystalline
forms. While instances of polycrystal habit frequencies as large as those reported by
Korolev et al. (1999) were observed in this study and the cirrus study performed by the
authors, the higher frequencies at times observed by Korolev et al. may re ect processes
which enhance polycrystal formation. Small pristine crystals may interact with aerosol
or supercooled droplets, transforming them into polycrystals while still smaller than
30 ¹m in size. Sublimation with subsequent regrowth may also transform a pristine
crystal into a more complex or irregular shape, though slow evaporation at a few percent
subsaturation may leave a crystal substantially faceted (Hallett et al. 2001). Unlike the
atmosphere, the diffusion chamber contains neither aerosols nor supercooled drops, and
it is never subsaturated with respect to ice except when the plates are isothermal where
the subsaturation is less than 0.5%. Frequencies of polycrystals approaching 100% were
seen with the static diffusion chamber when crystals nucleated and grew on the glass
Ž lament during evacuation of the chamber with the supersaturation near or above the
critical value for the particular temperature. The initial growth of crystals for only a
few tens of seconds under these conditions led to habits that were almost exclusively
polycrystalline and were dominated by plate-like crystals of the spatial type. It is likely
that all of these processes play a role in the formation of polycrystals under various
conditions, and this may explain the predominance of complex shapes often observed in
tropical cirrus, apart from those formed by aggregation.
While polycrystalline forms may not be aesthetically appealing, it is likely that there
has been a tendency in the past for investigators to preferentially report habits based on
more recognizable crystal forms such as pristine plates, prisms, columns, and sheaths.
An example of this type of skewed analysis has occurred before as in the case of classical
VAPOUR GROWN ICE CRYSTALS
1481
snow crystal observations (Korolev et al. 1999). The conclusion that polycrystalline
forms dominate the crystal habit under most atmospheric conditions certainly presents
more difŽ culties for modellers already challenged by the task of parametrizing crystal
shapes that are more complex than plates, columns or simple polycrystals. However,
there is now an overwhelming amount of data both from in situ observations and
laboratory studies that indicate this conclusion is indeed correct, and realistic optical
and radiative models of the ice crystals in the atmosphere must take into account the
predominance of polycrystalline forms with poorly developed or irregular facets.
8.
C ONCLUSIONS
The growth of crystals on silver iodide, kaolinite, and glass (silicates) with sizes in
excess of 50 ¹m presented in this study has provided a consensus between laboratory
and atmospheric observations. The habits from silver iodide particles adhering to glass
Ž laments show good correlation with previous laboratory results using silver iodide
smoke, and are substantially different from what is observed in the atmosphere except
under very speciŽ c conditions. Kaolinite and glass have produced habits that are most
like those seen in the atmosphere, habits that are different from those seen with silver
iodide.
The habits of crystals grown on glass and kaolinite are in agreement with in situ
observations for crystals larger than approximately 50 ¹m. These results indicate that
between ¡20 and ¡40 ± C the basic crystal habit is plate-like, with polycrystalline
forms dominating the habit distribution except at ice supersaturations below 2%. Pristine
crystals consist mostly of thin and thick plates, short columns appearing infrequently.
From a comparison of past experiments and the results from this study, it is concluded
that the ‘prism’ description of the habit between ¡25 and ¡40 ± C has been incorrectly
assigned, mainly due to a lack of comprehensive data between ¡30 and ¡40 ± C and
the use of the term ‘prism’ to describe non-columnar crystals. The habit changes from
plate-like to columnar at a temperature of approximately ¡40 ± C, the exact transition
temperature depending somewhat on the type of IN. However, for ice supersaturations
below 2%, the habit is approximately independent of temperature. For temperatures
below ¡40 ± C and ice supersaturations in excess of approximately 20%, the habit is
still dominated by polycrystals which appear predominantly as bullet rosettes.
In a future paper, the results presented here will be extended to crystal growth
rates, habit, and habit distribution for temperatures from ¡20 to ¡70 ± C together
with a comparison of in situ observations for temperatures below ¡40 ± C. This will
allow time constraints to be placed on growth characteristics, and will provide a
strategy for estimating updraught velocities in ice-phase clouds. SpeciŽ cally, in situ
aircraft observations of crystal size, concentration, habit and temperature, together
with laboratory measured growth rates and habit observations, will give estimates of
supersaturation from which updraught velocities may be inferred.
A CKNOWLEDGEMENTS
This research was supported by National Science Foundation, Physical Meteorology Program, Grant ATM-9900560 and National Aeronautics and Space Administration Grant NAG-1-2046.
1482
M. BAILEY and J. HALLETT
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