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Winter Microphysics Topics

Winter Precipitation Processes

1 The Ice-Crystal Process

1.1 Winter Clouds: Supercooled Droplets or Ice Crystals?
1.2 Initiation of Ice Crystals: The Nucleation Process
1.3 Ice-forming Nuclei, Activation Temperatures, and Impacts of RH
1.4 Ice Crystal Growth
1.5 Ice/Snow Crystal Habits
1.6 Operational Significance of the Ice-Crystal Process


0.1 Objectives

  • Explain the microphysics of snow crystal growth, the interaction of cloud water with cloud ice, and the important roles of dendrites and aggregation
  • Apply microphysics knowledge to operational settings


1.1 Winter Clouds: Supercooled Droplets or Ice Crystals?

1.1.1 The Behaviour of Water at Low Temperatures

Though bodies of liquid water freeze when their temperatures reach slightly below 0°C, water droplets in clouds behave quite differently. Observations in clouds have shown that at -10°C it is possible to have only 1 ice crystal per 1 million liquid water droplets. At -20°C, the ratio of ice to liquid can be less than 50%, yet some operational studies note a predominance of ice at these temperatures. In laboratory experiments, cloud temperatures can reach -40°C before all water droplets are found as ice crystals (Pruppacher and Klett, 1979). Water droplets that are found in temperatures below freezing are referred to as supercooled. Regions of a cloud transformed to ice crystals, where the cloud is saturated with respect to ice, are said to be glaciated.


1.1.2 Cloud Phase Transition: Glaciation

In mixed-phase clouds, glaciation, the transition in the cloud of supercooled liquid to ice, generally takes place rapidly. Glaciation tends to begin in the highest (coldest) part of the cloud, where ice is nucleated, and then work downward as ice crystals become larger and heavier and fall into the lower levels of the cloud. This increases the number of ice crystal surfaces available and further depletes the amount of water vapor in the cloud.



Glaciation can be initiated by conditions within the cloud or can result from clouds above the supercooled cloud seeding ice from above. This seeder/feeder effect is most prevalent with mid- or upper-level cirrus, cirrostratus, or alto-type clouds seeding lower-level stratus and stratocumulus clouds.

1.2 Initiation of Ice Crystals: The Nucleation Process

1.2.1 The Bergeron-Findeisen Process

The Bergeron-Findeisen process describes how ice crystals grow at the expense of supercooled water droplets in a water-saturated environment. This process is of particular importance in mid- to high latitudes where clouds routinely extend upward to subfreezing temperatures. Determining the ratio of supercooled droplets to ice crystals and the rate at which a cloud becomes glaciated aids a forecaster's ability to predict winter precipitation types and amounts. Making these determinations is based on an understanding of the Bergeron-Findeisen process.

In 1911, Alfred Wegener, a geologist and originator of the theory of continental drift, originally proposed a theory of ice crystal growth based on the difference in saturated water-vapor pressure between ice crystals and supercooled water droplets. In the 1930's, the Swedish meteorologist Tor Bergeron and the German meteorologist Walter Findeisen contributed further to the theory which became known as the Bergeron-Findeisen process.



Two Types of Nucleation Processes

In order for ice crystals to form in clouds, the water molecules making up the vaporous cloud droplets need a substrate on which to begin the formation of an ice crystal lattice. The initiation of this process can take place by homogeneous or heterogeneous nucleation.


1.2.2 Homogeneous nucleation

Homogeneous nucleation takes place at very cold temperatures in the absence of any ice-forming nuclei (IN). Nucleation takes place as water molecules within a droplet are cooled sufficiently to begin forming minute ice structures, called ice embryos. Surrounding molecules attach themselves to these ice embryos and add to the growing crystal lattice. This structure is prone to disruption due to thermal agitation, an event that is more likely to occur in smaller drops. Thus, the smaller the size of the droplet, the lower the freezing temperature needed to form ice crystals.

Laboratory studies have shown that 5-µm diameter droplets of pure water don't freeze until their temperature is brought down to -40°C. Larger droplets of 25 µm in diameter freeze at -36°C, a slightly warmer temperature due to the larger size of the droplet. This implies that any cloud at temperatures below -40°C will consist purely of ice crystals. From a forecasting perspective, due to the glaciating process, supercooled liquid droplets are relatively rare below -20°C.

Early on in the study of modern meteorology, homogeneous nucleation was ruled out as the prevalent process of ice crystal formation in the atmosphere. Based on observations of the amount of ice crystals found in most clouds and mean temperatures that are too high to support homogeneous nucleation, other processes for initiating freezing need to take place.


1.2.3 Heterogeneous nucleation

Heterogeneous nucleation is the predominant process of ice crystal initiation in the atmosphere. It takes place due to the presence of ice-forming nuclei (IN) in saturated, sub-freezing environments. There are 3 types of heterogeneous nucleation:

Water vapor condenses as ice directly onto IN surfaces without passing through the liquid phase.

IN contained within a droplet initiate freezing within that droplet.

IN initiate ice crystal formation upon contact with a droplet. This occurs through the collision of supercooled droplets with IN.


1.3 Ice-forming Nuclei, Activation Temperatures, and Impacts of RH

1.3.1 Common IN and their Activation Temperatures

As with cloud condensation nuclei (CCN), ice-forming nuclei (IN) are available in the atmosphere but at fairly low concentrations. IN are hygroscopic (water attracting) molecules. According to Rogers (1979), IN provide a hexagonal lattice structure resembling natural ice. However, they are not active until they reach a particular temperature below 0°C. As the temperature drops, more of these nuclei become active to initiate ice crystal formation. Thus, the concentration of active IN rises as the temperature drops.

Common IN and their Activation Temperatures
Substance Activation Temperature (°C) Prevalence
leaf bacteria
found in decaying leaf matter, possibly a prevelant source of IN
silver iodide
used for artificial cloud seeding
common clay mineral
copper sulphide
sodium chloride
sea water
volcanic ash
common aerosol
common clay mineral

Relative Humidity and IN Activation

The amount of IN activated in clouds increases with relative humidity (RH). Lowering the temperature of a cloud, which raises relative humidity, increases the number of activated IN. Therefore, heterogenous nucleation is more likely to occur in the coldest regions in a cloud, typically towards the top.


1.4 Ice Crystal Growth

1.4.1 Three Ice Crystal Growth Processes

Once an ice crystal has formed, it can grow through several processes:

  • diffusion deposition
  • accretion
  • aggregation

Depending on the temperatures and saturation levels in the cloud, the ice crystal can grow in various forms, or crystal habits. Of course, if the ice crystal travels to areas warmer than 0°C, it can also melt.


1.4.2 Ice Crystal Growth by Diffusion Deposition

Diffusion deposition occurs due to differences in the saturation vapor pressure between ice and liquid water. At a given temperature, the vapor pressure over a water surface is greater than that over an ice surface. If water droplets and ice crystals exist in the same environment (called mixed phase conditions), a vapor pressure gradient develops between the droplets and crystals. Due to this gradient, water vapor moves from the higher pressure surrounding the droplets to the lower pressure surrounding the crystals. Thus, the ice crystals grow at the droplets' expense. This process creates sub-saturation with respect to water, and the droplets evaporate to maintain water saturation, making additional water vapor available for ice crystal growth. Eventually the pool of liquid water diminishes and the cloud becomes glaciated.

The process of growth by deposition can also take place in a setting that is supersaturated with respect to ice as the ice crystals continue to grow through deposition of the free water vapor surrounding the ice crystals.

1.4.3 Ice Crystal Growth by Diffusion Deposition (cont.)

The rate of growth of ice crystals is most directly influenced by temperature and humidity. Lowering the temperature of a cloud causes conversion to ice and existing ice growth to speed up at the expense of any available liquid water.

The rate of diffusion is highest where the difference between the saturation vapor pressure between ice and supercooled liquid water is greatest. Optimal growth rates occur near -15°C.


1.4.4 Ice Crystal Growth by Accretion (Riming)

In a process called accretion, ice crystals can grow via collision with supercooled droplets. The supercooled droplet freezes on contact and sticks to the original ice crystal to form rime ice. This process is optimal in saturated layers with temperatures of 0 to -10°C (Staudenmaier, 1999). Excessive riming eventually results in the formation of graupel or snow pellets.



1.4.5 Ice Crystal Growth by Accretion (Riming) (cont.)

As ice crystals grow by accretion, rapid freezing of droplets can result in shattering to form multiple tiny ice particles. This process is called splintering and is a secondary source of ice-forming nuclei (IN). This can then speed up the accretion process and glaciation of the cloud.

In summer convective storms, hail is formed as heavily-rimed ice crystals are continuously swept up into regions of supercooled drops in the convective cloud, growing in the cycling process until their weight overcomes the updrafts.


1.4.6 Ice Crystal Growth by Aggregation

As ice crystals grow and collide, they can stick to each other in a process called aggregation. Liquid molecules on the outer surface of the crystals serve to increase bonding between two colliding crystals. As the crystal moves through warmer regions in the cloud, there will be more liquid molecules on the outside of the crystal, and larger aggregations can build to form large snowflakes. Stickiness is maximized at or near 0°C.

Crystals with dendritic characteristics can also mechanically interlock to form larger aggregates (Pruppacher and Klett, 1979).




1.5 Ice/Snow Crystal Habits

1.5.1 Effect of Temperature on Crystal Habits

The crystal habit, or shape, of a growing ice crystal is determined by the temperature and associated saturation vapor pressure difference between ice and supercooled water. The following chart and graph illustrate the habits that form at various temperatures at saturation with respect to liquid water.






Ice/Snow Crystal Habits at various temperatures at saturation with respect to liquid water

Initiation temperature (°C) Crystal Habit  
0 to -4
thin plate
-3 to -6
-6 to -10
hollow columns

-10 to -12
solid plates
sector plates
broad-branch plates

-12 to -16
(as well as most plates)
-16 to -22
solid plates
sector plates
broad-branch plates

-22 to -40
solid columns
hollow columns
bullet rosette



1.5.4 Underlying Shape: the Hexagon

A common element of most ice crystal habits is the underlying hexagonal shape. Whether formed in cold temperatures as hollow columns or in relatively warmer temperatures as dendrites, the underlying structure of the crystal is hexagonal. The hexagonal arrangement can be attributed to fundamental physical parameters of water molecules as they are cooled and take on their lowest energy form. The hexagonal configuration maximizes attractive forces between the molecules while minimizing repulsive ones.

The dendrite form is the most intricate and can be attributed to the local differences in the vapor pressure along the six outer points versus the inner edges of the crystal. Since conditions are the same at each point, this difference leads to further build up of the crystal along the points in a symmetrical fashion. The dendritic habit is most prevalent in the temperature ranges with greatest excess water vapor pressure with respect to ice, between -10 and -15°C.



1.6 Operational Significance of the Ice-Crystal Process

1.6.1 Ratio of Supercooled Cloud to Mixed Phase

Operationally, knowledge of the ice crystal process can help in forecasting precipitation type and amount and is essential in making aviation icing forecasts. The following topics summarize the operational implications of the ice crystal process.

Within clouds, at temperatures warmer than -10°C, supercooled liquid can be expected most often. Generally, with decreasing temperature, the likelihood of the cloud containing ice increases. At -20° C, fewer than 10% of the sampled clouds consisted entirely of supercooled liquid.



Optimal Temperatures and Conditions for Ice Crystal Initiation and Growth

Observation and laboratory experiments indicate that the optimal conditions for ice crystal initiation and growth are in cloud layers that are saturated with respect to liquid water, and thus supersaturated with respect to ice, with temperatures in the -10 to -18°C range.


1.6.2 Potential Presence of Ice Crystal Initiation

Potential presence of ice crystal initiation in clouds based on temperature
Temperature (°C) Potential presence of ice initiation
no initiation
no initiation
60% chance of initiation and presence of ice
70% chance of initiation and presence of ice
90% chance of initiation and presence of ice
100% chance of initiation and presence of ice

AWIPS can be used to highlight the liquid-to-ice temperature range in cloud tops. Based on cloud-top temperature, and the color regimes in the table below, the presence of ice in cloud tops can be determined through the use of IR.

Enhancement Color

Temperature Deg (°C)

Ice or Liquid

Optimal AWIPS color choices (RGB values)

Yellow 0 to -8 Liquid 255  255   0


-8 to -10

Likely Liquid

0     0     200

Light Blue

-10 to -12

60% Chance Ice is there

0   100   150


-12 to -15

70% Chance Ice is there

255  255  255


-15 to -20

90% Chance Ice is there

250  0 160 (-15°C)
interpolated to
250  80  170 (-20°C)

Black to White

-20 or less

ICE is there!

0  0  0 (-20°C)
interpolated to
255  255  255
(coldest temp)


1.6.3 Dendrite Zones and Heavy Snowfall

Regions of the atmosphere where upward vertical motion coincides with temperatures favorable for dendritic growth (-12 to -16°C with high RH values) are strongly correlated with heavy snowfall. Crystal growth rates and aggregation are maximized in these dendritic growth zones. Upward vertical motion in regions of high RH replenishes the supply of supercooled liquid water droplets and thus enables rapid dendrite growth to continue.

This cross section from Pueblo, Colorado to Cheyenne, Wyoming on 25 October 1997 shows an example of this effect. Heavy snows were observed in the Denver area, the central region of this cross section. Relative humidity throughout this region was > 90% at the time. Note the location of high vertical velocity both within and above the -12 to -16°C layers. In the layers above, where vertical velocity is highest and temperatures are lower (-17 to -20°C), smaller crystals would be forming and falling through the dendritic zone below, rapidly growing through aggregation and developing into larger dendrites.


1.6.4 Aviation Icing Threat

The presence of cloud layers composed of supercooled liquid poses a potential icing threat to aircraft passing through those layers. Cloud layers consisting of ice particles pose no icing threat.


1.6.5 Use of GOES 3.9 to Determine Cloud Phase

Clouds consisting of ice particles can be distinguished from clouds consisting of supercooled water droplets through examination of the GOES 3.9-µm infrared imagery. At this wavelength, reflectance by water droplets is much greater than the reflectance from ice particles. Therefore, lower clouds (stratus or stratocumulus) containing water droplets appear white, while the higher clouds (cirrus) containing ice particles appear dark. This is opposite from the normal appearance of high and low clouds on a channel-4 IR image. In this image, lower-level water droplet clouds are evident over northeast Wyoming, while higher-level ice crystal clouds are evident over southwest Wyoming, northern Colorado, and South Dakota.

It is important to note that upper-level ice clouds can obscure lower-level supercooled liquid clouds in satellite images.



Ahrens, C. Donald, 2000: Meteorology Today, An Introduction to Weather, Climate, and the Environment. Brooks/Cole Thomson Learning, 182-199 pp.

Baumgardt, Dan, Science Operations officer, NWS, La Cross WI., 1999: Wintertime Cloud Microphysics Review, online presentation materials, http://www.crh.noaa.gov/arx/micrope.htm

NOAA Snow Monitoring website: http://lwf.ncdc.noaa.gov/oa/climate/research/snow/snow.html

Pruppacher, H.R., and J.D. Klett, 1978: Microphysics of Clouds and Precipitation. D. Reidel Publishing Company, Boston, 714 pp.

Rodgers, R.R., 1979: A Short Course in Cloud Physics, 2nd Edition. Pergammon Press, New York, 116-133 pp. [also reference 3rd edition with M.K. Yau 1989]

Staudenmaier, Michael Jr. - NWSO Flagstaff, AZ Western Region technical Attachement, No. 99-12, June 22, 1999: The Importance of Microphysic in Snowfall: A Practical Example.

Aviation Icing Modules on Meted

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