A Comprehensive Habit Diagram for Atmospheric Ice Crystals ......laboratory studies (Bailey and Hallett 2002, 2004, here-after BH02 and BH04) and ﬁeld observations utilizing the

A Comprehensive Habit Diagram for Atmospheric Ice Crystals: Confirmationfrom the Laboratory, AIRS II, and Other Field Studies

MATTHEW P. BAILEY AND JOHN HALLETT

Desert Research Institute, Reno, Nevada

(Manuscript received 8 July 2008, in final form 26 November 2008)

ABSTRACT

Recent laboratory experiments and in situ observations have produced results in broad agreement with

respect to ice crystal habits in the atmosphere. These studies reveal that the ice crystal habit at 2208C is

platelike, extending to 2408C, and not columnar as indicated in many habit diagrams found in atmospheric

science journals and texts. These diagrams were typically derived decades ago from laboratory studies, some

with inherent habit bias, or from combinations of laboratory and in situ observations at the ground, obser-

vations that often did not account for habit modification by precipitation from overlying clouds of varying

temperatures. Habit predictions from these diagrams often disagreed with in situ observations at tempera-

tures below 2208C. More recent laboratory and in situ studies have achieved a consensus on atmospheric ice

crystal habits that differs from the traditional habit diagrams. These newer results can now be combined to

give a comprehensive description of ice crystal habits for the atmosphere as a function of temperature and ice

supersaturation for temperatures from 08 to 2708C, a description dominated by irregular and imperfect

crystals. Cloud particle imager (CPI) habit observations made during the Second Alliance Icing Research

Study (AIRS II) and elsewhere corroborate this comprehensive habit description, and a new habit diagram is

derived from these results.

1. Introduction

Since 2004, a comprehensive picture of atmospheric

ice crystal behavior has been revealed through both

laboratory studies (Bailey and Hallett 2002, 2004, here-

after BH02 and BH04) and field observations utilizing

the Spec. Inc. CPI (Lawson et al. 1998), a nonimpacting

cloud particle imager. This has been confirmed with ob-

servations made during the Second Alliance Icing Re-

search Study (AIRS II) and several additional studies

utilizing the CPI (Korolev et al. 1999, 2000; Baker and

Lawson 2006, hereafter BL06; Lawson et al. 2006a,b,

hereafter LB06a and LB06b). While those who read

atmospheric science journals might be aware of these

advances, most outside the field are not. Erroneous

descriptions of ice crystal habits are currently found on

the Internet, and most atmospheric texts have yet to be

revised (except for Wallace and Hobbs 2006), mislead-

ing those new to the subject who have an interest in or

need for accurate habit information.

A review of over 70 years of ice crystal studies reveals

a bewildering variety of habit diagrams describing ice

crystal shape as a function of temperature and ice su-

persaturation. The diagrams were drawn from labora-

tory studies, in situ observations, or combinations of the

two. These include combinations of separate laboratory

results as can be found in texts such as Pruppacher and

Klett (1997) in addition to field observations of snowfall

typically obtained at the ground (Magono and Lee

1966) as shown in Fig. 1. Although almost all the habit

diagrams agree on the behavior of ice crystals at tem-

peratures between 08 and 2188C, they differ substan-

tially for lower temperatures. Aircraft observations of

ice crystals in clouds offer still a different picture of ice

crystal habits at temperatures below 2208C, especially

with respect to laboratory studies and observations at

the ground.

Two significant misconceptions involving ice crystal

shape are common in the current habit diagrams. First is

the designation of a columnar habit beginning at 2208C.

This misconception evolved from an erroneous combi-

nation of biased laboratory observations (BH02) and

field observations at the ground and in clouds that did not

take into account the modification of habit distribution

Corresponding author address: Matthew Bailey, Desert Re-

search Institute, 2215 Raggio Parkway, Reno, NV 89512.

E-mail: [email protected]

2888 J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S VOLUME 66

DOI: 10.1175/2009JAS2883.1

� 2009 American Meteorological Society

that can occur during sedimentation, combining habits

from different temperatures. This led some in the past

(Chen and Lamb 1994) to erroneously propose an os-

cillatory habit behavior, from plates (08 to 248C) to

columns (248 to 288C) to plates (288 to 2228C) and

back to columns again (T ’ 2228C), a misinterpretation

that continues to appear in habit descriptions (e.g.,

Hashino and Tripoli 2007).

Second is an overemphasis on symmetric shapes.

While aesthetically appealing and offering a striking

subject for photography, the fact is that most ice crystals

are defective and irregular in shape to varying degrees

and are mostly polycrystalline at temperatures below

2208C. Many are so irregular that they were mistakenly

identified as aggregates in past studies. In their classic

book on snow crystals, Bentley and Humphries (1931)

noted that symmetric snowflakes or dendrites were the

rare exception, with poorly formed and irregular branched

dendritic crystals making up the overwhelming major-

ity, which anyone with a magnifying glass can confirm

on a snowy day. Tape (1994), in his book on halos ob-

served in the Antarctic, noted that, even under the

relatively stable growth conditions that produce halo

crystals, the vast majority of ice crystals are defective in

shape with rough surfaces. The fact is that ice is one of

the most disordered crystals in nature and takes on a

wide variety of complex shapes and habit themes as

a function of temperature, ice supersaturation, and at-

mospheric pressure. Although they are rarely the ide-

alized hexagonal shapes depicted in the habit diagrams,

they have their own aesthetic appeal and are fascinating

in their complexity, and it is time for a revised habit

diagram that reflects these characteristics and the new

revelations concerning habit, temperature, and ice su-

persaturation.

2. AIRS II and other field studies

Early laboratory studies were performed before the

modern era of aircraft observations of ice crystals, which

followed the invention of replicators, 2DC, and other

imaging probes. One exception to this was the pioneering

work of H. K. Weickmann (Weickmann 1945; aufm

Kampe et al. 1951) in Germany during the Second

World War, who flew an open cockpit plane through

clouds ranging from stratiform to cirrus. While holding

lacquer-coated shingles in the airstream, Weickmann

captured and replicated ice crystals, obtaining a fairly

accurate picture of in situ habits as a function of tem-

perature. Unfortunately for the atmospheric commu-

nity, Weickmann’s work went largely unnoticed while

more attention was paid to the work of Nakaya (1954),

Kobayashi (1957, 1961), and others, which at times was

skewed by experimental artifacts as discussed in BH02.

Weickmann’s field observations, together with subsequent

laboratory studies (aufm Kampe et al. 1951), indicated

the predominance of plates and platelike polycrystalline

forms at temperatures near 2208C and colder, with a

variable mix of hollow and solid columnar (‘‘prism’’)

forms, as opposed to the columnar habit designated in

many of the currently accepted habit diagrams, al-

though even Weickmann at times misidentified complex

polycrystals as aggregates.

In situ observations from aircraft employing replica-

tors suffered from the fact that many of the collected

crystals were shattered on impact, except for the more

robust solid crystals like thick plates, columns, and

compact polycrystals. The 2DC images suffered from

poor resolution; hence, habit identification was often

ambiguous or incorrect. For example, there are long,

narrow, platelike polycrystals, for example, ‘‘scrolls’’

FIG. 1. Habit diagrams from Pruppacher and Klett (1997) and Magono and Lee (1966).

SEPTEMBER 2009 B A I L E Y A N D H A L L E T T 2889

and ‘‘spearheads’’ (related to side planes) which grow

between 2208 and 2408C (BH04), that likely were

misidentified as columns or bullets in pixilated 2DC

images. Even with these limitations, it was soon evident

that in situ habit observations often differed substan-

tially from the laboratory results for clouds with tem-

peratures below 2208C, casting doubt on the relevance

of laboratory ice crystal studies at lower temperatures.

In 2002 the opportunity arose to examine thousands of

CPI images gathered by Korolev and Isaac (Environ-

ment Canada) from studies of continental, maritime, and

arctic stratiform and cumulus clouds [e.g., the First In-

ternational Satellite Cloud Climatology Project (ISCCP)

Regional Experiment Arctic Cloud Experiment (FIRE

ACE), AIRS, and the Canadian Freezing Drizzle Ex-

periment (CFDE III)]. Prior to this, the habit diagram

in Fig. 2 had already been established from newer lab-

oratory growth studies (BH02; BH04). The CPI images

revealed that at times, between 2208 and 2408C, bullet

rosettes, columns, and mixed rosettes (both bullet and

platelike components) appeared with platelike poly-

crystals (e.g., side planes, crossed plates, assemblages of

plates, etc.), while at other times predominantly plate-

like habits were observed for approximately the same

temperature regime. In the absence of nucleation bias

(BH02), ice crystal habit is known to be, first, a function

of temperature, with ice supersaturation playing a sec-

ondary role whose influence increases with increasing

relative humidity. Thus, the varying appearance of co-

lumnar and platelike forms at the same temperature

appeared contradictory. However, when flight log re-

cords were compared with crystal images, a pattern

emerged. Platelike polycrystals dominated the habit

distribution when cloud tops were warmer than ap-

proximately 2408C and, additionally, there were few or

no overlying cirrus clouds. When cloud top tempera-

tures were brlow 2408C or when overlying cirrus was

present, a mix of columnar and platelike polycrystals,

including bullet and mixed rosettes, was observed.

These observations agreed with the laboratory habit

results in BH04 and further indicated that overlying

cirrus clouds were often the source of the columnar

forms appearing in the platelike growth regime between

2208 and 2408C.

FIG. 2. Habit diagram drawn from laboratory observations (BH04) of ice crystals grown between 2208 and 2708C and examples of linear

growth rates as a function of ice supersaturation and pressure for selected crystals. ‘‘Sheaths’’ are hollow columns or bullets.


In the fall of 2003, the authors participated in the

Second Alliance Icing Research Study conducted in the

Great Lakes region with the NCAR C-130 and a num-

ber of other aircraft. The goal of the study was to obtain

meteorological and cloud microphysical measurements

of mixed phase clouds with substantial liquid water

content, which are responsible for aircraft icing. Mixed

phase clouds with temperatures from 08 to 2358C were

encountered during flights, as well as colder clouds with

temperatures as low as approximately 2508C. Hence,

the project provided an excellent opportunity for ob-

serving ice crystal habits under a broad range of condi-

tions for comparison with the laboratory results in BH04.

Among the instruments deployed for this project

were particle imagers, including a CPI, and both large

and standard format cloudscopes provided by the Des-

ert Research Institute (DRI). Also on board were a dual

wavelength lidar, two FSSPs, a counterflow virtual impac-

tor (CVI), and cloud condensation nuclei (CCN) counters.

Other instruments provided by DRI included a large and

small ‘‘T-Probe’’ (Hallett and Vidaurre 2006), an impac-

tion device that measures total water content (TWC) and

liquid water content (LWC) in a manner similar to the

Nevzorov probe, but with substantially different probe

geometries and collection efficiencies. Measurements with

many of these instruments have recently been shown to be

affected by particle shattering on instrument surfaces,

discussions of which can be found in Korolev and Isaac

(2005) and Isaac et al. (2006).

On one particular mission (1 December 2003) the

C-130 made several passes through the tops of cumulus

towers over Lake Huron with cloud top temperature

near 2378C. Brief episodes of supercooled droplets

were encountered in these towers at temperatures as

low as 2358C. Figure 3 shows a sample of lidar data and

CPI images obtained while passing through one of these

clouds, which is visible at the right side of the lidar figure

near 2035 UTC. Because ice crystal habit is first a

function of temperature, it can be seen that some of the

crystals observed have traveled a long way in updrafts

from the regions where they likely nucleated. For in-

stance, the large six-branched stellar plates in the lower

right quadrant, partially rimed, are habits that typically

nucleate at temperatures above 2208C. Except for a

small number of short columns, thick plates, and skel-

etal plates of varying complexity, the majority of the

habits observed were the complex platelike polycrystals

commonly observed in the laboratory between 2208

and 2408C near water saturation, as indicated in Fig. 2.

While a detailed habit frequency analysis was not per-

formed, the observed habit distribution was consistent

with those observed in situ by others under similar

conditions (Korolev et al. 1999, 2000; BL06; LB06a,b).

While some of the complex platelike assemblages in

Fig. 3 contain columnar components (‘‘mixed rosettes’’),

there were a few rosettes (below center) that may have

been true bullet rosettes; however, some of the com-

ponents appear to be platelike, rendering them mixed

rosettes. Visually, the sky above cloud top was clear at

the time, and lidar data showed no significant back-

scattering that would indicate the presence of overlying

cirrus that could have been the source of columnar

forms falling into these clouds (a weak signal from an

altitude of 6–7 km might indicate the presence of sub-

visual cirrus). These observations essentially confirm the

hypothesis that true bullet rosettes and columnar forms,

in general, are not likely to nucleate at temperatures

higher than approximately 2408C, as in the laboratory,

although short columns appear with low frequency

throughout the platelike region between approximately

2208 and 2408C (BH04).

These habit results are also corroborated by extensive

observations of snowfall at the ground in the Arctic by

Kajikawa et al. (1980) and in the Antarctic by Ohtake

and Inoue (1980), often precipitating in clear skies and

after an intrusion of moist air aloft at heights of 1000 to

4500 m. When soundings showed upper air tempera-

tures between 2228 and 2288C, thin plates were mainly

observed. Between 2308 and 2458C, complex side plane

crystals in addition to bullets and columns were ob-

served, and between 2408 and 2558C, bullet rosettes

were typically observed.

Similar results were recently reported, in BL06 and

LB06a,b of ice crystals observed in wave, cirrus, and

arctic clouds. In these cases, they found that true bullet

rosettes were rarely observed when cloud top tempera-

tures were no colder than 2408C; and at warmer tem-

peratures, 95% of rosette-shaped crystals were not true

bullet rosettes, containing side planes and other com-

plex growth structures. Examples of similar crystals

observed during AIRS II are shown in Fig. 4. However,

they did note that true bullet rosettes were observed on

rare occasions when air or cloud temperatures were

above 2408C (2368C in one wave cloud and 2338C

for crystals collected at the ground at the South Pole),

with no apparent overlying cirrus clouds. In the labora-

tory, bullet rosettes generally nucleate at a temperature

right at, or just below, 2408C and at an ice supersat-

uration intermediate to ice and water saturation, typi-

cally around 25% with respect to ice at 2408 to 2458C

(BH04). Thus, either 2408C is a true habit transition

from predominantly platelike to columnar forms or

a particular mode of nucleation, differing from the

case in BH04, causes true bullet rosettes to appear on

rare occasions at temperatures higher than 2408C, al-

though subvisual cirrus ‘‘seed’’ crystals cannot currently


be discounted as a source of columnar forms when

cloud top or air temperatures are higher than but near

2408C.

It is worth noting, as in LB06a, that supercooled liquid

water is rarely observed at temperatures below 2378C.

Most droplets that freeze at 2208C are polycrystalline

in nature, and from in situ observations of polycrys-

talline ice habit frequency, this trend increases with

decreasing temperature, with polycrystalline bullet

rosettes and mixed rosettes continuing to dominate at

FIG. 3. CPI images collected from the top of a convective tower at 2378C over Lake Huron during

AIRS II. Images correspond to the lidar measurements obtained while penetrating the cloud indicated

at the right of the upper figure. No visible overlying cirrus were present at the time, although lidar data

might indicate the presence of subvisual cirrus at an altitude of 6–7 km. Crystals presented are to

illustrate habit and are not on the same size scale.


temperatures below 2408C in the absence of liquid

water. Hence, polycrystalline forms are dominant under

most conditions below 2208C; however, they exhibit the

characteristics of the particular habit regime in which

they grow, that is, platelike versus columnar. It should

also be noted that in BL06 and LB06a,b the general

term ‘‘rosette shape’’ includes bullet rosettes, mixed-

habit rosettes, and platelike polycrystals with a rosette

shape, so bullet rosette shapes should generally not be

used to model crystals at temperatures above 2408C.

3. The new habit diagram

Figure 5 shows the new habit diagram drawn from

laboratory results and from CPI images obtained during

AIRS II for temperatures from 218 to 2508C (a picto-

rial glossary of complex polycrystalline forms and a

discussion of their supersaturation dependence can be

found in BH04). This new habit diagram retains the

description of habits from the older diagrams for tem-

peratures above 2188C, that is, from plates (08 to 248C)

to columns (248 to 288C) to plates (288 to 2228C).

Crystals at lower temperatures shown in Fig. 5 are taken

from 2508C data and are representative of what is

confirmed observationally by LB06a for temperatures

down to 2618C and in the laboratory (BH04). The habit

results for 2708C at the left side of the figure are similar

to those at 2608C, characterized by ‘‘budding’’ rosettes,

small compact irregular polycrystals, small columns,

plates, and ‘‘spheroids’’—particles that are too small to

be resolved in CPI particle images and appear quasi-

spherical (discussed in section 5) but are usually faceted

in the laboratory. Bullet rosettes are typically not ob-

served in cirrus at 2708C, but do occur on rare occasions

as observed during the Costa Rica Aura Validation

Experiment (CR-AVE) by Lawson (P. Lawson 2008,

personal communication). The absence of in situ ob-

servations of well-developed rosettes in comparison

with the laboratory results appears to indicate that ef-

fective in situ ice supersaturation at 2708C, including

ventilation effects, rarely exceeds 40% with respect to

ice (RHice 5 140%) and is often considerably lower,

which is consistent with the majority of measurements

in cirrus clouds at low temperatures as concluded by

Ovarlez et al. (2002) and as recently measured by

Garrett et al. (2005) during the Cirrus Regional Study of

Tropical Anvils and Cirrus Layers–Florida-Area Cirrus

Experiment (CRYSTAL-FACE). Habits similar to those

observed in the laboratory at 2708C were also observed

in subvisual cirrus at temperatures of 2758C and colder

by Lawson et al. (2008) during CR-AVE, although in-

strumental measurements of the ice supersaturation at

which these habits appeared exhibited large discrep-

ancies (Jensen et al. 2008).

The term ‘‘budding rosette’’ (BL06) is a description of

a class of barely developed bullet, plate, or mixed habit

rosettes that appear in CPI data and the laboratory for

temperatures lower than 2308C or so. Because they are

small, it is often unclear whether the ‘‘buds’’ are col-

umns, thick plates, or something more complex. The

buds likely follow the general habit as a function of

temperature, although mixed habit rosettes are appar-

ent in CPI data and in the laboratory for temperatures

from 2308 to 2408C, at all but the lowest ice supersat-

uration, and down to 2608C for ice supersaturation of

10%–25% (Fig. 5, lower left).

FIG. 4. Mixed habit rosettes and side plane observed at 2308C during AIRS II.


4. Modification of columnar and rosette habitsprecipitating from cirrus clouds

While studying the CPI images provided by Korolev

and Isaac, another pattern emerged with respect to bullet

rosettes and columns. Rosette ‘‘bullets’’ and columns

grown in the laboratory and observed at in situ at tem-

peratures lower than 2408C typically have aspect ratios

of 3–4 or greater, except in the case of single columns

growing at low to moderate ice supersaturations (below

FIG. 5. Habit diagram in text and pictorial format for atmospheric ice crystals derived from laboratory results (BH04)

and CPI images gathered during AIRS II and other field studies. Diagonal bars near the middle of the upper diagram are

drawn to suggest the possibility of the extension of the bullet rosette habit to temperatures slightly higher than 2408C.


approximately 20%). However, the in situ bullets and

columns observed at temperatures between 2208 and

2408C were often much more robust and stout in ap-

pearance; that is, they were larger but had considerably

smaller aspect ratios, similar to those of short columns

that occasionally nucleate and grow in this temperature

region, although typically at low to moderate ice super-

saturation and with relatively low frequency (BH04).

Also, at times, some in situ bullets and columns ob-

served in this warmer temperature region have ends

that are capped or skirted with platelike structures, and

quite often are hollow to some extent. Since such habits

and habit details were rarely observed in the laboratory

for crystals that nucleated and grew between approxi-

mately 2208 and 2408C (a regions with a low frequency

of columnar forms), there was reason to suspect that

these changes were due to the change in growth regime,

from columnar to platelike, that would be experienced by

a cirrus crystal precipitating to lower altitudes where

platelike habits dominate. Hence, a series of experiments

was performed in the laboratory with a static diffusion

chamber, which simulated this process.

Columns and bullet rosettes were initially grown at cirrus

temperatures (T , 2408C) and pressures (P ’ 300 mb)

with ice supersaturation in excess of 25%. Then the dif-

fusion chamber temperature and pressure were slowly

increased at a rate approximating the changing conditions

of a precipitating cirrus crystal to warmer temperatures

(e.g., 2308C , T , 2208C, 400 mb , P , 550 mb). The

ice supersaturation was varied from low values up to

water saturation to simulate the possible intracloud and

intercloud conditions. The results are summarized in

Fig. 6. At ice supersaturations approximately interme-

diate to ice and water saturation, transitioning bullets

remained solid or grew plate ‘‘caps’’ on their ends. Near

water saturation, bullets became hollow to varying de-

grees, sometimes growing platelike skirts around the

open end, but in all transition cases, aspect ratios de-

creased as the bullets grew in width, as can be seen in

Fig. 6. This is the response of a column growing in a

platelike growth regime.

The shift in habit characteristics due to fall is sum-

marized in Fig. 7 with AIRS II CPI images of bullet

rosettes and mixed rosettes gathered at various tem-

peratures. As noted in LB06b, most of the rosettes ob-

served at temperatures above 2408C are mixed rosettes

with platelike components. Since this is a platelike

growth regime, these secondary growth components can

have relatively fast linear growth rates compared with

columns, especially near water saturation, often over-

taking the thickening bullets in size and length.

Another artifact of the change in rosette habit can be

seen by comparing ‘‘cold’’ bullet rosettes below 2408C

with mixed rosettes at higher temperatures. The true bullet

rosettes have bullets that are usually interpenetrating

with respect to the core or kernel of the rosette, whereas

FIG. 6. Center region of the habit diagram showing the transformation of bullet rosettes that have

transitioned to the platelike growth regime warmer than 2408C.


bullets at warmer temperatures, especially those which

have grown near water saturation, have pinched or tapered

bases (see Fig. 6). This can be explained as a response to

changing growth conditions. Bullets in the platelike

growth regime start to grow in width; however, the bases

of the bullets are recessed in the rosette structure and

hence are shadowed from the level of ice supersatura-

tion experienced by the exposed ends. So, the base of

the bullet grows more slowly, resulting in a tapered

shape. It is plausible that the growth of bullet rosettes at

high ice supersaturation and temperatures below 2408C

might also exhibit tapered bullets, although this could

be an indication of bullet rosettes that have cycled

through the 2408C level owing to sedimentation and

updrafts.

5. Additional habit observations

The traditional habit diagrams have generally been in

close agreement with respect to the habits for tempera-

tures warmer than the dendrite growth regime, which

terminates at approximately 2188C, and it is only this part

of the old habit diagrams that is retained in the new one.

Below this temperature are two habit regimes that are

dominated by polycrystalline forms at nearly all ice su-

persaturations: a platelike growth regime between 2208

and 2408C and a columnar regime between approxi-

mately 2408 and 2708C. The intermediate ice supersat-

uration regime reveals a mix of mostly polycrystals with

some columns and plates, with bullet rosettes nucleating

at ice supersaturations of around 25% for temperatures

below 2408C, according to BH04. As described in BH02

and BH04, the habit at low ice supersaturation (bottom of

Figs. 2 and 5) forms a subset of ice crystal habits that

shows little temperature dependence, except for a small

increase in short columnar forms with decreasing tem-

perature. In the habit diagrams, this is often described as a

region of ‘‘equilibrium shapes,’’ another mischaracteri-

zation of ice crystal shape. At nearly all temperatures

below 2208C and for low ice supersaturation, the habit

FIG. 7. Transformation of bullet rosettes that fall to the platelike regime warmer than 2408C.


is a mix of plates and short columns (often distorted to

some extent) but more frequently composed of compact

faceted polyhedra, crystals referred to as ‘‘irregulars’’ or

spheroids in BL06 and LB06a,b.

The bottom left region of Fig. 5 describes crystals as

compact faceted polycrystals. Small crystals of these

types are like the small irregulars in BL06 and LB06a,b

with sizes less than approximately 50 mm. Examples of

these can be found in the lower left corner and along the

bottom of Fig. 5. In the laboratory these occur at very

low ice supersaturation (1%–2%) up to around 10%–

15%, and smaller versions of these crystals are likely the

unresolved spheroids shown in those papers. However,

the term ‘‘spheroid’’ should not be misconstrued to

imply quasi-spherical or spherical ice particles, a shape

that has often been used in past models of high-level

cirrus particles. Except for the possible case of frozen

solution drops, they are, in fact, compact polyhedra with

many small facets and irregular structures on their

surfaces. Crystals of this type with sizes of 5–20 mm have

been produced in the laboratory with the DRI fall tower

at temperatures of around 2428C and are not spherical,

as can be seen in Fig. 8. These crystals were grown from

5-mm drops generated with a sonic nebulizer that nu-

cleated, either homogeneously or by contact with

background aerosol, while falling 5 m in the refrigerated

fall tower into a large cold box. Microscopic images

were obtained with the DRI high-resolution cloud-

scope, which can resolve crystals down to a size of about

3 microns. Most of the particles that look quasi-spherical

at lower magnification are faceted or show emerging

facets at a size of 5–10 mm, while somewhat larger com-

pact shapes are clearly faceted or are budding rosettes.

Very few of the 5-mm particles were frozen spheres.

6. Conclusions

The laboratory habit observations of BH02 and BH04

are in agreement with the analysis of well over a million

CPI images of in situ ice crystals collected from a broad

range of temperature, ice supersaturation, and cloud

types. The consensus between laboratory and in situ

observations has resulted in a comprehensive habit di-

agram that effectively describes the shapes of atmo-

spheric ice crystals as a function of temperature and ice

supersaturation. Knowledge or estimates of cloud top

and bottom temperatures in comparison with the new

habit diagram can be used to estimate ice crystal habit

for many scenarios.

The new habit diagram retains the well-established

descriptions of habits from the older diagrams for

temperatures above 2188C, that is, from plates (08 to

248C) to columns (248 to 288C) to plates (288 to

2228C). However, it diverges from the previous habit

diagrams for lower temperatures and reveals that the

habit is dominated by polycrystals of various forms with

FIG. 8. (top) CPI spheroid particles gathered in cirrus clouds with sizes from about 30 to 70 mm from BL06.

(middle) High-resolution cloudscope images of 5–20-mm ice crystals grown in the DRI fall tower near 2428C.

(bottom) Magnification of the middle strip. On close inspection, many of the 5-mm and larger quasi-spherical

particles exhibit facets.


two distinct habit regimes: platelike from 2208 to 2408C

and columnar from 2408 to 2708C. The new diagram

also emphasizes the fact that most individual crystals are

complex, irregular, and imperfect in appearance to some

extent, in agreement with Korolev et al. (1999, 2000),

and includes single crystals such as plates and columns

that are most common at low ice supersaturation. Ad-

ditionally, the majority of very small crystals growing at

low ice supersaturation are compact faceted polycrys-

tals as opposed to spheroids.

The laboratory habit diagrams in Fig. 2 and the top of

Fig. 5 were constructed from crystals observed under

static growth conditions and match the in situ observa-

tions as long as sedimentation or updrafts do not sub-

stantially impact the overall habit distribution. This

would be the case for crystals whose growth was re-

stricted to either the platelike or columnar growth re-

gimes. It is clear, however, that substantial changes in

habit can occur due to vertical motions, and a history of

nucleation and changing growth conditions is often pre-

served in the habit details of modified crystals. Simula-

tions in the laboratory have reproduced a commonly

observed transformation in habit that occurs when col-

umns and bullet rosettes precipitate from temperatures

below 2408C into the warmer platelike growth regime

at temperatures from 2208 to 2408C, as often occurs in

the atmosphere. Bullet rosettes typically become mixed

rosettes with platelike components, with bullets and

columns experiencing a reduction in aspect ratio while

growing larger and wider—sometimes becoming hollow

or capped, depending on ice supersaturation. The trans-

formation of shape and habit for bullet rosettes and other

crystal habits due to sedimentation indicates that further

laboratory growth studies involving changing growth

conditions would add significantly to our knowledge of

ice crystal shape and mass in clouds.

Acknowledgments. This research was supported by the

National Science Foundation, Physical Meteorology Pro-

gram (ATM-9900560) and NASA (NAGS-7973). AIRS II

research was sponsored by NSF (ATM-0313581). Fall

tower experiments were supported by the U.S. Air Force

Office of Scientific Research (F49620-00-1-0215).

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