Supercooled Fog, Ice Fog and Snowfall on a Calm and Cold Day in Asahikawa* By Akira ...

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236 Journal of the Meteorological Society of Japan Vol. 49, No. 4
Supercooled Fog, Ice Fog and Snowfall on a Calm
and Cold Day in Asahikawa*
By Akira Yamashita, Yoichi Fujiki and Chuji Takahashi
Geophysical Institute, Tokyo University, Tokyo
(Manuscript received 15 May 1969, in revised form 22 April 1971)
Abstract
The observations at the city and the suburbs of Asahikawa disclosed the areal distributions
of supercooled dense fog and ice fog. At the central part of the city a supercooled dense fog
seemed to be converted to an ice fog by artificial ice nuclei even at -14*-17 °C, when in the
suburbs sometimes stable supercooled fog was observed at -26°C. It was also confirmed that
ice crystals near the vapor sources grew sometimes so large that they might be called snow crystals.
1. Introduction
The city of Asahikawa is a typical local city
having the population of about 250,000 in Hok-
kaido. Affected by geographic barriers, the tem-
perature in the city falls rapidly at fine and calm
night in winter and dense fogs containing water
droplets and ice crystals are frequently observed.
Several workers have attributed the frequent gen-
eration of fog in the city to warm water discharged
from a paper mill into the River Ushubetsu.
Among them Sakurai (1968a, 1969) has studied
ice crystals in a supercooled fog at the yard of
Hokkaido University of Education and Yamamoto
(1969) has pointed out that, in addition to the
exhaust water, exhaust steam from the paper mill
is so noteworthy as to produce snow crystals.
However, they made observations at limited places
and little has been known about how a super-
cooled fog converts to an ice fog and how ice
crystals grow in a supercooled fog.
Our observations were carried out from 24
January to 10 February 1968 covering wide area
from the central part of the city to the suburbs
for the purpose to know how the conversion of
fog was occurring. There were also some hopes
to get some knowledges about the phase change
in natural clouds from the careful observations
of the phase change in fogs.
Fig. 1. A simple particle sampler. The square
rod (0.8 cm * 3 cm * 90 cm) is swung as
2. Procedure of observation
fast as possible to collect particles. The
The replication method (Schaefer, 1956) was small stick, on which a slide glass is
fixed can be easily exchanged. (b) is an
* Division of Meteorology , Contribution No. 198. enlarged view of a part of (a).
August 1971 Akira Yamashita, Yoichi Fujiki and Chuji Takahashi 237
employed to sample both ice crystals and water sampled were larger than 30*. However, it was
droplets and the water-blue method (Okita, 1958; found to be impossible to determine the size
Yamashita, 1969) was used as a subsidiary method. distributions of water droplets in the air from
The collection of fog droplets and ice crystals was the observed size distributions, because they con-
made by using a light square rod of 90 cm long. sisted of too many droplets of smaller than 5*
Its one end is made to support a slide (or a film) in diameter. Consequently, these observed size
of 8 mm wide as shown in Fig. 1 and the other distributions were employed to get relative abun-
end is used as a grip. If it is swung by an dance of water droplets. In case of the replication
observer the slide collects particles on its surface method a part of the solution is possible to be
at the mean speed of about 25 m/sec (the maximum blown off by swings and, for the particles which
speed being 40 m/sec). In case of using the re- collide on the film by later swings, there is also
plication method a few drops of the Formvar a impossibility of being replicated. Calibration
solution (2% in C2H2C12) are dropped on the due to these two possible factors was made by
surface of the slide just before swinging the comparing the numbers of water droplets captured
sampler. by the replication method with those of the water-
Estimation of collection efficiencies were made blue method. Comparing about ten cases it was
by the same way as the aircraft observation found that one half of the particles collided on
(Yamashita, 1969). With ice crystals the efficiency the slide glass could be replicated.
is nearly 100% because almost all ice crystals Observations were made moving by a car or
Table 1. List of the observation
*1 The maximum air temperature on the former day and the minimum air temperature in the morning
of the latter day in °C observed at Asahikawa Local Meteorological Observatory in the area A.
*2 A; the central part of the city
, B; the central part of the city near the unfrozen rivers, C; near the
unfrozen river in the suburbs, D; the lee area of the paper mill, E; the suburbs, I; ice fog (where
ice crystals were observed in the air), S; supercooled dense fog (always ice crystals were growing in
the fog), Il; where large ice crystals were falling (snowfall), ( ); not based on observations but on
estimation, -; nothing was found in the air.
*3 ; including surveys by car or on foot .
*4 ; a few hours before dawn or half a day from about a few hours after the sunset till the next morning .
*5 ; Supercooled dense fog was also observed early in the morning .
238 Journal of the Meteorological Society of Japan Vol. 49, No. 4
Fig. 2. Typical temperature distribution on a cold day. (Dotted
areas are heat islands formed at the central parts of the
city and towns. The River Ishikari and the downstream
of the River Ushubetsu in the city are two main unfrozen
rivers. Air temperature was recorded continuously by a
0.3 mm diameter C-C thermocouple mounted on a car.)
Fig. 3. Dense fog over the river and a cloud (d) produced by the exhaust steam of the
paper mill. The left portion of the lower picture is connected to the right end
of the upper one. a, b, c and e in this photograph are the same places as
a, b, c and e shown in Fig. 2 respectively. Photographs were taken from the
city office at the center of the city (area A) at 6 h 30 m on 2 February 1968.
August 1971 Akira Yamashita, Yoichi Fujiki and Chuji Takahashi 239
on foot carrying a square rod, a mercury thermo- sources in the city. One of the most important
meter and a sampling box which stored many vapor sources was undoubtedly the River Ishikari
slides and the other necessaries keeping at about and the River Ushubetsu which carried the dis-
the outdoor temperature. In an ice fog the rod charged water of the paper mill, and supercooled
was swung usually ten times and the replication fogs were supplied constantly above the running
method was used. In a supercooled fog it was water of 0*+8°C (Kushizaki et al, 1966) when
swung once or twice and the replication method air temperature was lower than about -15°C.
and the water-blue method were employed at the Another important vapor sources was exhaust
same time as far as possible. After the sampling vapor of the paper mill as proved by Fig. 4. In
the slides or films on small sticks were stored in addition to these two sources the combustion for
the sampling box. Our observations could cover heatings will also be important because the beat-
the city and its suburbs in comparatively short ings and other activities as a whole burns as
time by one or two groups of observers, because much fuel as the paper mill (which consumed
an observation at one place could be completed 850 tons' per day of coal in 1962) in winter
within three minutes by designing the observa- (Yamamoto, 1967-8).
tional procedure as simple as possible. The other type of supercooled fog was observed
early in the morning on 5 February 1968, that is,
3. General features dense supercooled fog covered wide area in the
From 24 January to 10 February 1968, we city and the suburbs at about -11*-12°C and
made four moving observations covering the city a few ice crystals were observed falling in the fog
and its suburbs and additional six observations
of characteristic areas. We also made samplings
of fog particles at a fixed station on the bank of
the Ishikari and several moving surveys by a car
only carrying a mercury thermometer. In Table
1, all of these are summarized briefly.
There were seven days when fog appeared in
the city from about the sunset till the next
morning, and three days when fog appeared only
early in the morning in a similar way as those
seven days. On those seven days, it was fine
and wind was very weak with the daily mean
wind velocities observed at Asahikawa Local
Meteorological Observatory (located almost at
the center of the city) were 0.5-1.0 m/sec. After
the sunset temperature fell rapidly and a few
hours later supercooled fog was found when air
temperature at the central part of the city reached
about -15°C. At midnight the temperature dis-
tribution of the city came to be quite characteristic
as an example is shown in Fig. 2, and from mid-
night till dawn temperature fell gradually at all
places. The temperature distribution in Fig. 2
explains our observed wind directions in the sub-
urbs, which were almost always toward the central
part of the city although the wind was so weak
Fig. 4 Clouds produced by the paper mill.
as only confirmed by observing the moving direc-
(a) At 11 h 00 m, 8 February 1968. (Air tem-
tion of floating tiny ice crystals in the air. The
perature; -8°C in the city. Cloud top; 600m
distributions of supercooled fogs and ice fogs on
high, horizontal diameter; 350m)
these days which are roughly understood by Table
(b) At 20 h 00 m, 29 January 1968. Air tem-
1 and also by Fig. 3 visually, suggests that super- perature; -14°C in the city and -17°C in
cooled fogs are mainly formed by manmade vapor the suburbs.
240 Journal of the Meteorological Society of Japan Vol. 49, No. 4
Fig. 5. A typical areal distribution of an ice fog (*) and a supercooled fog (*) at night.
Only mean crystal sizes are shown schematically because ice crystals at one station
were comparatively of uniform size. Snow crystals observed near the paper mill
were too large to be shown in the same way. Dotted areas show the city.
at the central part of the city. This unusual fog be shown in detail in the following section.
formation, which was also experienced once in Ice crystals observed at one station were com-
1969 during our stay of a month, can be attributed paratively of uniform sizes (this is the reason why
to water vapor stored in the air by abnormally we show only mean sizes in the figures and tables)
high air temperature on the previous day. and there were only several observations in which
As regards supercooled fog and ice fog the city ice crystals had clearly two maxima in their size
and its vicinity can be divided into five areas distribution. Water droplets were also of uniform
having different features as follows : the central sizes (dmax=15*20*, d = 5*) and they were always
part of the city (area A), the riverside of the in dissipating stage except near the vapor sources.
Ishikari and the Ushubetsu in the city (area B), Fog water content of the five areas estimated from
the riverside of Ishikari in the suburbs (area C), the number of sampled ice crystals and water
the lee of the paper mill (area D) and the suburbs droplets was in the following order: D (about 3
(area E). These areas are shown in Fig. 2 and x 10-2 g/m3)>A (about 1*10-2 g/m3)*B>C>E.
characteristics of these five areas are seen in Fig.
5 and Table 1. Observations in these areas will
August 1971 Akira Yamashita, Yoichi Fujiki and Chuji Takahashi 241
the fog, on the other hand in the area A ice fog
4. Features of five characteristic areas
was almost always observed.
(1) The central part of the city (area A) and The overall results of the observations are briefly
the riverside of the Ishikari and the Ushu- summarized in Table 2. It is interesting to note
betsu in the city (area B)
the following characteristics in these areas, al-
Among many observations made in these areas though only mean sizes and mean numbers of
the results of a typical series made on 9 February ice crystals and the presence of supercooled water
are shown in Fig. 6. The figure shows the dis- droplets are presented in the table. At the central
tribution of supercooled fog and ice fog at about part of the city (area A) tiny ice crystals (see Fig.
-16°C, and this type of the distribution, which 7) were always found when the air temperature
is also proved by the photograph shown in Fig. was lower than -14°C and it was noticed that
3, is thought to be common as long as the air as the temperature fell in the city the number of
temperature is about -15*-17°C. In the area ice crystals increased and their size became smaller.
B dense supercooled fog was always found and a No supercooled droplets were detected except when
small number of ice crystals were found falling in the station was thought to be directly affected by
Fig. 6. An areal distribution of an ice fog (*) and a supercooled fog (*) in the city.
Only mean crystal sizes are shown schematically because ice crystals at one station
were comparatively of uniform size. When size distribution curve has two
maxima, two sizes of the maxima are shown instead of the mean size.
242 Journal of the Meteorological Society of Japan Vol. 49, No. 4
Table 2. Summary of observations. The mean values of several observations
at almost the same condition are shown.
* + observed, - not observed and * dense supercooled fog (Number of water droplets >10 , 000/liter)
** supercooled fog only just over the running water
ice crystals larger than 300 * were observed ***
the air coming from the area B. At the riverside tory experiment (Yamashita, 1971). As the air
of the Ishikari and the Ushubetsu in the city temperature fell to lower than -20°C quite a lot
(area B) supercooled water droplets, which were of ice crystals of about 70* in size appeared while
constantly supplied over the surface of the running the concentration of supercooled water droplets
water, were observed when the air temperature decreased
was lower than about -14°C. When the air The difference of general features of fog between
temperature fell to lower than -20°C early in the areas A and B will be explanied as follows.
the morning, the place where supercooled fog Supercooled fog, mainly generated over the rivers
covered began to decrease by its conversion to and subsidiary by other sources in the city, are
ice fog and when the air temperature was as low incessantly dissipated over these areas due to the
as -25°C supercooled water droplets were ob- sublimation of water vapor onto ice nuclei and
served only over the running water. As to ice changes the area of fog cover as the air temper-
crystals, when the air temperature was about -14 ature falls. These ice nuclei are thought to be
18°C a small number of them grown to 300** the city origin by considering the differences of
700* were found falling in supercooled fog * (see fogs in the city and in its suburbs.
Fig. 8 (a)). The possibility of rapid growth of
the ice crystals into those having this several hun- (2) The riverside of the unfrozen river in the
dred micron size range during their fall through suburbs (area C)
a few tens of meters' was confirmed in a labora- At the riverside of the River Ishikari in the
August 1971 Akira Yamashita, Yoichi Fujiki and Chuji Takahashi 243
Fig. 7. Ice crystals at the central part of the city: (area A)
(a), (b): Air temperature -15.5°C, 9 February at the city office.
(c), (d): Air temperature -14.7°C, 9 February at the railway station.
(e) : Air temperature -22.4°C, 28 January at the city office.
(f) : Air temperature -19.0°C, 27 January at the city office.
suburbs (near the Nagayama Bridge) two series
of detailed observations and several simple obser- (3) Snowfall at the lee of the paper mill (area
vations were made on typical cold days. At D)
-17°C supercooled fog was found only near the This area is almost the same as the area A in
surface of running water. When the air temper- its location, but it has the special characteristics
ature fell lower than -20°C, supercooled fog that a cloud of exhaust vapor of the paper mill
formed by the unfrozen running water grew as may affect the fog near the ground.
high as 10*20 meter and migrated from place Ice crystals observed on the street at about
to place near the river. As ice crystals were also 300 m lee of the paper mill were so large as to be
formed in the fog incessantly, at the circumfer- called snow crystals. On typical cold days this
ences of supercooled dense fog it was found to snowfall was always observed at almost the same
convert into dense ice fog. Air temperature at area on a street 50*100 meter in width and it
the riverside was warmed by 2*6°C by the river. was estimated that it would be snowing at the
Prevailing wind directions were almost always narrow fan-shaped area under the artificial cloud
towards the central part of the city. such as an example shown in Fig. 4 (b). For
Comparing the feature of fog of this area with example at midnight on 28 January snowfall was
that of the area B, it was found that the formation observed at the two stations 300 and 1,200 m lee
of supercooled fog in this area was less vigorous of the paper mill. Only the results obtained at
and the temperature at which supercooled fog the stations where the largest crystals were sampled
converted to ice fog was lower than in the area in each series are shown in Table 3. In the series
B. These facts can be easily understood naturally the snow crystals of about 1 mm were observed
if we consider the difference of the temperature with the concentration of 1*2 per liter in the air
and flow rate of running water and the estimated with one exception in the morning on 28 January
difference of ice nuclei concentration between the when only quite a lot of tiny columnar ice crystals
areas. (more than 1,000 per liter) were observed at -26
244 Journal of the Meteorological Society of Japan Vol. 49, No. 4
Fig. 8. Ice crystals at the riverside of Ishikari in the city: (area B)
(a): Air temperature -16°C, 9 February.
(b) : Air temperature -18.5°C, 27 January.
(c): Air temperature -24.8°C, 26 January.
(d): Air temperature -22.0°C, 28 January.
Table 3. Snowfalls at the lee of the paper mill named "factory snow", lay 0.5*1 mm deep on
the ground near the paper mill during a night
and was not snow white but a little dark. The
snow crystals were dendritic crystals of 800*
1,500* (see Figs. 9 and 10) and did not form
flakes. These large crystals grown in the exhaust
vapor of the paper mill are of reasonable size
considering their growth in the vertical distance
of 100 to 300 m (estimated from the photograph
shown in Fig. 4 b). Their growth is also regarded
(a) Temperature at the place where snowfall was
observed (about 500 meter apart from the as reasonable from their crystal shape considering
center of the paper mill). the estimation that the air temperature at about
(b) Temperature at the place where is not affected 300 meter high might be about 5°C higher than
by the paper mill (about 500 meter apart from the air temperature at the ground (about -20°C),
the place (a)). although the measurement of vertical temperature
°C (Table 2)
. This suggests that the paper mill distribution on such a cold day were made only
produce abundant ice nuclei active at about -26 at the riverside (Okita, 1954; Sakurai, 1968b).
°C whereas it produces not so much ice nuclei Their unsymmetrical forms will be attributed to
active at about -20°C. soot particles in polluted air, because almost all
This type of snow, which Yamamoto (1969) sampled crystals have collected black particles.
August 1971 Akira Yamashita, Yoichi Fujiki and Chuji Takahashi 245
Fig. 9. Ice crystals at the lee of the paper mill: (area D) All of these were sampled
by a simple sampler on 1 February near the paper mill.
(a) : Air temperature -21.3*C (b) : Air temperature -21.5*C
(c) : Air temperature -21.6*C (d) : Air temperature -22.8*C
suburbs at -20*-22*C are shown. In this
(4) Subrubs (area E) area, once a supercooled fog moved from some-
The place classified suburbs is not a particular where or formed in the air, they would not easily
area, but observations made in the field apart be dissipated contrary to the areas A and B.
from the unfrozen rivers are summarized in Table This can be explained by the lack of ice nuclei
2 for convenience. On typical cold days tiny ice sources in the suburbs.
crystals were observed in the air when air temper-
ature fell to, lower than -20*C. Generally their 5. Discussion
sizes were small and they were comparatively few, Features of fog in five areas were quite different
although both sizes and numbers varied consider- and it was clearly shown that these features were
ably from place to place. Perhaps they were closely related with the vapor sources and with
formed by small vapor sources scattered in the the supposed number of ice nuclei (Table 2).
field or they were migrated from the riverside The vapor sources were mainly unfrozen water
(area C) by a light wind. What was worthy of and discharged warm water (3 m3/sec) flowing in
notice was that even at -26*C supercooled dense the area B and exhaust steam mixed with the
fog was observed covering very large area in the smoke of the paper mill (850 ton/day of coal
suburbs early in the morning on 28 January. was consumed in 1962 at the mill). Total water
The fog was about 5*15 m high and was com- vapor supplied from various kinds of combustions
posed of many supercooled water droplets and a in the city could not be ignored, although this
small number (10 per liter) of columnar ice crys- may be at the least at midnight.
tals. In Fig. 11 ice crystals sampled in the It is estimated that there was distinct difference