ICE CRYSTALS
'
_33
supersaturation by the temperature of the water vapour source. The crystals observed
are snmlar to snow crystals growing in the atmosphere, and exhibit a remafkable change
of habit (dened as the relative growth rate along “a” and “0” axes) with temperature.
The form also changes with supersaturation (table 1, gure 13). Small traces of impurity,
especially polar substances, can exert a striking inuence on the crystal habit (gure 14).
Here plate and dendr1te growth have been completely replaced by prismgrowth when a
trace of camphor is added to the chamber. The regime for is0butyl alcohol is shown in
gure 15.
Some 1ns1ght mto the reason for these habit changes can be obtained from
experimental observation of the growth of thin ice crystals growing epitaxially on freshly
cleaved covelhte, natural copper sulphide. In vertical lumination these crystals show
brilliant interference colours which enable their thickness to be measured to an accuracy
‘
of:}: 250À (gure 16 a). Providing the supersaturation does not exceed about 100 %
some crystals of diameter up to 100 microns fail to thicken and only grow ’horizontally.
Thickening does occur if the surface is damaged (gure 16 17) or when crystals of dierent
thickness grow together when layers spread from the point of contact over the thinner
crystal (gure 16 0). These observationsimply the existence of ice crystals which are
TABLE 1. VARIATION OF ICE CRYSTAL HABIT IN GROWTH FROM THE VÀl’OUR iN AIR
tu
t'
-
_
—
—
supersa “‘ ‘°n
up to 5%
'
5 to 15%
‘
15 to 100%
‘
greater than 100%
Tem.…
perature
interval °C
0
_
2
Plates
Plates
|
Dendtes
Dendrites
—
'
_
2
__
3
‘
Plates
,
|
Plates
’
_
3
_
4
Solid Prisms
Hollow Prisms
‘
Hollow Prisms
|
Hollow Psms
…
——4
—
5
‘
«'
'
‘
Needles:
,
l
"
' Spikes
at 22° to“c”axis
_
_
8
.
Prmms‘Ho0WPsm5 |
.Hollow Psn‘as'_
—
8
——12
Plates
.
._
IP1&ŒS
3
|
/
,
-—
12
——
16
.
;-| Dwdntes‘ «
Dendtes
_
_,
'—PrismS and nœdleshavegrownmoæalongcthaaaSduecüons
,
,
.
_
'
_33
supersaturation by the temperature of the water vapour source. The crystals observed
are snmlar to snow crystals growing in the atmosphere, and exhibit a remafkable change
of habit (dened as the relative growth rate along “a” and “0” axes) with temperature.
The form also changes with supersaturation (table 1, gure 13). Small traces of impurity,
especially polar substances, can exert a striking inuence on the crystal habit (gure 14).
Here plate and dendr1te growth have been completely replaced by prismgrowth when a
trace of camphor is added to the chamber. The regime for is0butyl alcohol is shown in
gure 15.
Some 1ns1ght mto the reason for these habit changes can be obtained from
experimental observation of the growth of thin ice crystals growing epitaxially on freshly
cleaved covelhte, natural copper sulphide. In vertical lumination these crystals show
brilliant interference colours which enable their thickness to be measured to an accuracy
‘
of:}: 250À (gure 16 a). Providing the supersaturation does not exceed about 100 %
some crystals of diameter up to 100 microns fail to thicken and only grow ’horizontally.
Thickening does occur if the surface is damaged (gure 16 17) or when crystals of dierent
thickness grow together when layers spread from the point of contact over the thinner
crystal (gure 16 0). These observationsimply the existence of ice crystals which are
TABLE 1. VARIATION OF ICE CRYSTAL HABIT IN GROWTH FROM THE VÀl’OUR iN AIR
tu
t'
-
_
—
—
supersa “‘ ‘°n
up to 5%
'
5 to 15%
‘
15 to 100%
‘
greater than 100%
Tem.…
perature
interval °C
0
_
2
Plates
Plates
|
Dendtes
Dendrites
—
'
_
2
__
3
‘
Plates
,
|
Plates
’
_
3
_
4
Solid Prisms
Hollow Prisms
‘
Hollow Prisms
|
Hollow Psms
…
——4
—
5
‘
«'
'
‘
Needles:
,
l
"
' Spikes
at 22° to“c”axis
_
_
8
.
Prmms‘Ho0WPsm5 |
.Hollow Psn‘as'_
—
8
——12
Plates
.
._
IP1&ŒS
3
|
/
,
-—
12
——
16
.
;-| Dwdntes‘ «
Dendtes
_
_,
'—PrismS and nœdleshavegrownmoæalongcthaaaSduecüons
,
,
.
_