Download Physics and chemistry of snowfall and snow

Physics and chemistry of snowfall
and snow distribution
T.L. Richards
A tmosphe rie Enviranment Servie@,
Environment Crmada, Toronto, Onta2rio
ABSTRACT: Snow, whether falling through the air or accumulating on
the ground, is a most important phase of the hydrologic cycle. The
initial snowfall is produced almost entirely by means of synoptic
scale or large mesoscale physical processes associated with
travelling low pressure areas or orographic or convective lifting.
The ultimate distribution and physical characteristics of the snow
cover on the ground is, however, highly dependent upon local
variations of wind caused by small-scale topographic features and by
even smaller scale energy processes in the snowpack itself. Although
there are chemical aspects to the nucleation process the major
chemical constituents of the snow cover are the result of the
scavenging effects of the falling snow and dry fallout on the snow
itself.
' ,
RESUME: La neige est une des plus importantes phases du cycle
hydrologique. La première chute de neige qui est produite presqu'
entièrement au moyen de processus physiques d'échelle synoptique ou
de large mésoéchelle, est associée avec une aire de basse pression ou
avec une ascendance orographique ou avec une ascendance de
convection. La distribution finale et les caractéristiques physiques
de la couche de neige,sont grandement dépendantes des effets locaux
du vent causés par de's aspects topographiques de petite échelle et
aussi mdme par des processus d'énergie de plus petite échelle dans
l'accumulation de la neige elle-même. Bien qu'il y ait des aspects
chimiques dans le processus de nucléation, les constituants chimiques
de la couche de neige sont le résultat de l'effet de lavage de la
neige tombante et de retombées de particules sur la neige elle-même.
1. INTRODUCTION
Snow, whether falling through the air or accumulating on the
ground, is a most important phase of the hydrologic cycle and is a
major source of moisture which affects both the physical quantity and
the chemical quality of the earth's fresh water supply. In addition,
the depth, density and melting characteristics of the snow cover
dictate the flow regimes of most of the rivers of the temperate
latitudes of the world.
The rather complex and all-inclgsive subject of the physics and
chemistry of snowfall and snow distribution falls naturally into
several separate and, in some cases, almost independent divisions.
First is the evident one between falling snow and snow on the ground
or snow cover, and second is the even more distinct division between
physical and chemical processes and properties. Such divisions
suggest an approach that would first consider the physical and
chemical aspects of the natural production of snowfall to be followed
by discussions of the physical, and then the chemical,
characteristics of the snow cover or older snow on the ground.
Because of the broad scope of the subject it is apparent that a theme
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paper must, of necessity, be in the nature of an introductory review
of the state of the science. It is left for ensuing papers to undertake more detailed discussions of specific aspects of the subject or
to push forward towards new horizons.
2.
PHYSICS OF CLOUD FORMATION
It is recognized that the physics of snowfall is a special case
of the more general subject of the physics of precipitation. For
this reason it is necessary to first introduce the broader subject of
the physical processes involved in the formation of clouds and
precipitation in order to reach the special case of snow.
Clouds, and the resulting precipitation which may be either
liquid or solid, form in the free atmosphere almost entirely as a
result of the lifting and consequent expansion and cooling of
ascending air. In the simplest of terms, a parcel of air as it is
lifted, expands and cools to temperatures lower than the air
surrounding it. This adiabatic cooling process may continue until
the dew-point temperature of the parcel of air is reached and
condensation takes place.
The required vertical motion for such a process may be found in
the atmosphere under four general and relatively well-known
conditions:
(i) Widespread gradual lifting of low pressure areas and
frontal systems.
This type of vertical motion is prevalent with the travelling
synoptic-scale cyclonic storms or low pressure areas most frequently
found in temperate latitudes. As illustrated in Figure 1 such
lifting is actually the result of two separate actions: - the
convergent flow found associated with low pressure areas; and the
lifting of the warm moist air of the warm sector provided by the
associated warm and cold fronts. In the typical warm front situation
the somewhat warmer and lighter air is lifted very gradually over the
heavier cooler air. With a cold front the warm air is lifted much
more vigorously by the advancing colder air. Both these processes
will produce snow under the proper set of circumstances primarily
below freezing temperatures and a sufficient supply of moisture.
(ii) Orographic lifting.
When an airmass is thrust against a mountain barrier, or even
against a line of lesser hills, some of the air is forced to rise and
this can result in the same adiabatic lifting and cooling which will
form an extensive sheet of cloud and may ultimately provide rain or
snow,
a prime example is the large snowfall on the windward side of
the Rocky Mountains of North America (Figure 2).
(iii) Convective lifting.
Convective currents resulting from a warming of the lowest layer
of the atmosphere lead to the heaped-up cumuliform clouds most
readily identified with thunderstorm activity. However, under
freezing conditions these clouds can also produce heavy showery or
squally types of snowfall that in some areas form a large part of the
annual snow production (Figure 2).
Frêquently such low-level warming comes from the passage of cold
air over large bodies of relatively warm water. Very often this
snowfall is intensified by orographic lifting just to the lee of the
open water; for example, the greatest snowfalls in the United States
east of the Rocky Mountain areas may be found in the Tug Hill snowbelt area south and east of Lake Ontario. There, heavy snowfalls are
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due to convective lifting of the atmosphere resulting from the
passage of cold air over open, and therefore relatively warm, Lake
Ontario and further orographic lifting caused by the Adirondak
Mountains.
(iv) Lifting of mechanical turbulence.
Another cloud forming mechanism may be found in the random
stirring by ground friction of a layer of air below an inversion of
temperature. This cloud is typically very thin and rarely yields
more than a light drizzle or fine snow.
3.1
3. THE PHYSICS AND CHEMISTRY OF NUCLEATION
The Physics of Condensation
As indicated earlier, when air expands and is cooled adiabatic-
ally its relative humidity is increased. When the process continues
to the point of 100 per cent (or higher) saturation, condensation and
cloud formation begins. Condensation is a process that does not
occur easily in the pure environment. For pure water vapour at room
temperature the vapour pressure must be about four times its
saturation value before condensation occurs. Fortunately, however,
the atmosphere is not an entirely pure environment. Invariably it
contains very small particles (aerosols) some of which are hygroscopic crystals or droplets of various chemical solutions.
Condensation occurs very readily upon such aerosols with only a very
small degree of supersaturation required.
3.2
The FnemicaZ Composition of Condensation Nuclei
Although there is still much to be learned relative to the
chemistry of condensation nuclei it appears evident that there is an
adequate supply available in the earth's atmosphere. In this
connection the following sources have been identified:
(i) Sea salt nuclei.
These are prevalent not only over the sea but also over the
continents. Recent investigations have revealed that salt nuclei are
readily formed by bursting of salt water bubbles in the form of
breaking waves.
(ii) Continental dust particles.
Soil and dust particles from the e m r s surface are prevalent
throughout the atmosphere.
(iii) Nuclei created by combustion and chemical reaction i.e., sulphur, ammonia, oxides of nitrogen, chlorine and sodium.
3.3
The Physics of SzcbZimation and Freezing
When dealing exclusively with snow, prime interest is naturally
restricted to the cloud forming processes that take place in belowfreezing temperatures. Just as pure water vapour, when it becomes
supersaturated, does not condense spontaneously until a very large
degree of supersaturation has been reached, so also a pure liquid
will not freeze spontaneously until supercooled well below its
equilibrium freezing temperature. In the case of water, supercooling
to -40°C is required for such homogeneous freezing. However, as in
the case of condensation, suspended particles may act as nuclei for
the freezing process and if such nuclei are present freezing may
occur with only a few degrees of supercooling.
There is also another process by which ice crystals are formed
in the atmosphere. This is by direct sublimation from the vapour in
the same way that condensation droplets are formed. This process can
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occur spontaneously and when suitable nuclei are present it is
thought to be an important one.
3.4 The Chemical Composition of Ice-Forming Nuclei
Although ice-forming nuclei occur naturally in the atmosphere
their origin and composition are far from clearly established. One
reason for this is that they are relatively scarce and difficult to
identify since a cubic metre of atmospheric air at -10°C may contain
as few as ten ice nuclei among perhaps 1O1I other particles.
The following three quite different approaches have been taken
by investigators to study ice-forming nuclei:
(i) Samples of naturally occurring dust and smoke have been
examined for nucleation ability. Most investigators suggest that
clay-silicate particles (mainly kaolinite) play a large part in ice
nucleation. Industrial sources are also available but are not
considered to be more than locally importmt.
(ii) Snow crystals have been examined and studies have been
made of particles contained within them. Weickmann [l] using aircraft observations was a pioneer in this field. Again in this
approach, clay particles appear as the most likely agents.
(iii) Alternatively, Bowen [Z] in 1953 suggested that nuclei
enter the top of the atmosphere as meteoric dust. Although this
theory has drawn much public and scientific attention Mason [3] and
others have suggested that conclusive evidence is still lacking.
It should perhaps also be noted that artificial nuclei, such as
silver iodide, are well known and widely used in precipitationinducing attempts. However they are not considered to be part of the
natural process of snow production and so will not be discussed
further.
4. THE FORMATION OF SNOWFLAKES
Recent studies have classified the predominant ice crystal forms
found in different atmospheric temperature ranges and cloud types.
One such classification by Mason [3] considers the results of a
number of investigations and may be seen in Table 1. Beyond this
point, however, it must be admitted that in spite of considerable
experimental and theoretical work, the physical mechanisms underlying
the formation of intricate snowflake patterns from the original ice
crystals are still largely unknown. About all that can be said at
this time is that the growth occurs by either direct vapour transfer
from a cloud droplet to the crystal because of the difference in
vapour pressures over ice and water or by the interaction between ice
crystals to form aggregate snow known as snowflakes. The overall
result is that the snowflake is an aglomerate of individual crystals
in which the star-shaped dendrites are generally predominant but in
which needle and plate forms may also be found.
TABLE 1
CRYSTAL TYPES ASSOCIATED WITH TEMPERATURE RANGES
(from Mason [3])
Temperature Range
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-
3 to
8°C
- 8 to -25°C
-10 to -20°C
-20°C
-30°C
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Crystal Types
Needles
Plates, sector stars
Stellar dendrites
Prisms, single crystals, twins
Clusters of hollow prisms
Collecting, photographing and classifying snowflakes is a
science to some and an art form to others and is well reported in the
literature. However, a review of this aspect of the subject would be
lengthy and is not considered to be a necessary part of this paper.
5. THE PHYSICS OF SNOWFALL
In summary, the formation of snow, whether ít be a flake, a
flurry or a full-blown storm requires several well known
prerequisites:
(i) An atmospheric lifting process must be available either in
the form of a low pressure area or convection or orography, or any
combination of these processes. As a result, greater than average
snowfall should be expected along favoured s t o n tracks, to the
windward side of mountain barriers and to the lee of open water.
(ii) A sufficient amount of water vapour must be available so
that saturation or supersaturation may be reached in the lifting
process. If all other conditions are equal it is evident that the
greater the water vapour content of the lifted air, the greater will
be the snowfall.
(iii) Nuclei must be available in the atmosphere to ensure
condensation and the formation of ice crystals and snowflakes.
Although relatively little is known of this process in nature it may
be assumed that heavier snowfalls will be associated with greater
numbers of nuclei.
(iv) Finally, temperatures must be at, or below, freezing.
Since temperature is a function of both latitude and altitude it
follows that if all other conditions were equal, more snow would be
expected at higher latitudes and higher elevations.
The importance of the above-noted prerequisites may be verified
by inspecting the accompanying snowfall maps of the North American
continent (Figures 3 and 4). It is readily noted that there is more
snow in the north than in the south
a temperature (latitude) effect.
More snow is also to;be found in the mountain areas. This is the
effect of both tempeqature (elevation) and orography. More snow is
indicated in mid-northern latitudes than in the far north, this is
due partly to favourea storm tracks being in mid-latitudes and partly
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to a general deficiency
in the moisture supply in the far north.
More snow may also be ',foundto the lee of open waters and in the upslope areas such as the west slopes of the Rockies and the snowbelt
areas of the Great Lakes. As indicated earlier this is the result of
both convective and orographic lifting.
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6.
THE PHYSICAL PROPERTIES AND PROCESSES OF SNOW COVER
6.1 S n m Depth
On the synoptic or very broad scale the distribution of snow
cover as measured by depth is a function of the meteorological
controls of snowfall just outlined. However, superimposed upon the
above-noted relatively large scale distribution of snow cover are
patterns of snow depth that may be described as small meso-scale.
These are due to the modifying influence of transport by wind and to
variations in energy exchange within the snow cover itself.
Transport
Transport by blowing and/or drifting snow may result in a
decrease or increase in snow cover depending on the relative local
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wind speed which, in turn, is determined by local obstructions to
airflow. Wherever surface winds increase, as between hills or on the
crest of a ridge, erosion occurs. Conversely, wherever winds decrease, snow is deposited by the air stream. Favoured deposition
areas are the lee sides of ridges, in depressions and along the edges
of forests and urban areas.
As might be expected, different surfaces of the earth exhibit
quite different abilities to catch and retain snowfall. Kuzmin [6]
has made extensive studies in this area finding for example that
there is 30 to 40 per cent greater depth of snow cover in forest
areas than on open virgin soil. A summary of his findings is shown
in Table 2. The same author has also found variations of snow
accumulation in forest areas which are related to the specific kinds
of trees found in the forest; for example he reports a 15-year
average of 134 mm (water equivalent) of snow in a birch forest as
compared to only 85 mm in a fir forest.
TABLE 2
SNOW RETENTION COEFFICIENTS FOR VARIOUS SURFACES
RELATIVE TO VIRGIN SOIL
(Kuzmin, 1960 [6])
Surface
Coefficient
Virgin soil
Open ,ice
Arable land
Hilly land
Large forest tracts
River beds
1.0
0.4 to 0.5
0.9
1.2
1.3 to 1.4
3.0
Although possibly not directly related to hydrology, a survey of
the subject of drifting snow cannot be considered complete without
reference to Theakston's [7] laboratory investigations in which he
employs scale models exposed to simulated snowstorms created by light
white sand in an open-channel water flume. This work relates the
degree and pattern of drifting directly to the local physical
characteristics of the topography and of man-made obstructions, as
well as, of course, to the speed and direction of the local winds.
Energy exchange
Variations in the energy exchange within the snowpack and at the
snow-atmosphere interface may also result in major variations in the
snow cover. Shading, variations in slope, unequal pollution on the
snowpack by dust, variations in vegetative cover, and differences in
the depth of snow cover can all affect the uniformity of the energy
balance. MacKay [8] and others have discussed this process in detail
and the subject will no doubt receive further attention later in
these Symposia.
6.2 Snow Cover Densitg
An indication of depth alone does not provide an adequate measure of snow cover because of the variability of the density of the
snowpack on the ground. Even in freshly fallen snow densities vary
markedly in both time and space. Potter [9] reports that although
annual averages of snow-water equivalent for observing sites across
Canada fall within the relatively narrow range of .O74 to .lo7 the
water equivalent of individual snowfalls ranged all the way from .O2
to .28.
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The density of freshly fallen snow depends upon both the form of
the snow crystals and such meteorological factors as wind,
temperature and available moisture during deposition. Mellor [lo] ,
quotes a density of 0.1 g/cm3 with fluffy snow and no wind as
compared to a density of 0.3 g/cm3 under conditions of winds strong
enough to break the crystals into small particles. Other studies
suggest maximum densities at just above the freezing point'when the
snow is wet and melting and again at very low temperatures when the
crystals are small and readily fractured and compacted.
The density of snow also changes after it lies on the ground.
Prior to the spring thaw, the snowpack density is a function of the
wind, the pressure of the overlying snow, and heat exchanges due to
convection, condensation, radiation and conduction. However such
metamorphosis of the snowpack is a subject which will receive
specific attention in a later session of the Symposia.
CHEMICAL COMPOSITION OF SNOW
C h e ~ s t qof Falling or Freshly Fallen Snow
7.
7.1
As noted earlier, each snowflake begins its fall to earth with a
chemical composition resulting from the very necessary nuclei of
condensation, ice formation and/or sublimation. However as the snow
falls through the air additional chemical constituents are added as
the flakes pick up a share of the many aerosols present in the
atmosphere. The result of this scavenging effect is a combination of
chemical constituents primarily bicarbonates, sulphates, chlorides,
calcium, sodium, potassium, magnesium and silicas, and clays. The
actual proportions of chemicals identified in falling snow have been
found to be a function of:
(i) Air mass origin more salt particles are found over and
near the sea, more clay particles are found over the continental
areas.
(ii) Physiographic effects - it is generally accepted that
there are few aerosols at higher altitudes.
it has been found that more aerosols are
(iii) Local climate
present in dry climates than in humid climates.
industrial sources are important but it is
(iv) Local sources
thought that their effects are relatively local. Volcanoes, which
may also be considered as local sources, have been known to provide
widespread, but only periodic, chemical input to the atmosphere.
(v) Lightening, meteoric dust and solar thermal-nuclear
reaction - these have all been mentioned as possible sources of
aerosols but the importance of such sources is still a matter of
investigation.
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7.2 Chemistvy of the Snow Cover
The prime difference between the chemical content of freshly
fallen snow and that of the older snow on the ground is that the
concentrations of chemicals present in the older snow are likely to
be higher due to additions from dry fallout from the air. It is
therefore not unexpected that the largest increases in chemical
concentrations have been found relatively close to industrial sites.
Table 3 condensed from Mellor [lo] provides an indication of the
concentrations of chemicals found in snow on the ground at several
widely dispersed points around the world. The very wide ranges in
concentrations found in the same areas and the apparent lack of
consistent geographical distribution of chemical constituents
7
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indicates a need for many more such observational programs
distributed in both time and space. Indeed it is emphasized by
scientists working in the field that there is still much to be
explored in the chemistry of both falling snow and snow on the
ground. In both cases extensive world-wide observing networks are a
prime requisite to an improved understanding.
8.
SUMMARY
In general, there is little in common between the physical and
chemical aspects of snow. The physical processes discussed in the
paper dictate the amount of snow that will fall on any given area and
the final pattern of snow distribution on the ground. The physical
properties that have been considered dictate in part, the quantity
and characterissic of local snowmelt runoff. The chemical
composition of snowfall and snow on the ground will of course,
contribute to the chemical constituents of the runoff water. Only in
the nucleation phase o€ snow production do the physics and chemistry
of snow become intimately associated and here there is a frontier of
science that invites much further investigation.
9. ACKNOWLEDGMENTS
In developing the chemical aspects of this paper, the author is
indepted to Dr. M.T. Shiomi of the Chemical Limnology Section of the
Canada Centre for Inland Waters, Environment Canada, who kindly
provided a number of recent references. Two o€ these, a review of
Russian work by Posokhov [Il] and one of North American work by Feth
et a2 [U]although not quoted directly, provided a wealth of background material. In developing the physical aspects of the paper the
author also made extensive use of the well-known text books by
Mason [3] and Fletcher [13].
10. REFERENCES
WEICKMA", H.K. (1947). Die Eisphase in der Atmosphare
Reports and Translations No. 716, Ministry of Supply (A)
Volkenrode, London.
BOWEN, E.G. (1953). The influence of meteoric dust on rainfall.
Aust. J. Physics 6.
MASON, B.J. (1971).
The physics of clouds
Clarendon Press, Oxford.
-
second edition,
VIOMAS, M.K. (1964). Snowfall in Canada. Department of
Transport, Meteorological Branch, CIR-3977, TEC-503, 1964.
UNITED STATES DEPARTMENT OF COMMERCE. (1968). Climatic atlas
of the United States. Environmental Data Service, Environmental
Science Services Administration, 80 pp.
KUZMIN, P.P. (1960). Snow cover and snow reserves.
Gidrometeorologicheskoe Izdatelsko, Leningrad. Translation
National Science Foundation, Washington, D.C., 1633, pp. 1-84.
9
[7]
THEAKSTON, F.H. (1967). Advances in the use of models to
predict behaviour of snow and wind. Presented at the 60th
Annual Meeting, American Soc. of Agric. Engineers meeting
jointly with the Canadian Soc. of Agric. Engineering, Saskatoon,
Saskatchewan, June 1967.
[8]
MacKAY, G.A. (1968). Problems of measuring and evaluating snow
cover. Proceedings of Workshop Seminar on Snow Hydrology,
sponsored by Can. Nat. Com. for the IHD , pp. 48-62.
[9]
POTTER, J.G. (1965). Water content of freshly fallen snow,
CIR-4232, TEC-569, Meteorological Branch, Can. Dept. of
Transport.
[lo]
MELLOR, M. (1964). Properties of snow. Cold Regions Science
and Engineering, Part III Engineering, Section A: Snow
Engineering. U.S. Army Material Command, Cold Regions Research
on Engineering Laboratory, Hanover, N.H., pp. 1-105.
[ll]
POSOKHOV, E.V. (1967). Factors effekting the formation of the
chemical composition of atmospheric precipitation.
Gidrokhimichesky Institut (Novocherkasek) Vol. XLVI , an
unedited draft translation of the Foreign Language Division of
the Canada Dept. of the Secretary of State (1969).
[12]
FETH, J.H., ROGERS, S.M., and ROBERSON, C.E. (1964). Chemical
composition of snow in the northern Sierra Nevada and other
areas, Geochemistry of Water. Geol. Sun. Water Supply Paper
1535-5, U.S. Dept. of the Interior.
[I31
FLETCHER, N.H. (1966). The physics of rain clouds. University
Press, Cambridge, England.
10
FT
M
20,000 6000
10,000 3000
O
B
Fig. 1. Typical low pressure area showing cross-section
of warm and cold fronts
Fig. 2.
Orographic and convective lifting
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Fig, 3. Snowfall Map of Canada 1931-60 (M.K. Thomas, [3])
Fig. 4. Snowfall Ma? of United States 1931-60 (U.S. Department
of Commerce [5])
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DISCUSSION
W.T. Dickinson (Canada) - Our experience in analyzing the chemical constituents of rain and snowfall near Guelph, Ontario, Canada,
points out that much care is required in the sampling techniques.
The samples, which were sent to a laboratory for analysis, indicated
a rather high content of cadmium. This information was released to
the press before it could be confirmed by further testing which
showed the cadmium to be the consequence of sampling equipment and
sample preparation prior to analysis.
-
E.J. Lmzghm (Canada) Table III of your paper mentions that
samples were taken from snow on the ground. Is it possible to relate the values in this table with the chemical content of falling
snow? In particular, do you believe the lead found in snow is
simply a local low-level fallout or is there evidence that lead has
diffused to a sufficient height in the atmosphere to have become involved in the snow formation process?
T.L. Eichards (Canada) - According to the literature, the bulk
of the chemical constituents in snow on the ground come from dry
fallout. The longer snow remains on the ground, the higher its concentration of chemical consistuents. I have no feeling for the
extent of lead diffusion in the atmosphere. Perhaps the lead comes
from local sources near urban areas, from automobile exhausts.
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M.C. Quick (Canada) Professor Warren of the University of
British Columbia has measured lead concentrations of minute amounts,
especially as related to health problems. He found large differences between lead concentrations in cities, such as Vancouver,
British Columbia, and those in open country. He is also studying
many other trace elements and this work might be very useful to anyone studying chemical composition of snowpacks.
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