Clouds consist of tiny water droplets or ice crystals suspended in the air by updrafts.
Warm cloud consists entirely of liquid water droplets above -15 C (or 5 F).
Below -15, cold cloud consists of ice crystals either exclusively or mixed with supercooled water droplets.
Precipitation (rain, snow, hail, etc.) consists of heavier falling particles known as hydrometeors (meteorology takes
its name from the study of hydrometeors).
In order for precipitation to form, cloud drops or ice crystals must grow considerably.
A typical raindrop is 100 times larger than a typical cloud droplet (see diagram at above).
A typical raindrop is 100 times larger than a typical cloud drop
Cloud Nuclei
Cloud nuclei are microscopic dust particles that attract water vapor.
Cloud drops arise from water vapor condensing on small solid
airborne particles called condensation
nuclei.
Ice crystals in clouds arise from water vapor depositing on ice-forming nuclei.
Cloud nuclei are most abundant in lower troposphere over urban areas and are quite small
relative to a rain drop or cloud droplet (see image at right).
Cloud nuclei are hydroscopic (attract water molecules) and
are created from/by:
dust
volcanoes
factory smoke
forest fires
ocean salt
sulfate particles
from phytoplankton in ocean
Please note the differing sizes of raindrops, cloud droplets, and cloud nuclei at left.
Artifical cloud nuclei are used in weather modification. Introducing silver iodide
into newly formed updrafts aims to spread condensation and deposition over a larger
number of cloud drops and crystals. This is used to control the size of hailstones in severe thunderstorms.
Precipitation will fall as many smaller, less damaging hydrometeors as opposed to relatively few larger
hailstones.
Experiment demonstrating the formation of cloud in a bottle. Releasing the bottle leads to adiabatic cooling
and the environment becomes saturated.
Water vapor inside bottle condenses only when smoke particles are present to act as condensation nuclei.
Cloud Droplet Growth
Processes for Cloud Droplet Growth
To get a droplet (0.02 mm) to grow to raindrop size (2 mm) it must increase in size by a factor of 100.
Cloud droplet growth occurs by:
condensation
collision/coalescence
Condensation
Condensation is important in the early stages of droplet growth.
Because cloud droplets are spherical, they expose an increasingly larger surface to the surrounding air as they grow.
The condensation (green lines) is therefore increasingly balanced by evaporation (orange lines), slowing growth.
This is know as the curvature effect. in order for the droplet to continue to grow, the atmosphere must
be supersaturated (relative humidity around drop greater than 100%).
Droplet fall speeds
Cloud droplets are suspended in the air by updrafts.
The droplet fall speed is called the terminal velocity. The larger the droplet, the larger the terminal velocity.
If the updraft speed is larger than a droplets terminal velocity, the cloud droplet will stay suspended.
given a growing cumulus cloud with an updraft strength of 4 miles/hr:
if terminal velocity is 2 ms-1, the particles fall speed is 2 miles/hr upward
if terminal velocity is 4 ms-1, the particles fall speed is 0 miles/hr
if terminal velocity is 6 ms-1, the particles fall speed is 2 miles/hr downward
The longer the droplet is suspended in a saturated environemnet, the larger it will grow. Therefore,
stronger updrafts produce larger raindrops.
Collision/coalescence
Because an updraft is turbulent, cloud droplets will begin to collide (run into each other) and coalesce (stick
together to form larger droplets).
this is a dominant process for precipitation formation in warm clouds (tops warmer than about -15 C)
Some cloud droplets will grow large enough to have a terminal velocity greater than the updraft velocity
and will start to fall in the cloud
These bigger drops will "collect" smaller drops and grow even bigger.
This process occurs within 30 minutes in strong updrafts.
Though bodies of liquid water at the Earth's surface freeze when their temperature reaches slightly below 0 degrees C,
This temperature is threfore considered to be the freezing point of water.
This is not true in all cases. Water will freeze at 0 degrees C (32 degrees F) only if it comes into contact with a rough, solid surface. Cloud droplets,
however, are suspended in the air.
Water droplets that remains liquid below 0 C is referred to as supercooled.
The demonstrated at right shows water cooled to below freezing.
This can be done by immersing the bottle in a bowl of salty ice water.
Care must be taken to choose a smooth container
and not to agitate the water. As soon as it is agitated or comes into contaxt with ice it freezes.
Exeriment demonstrating the behavior of supercooled water (liquid water below freezing 0 C).
Water freezes instantly when in contact with solid surface or if agitated.
Cloud Phase vs. Temperature
Cloud phase in a saturated environment is determined largely by temperature.
Below -40 C, all cloud is made of ice crystals.
Between -40 and -15 C, both ice and supercooled water droplets are present.
At -20 degrees C, the ratio of ice to liquid is usually about 50%.
Above -15 C, cloud is made almost exclusively of supercooled water.
Observations in clouds have shown that at -10 degrees C it is possible to have only 1 ice crystal per
1 million liquid water droplets.
Above 0 C, water droplets are no longer supercooled. They will not freeze under these circumstances.
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 supercooled droplet cool sufficiently to begin forming minute
ice structures, called ice embryos.
Surrounding molecules attach themselves to these ice embryos and add to the growing crystal.
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.
Initial ice crystals are generally hexagonal (6-sided) in shape.
There are 3 types of heterogeneous nucleation:
Deposition - Water vapor condenses as ice directly onto IN surfaces without passing through the liquid phase.
Freezing - IN contained within a droplet initiate freezing within that droplet (see illustration at left)
Contact - IN (usually falling from above) initiate ice crystal formation upon contact with a droplet. This occurs through the collision of supercooled
droplets with IN. (see illustration at left)
Beregeron-Findeisen Process: Diffusion Deposition
The Bergeron-Findeisen process describes how ice crystals grow at the
expense of supercooled water droplets in a water-saturated environment. It is responsible for forming snow.
Water vapor is more prone to deposit itself on ice crystals than water droplets. As a result, water vapour deposits itself on
cloud ice crytals while the cloud water droplets evaporate. The result is snow .
This process is of particular importance in mid- to high latitudes where clouds routinely extend upward to subfreezing temperatures.
In an updraft, cloud droplets and ice crystals intitially form together. At temperatues close to freezing (0 C), the growth of liguid
cloud drops is favoured initially.
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. Water vapor difusses from around the water droplets
to the icecrystals.
Glaciation
Glaciation refers to the formation of ice crystals either by freezing or deposition.
Glaciation tends to begin in the highest (coldest) part of the cloud. Homogeneous and and heterogeneous
nucleation initialize the glaciation process.
As the larger crytals fall into the region of suprecooled water droplets, the Begeron-Findeisen process
encourages diffusion deposition, and the crystals grow into snowflakes.
If the snowflakes fall into a region that is above freezing, they will melt and form raindrops. This is by
far the most common method of rain formation in the middle latitudes.
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 chart and graph illustrate the habits that
form at various temperatures at saturation with respect to liquid water.
