LIST THE TWO PROCESSES INVOLVED IN THE FORMATION OF
PRECIPITATION
Bergeron effect: The Bergeron effect is the process in which water
evaporates off a droplet and deposits on ice crystals in the cloud in the mid to high levels.
Collision process: The collision process can occur after the Bergeron effect
has taken place. As particles in a cloud form, they collide with each other,
creating larger particles. The particles grow until they have acquired enough
mass to fall as precipitation.
Describe the Bergeron effect
The Bergeron effect involves precipitation forming in the middle and high
latitudes where the temperatures are below the freezing point of water. In
these clouds, water exists in its liquid form even though the temperatures
are cold enough to freeze water. Water that has a temperature below
freezing but is still in a liquid state is called supercooled water.
Water can exist in such a state in extremely small amounts (such as cloud
droplets). Ice crystals co-exist with the supercooled water in cold clouds.
When this occurs, the ice crystals will grow at the expense of the water
droplets.
Why? It is more difficult for water vapour to leave an ice surface than a liquid
surface. Ice is more rigid with less energy compared to liquid. Since ice has
less energy, it is more difficult for an ice molecule to gain the energy needed
to leave the ice surface as water vapour. This causes the saturation vapour
pressure over liquid to be greater than the saturation vapour pressure over
ice at the same temperature.
Describe the collision process
Collision process
The collision process applies to warm clouds. Warm clouds are those that
form at altitudes where the air temperature is above freezing. For
precipitation to form under this model there needs to be a variety of
condensation nuclei of different sizes.
Large condensation nuclei will create large water droplets, while smaller
condensation nuclei create small ones. In order for the droplets to fall to the
surface, they have to be heavy enough to overcome the resistance imposed
by upwardly rising air that is fuelling the development of the cloud.
The smaller, lighter droplets are easily suspended in the updrafts of air; the
larger heavy collector droplets fall and collide with the smaller ones. Upon
collision, the droplets coalesce into a bigger droplet. As the droplet falls,
resistance by the air flattens the droplet to the point where it becomes
unstable and breaks apart. With enough collisions, the droplet achieves a
size sufficient to fall all the way to the surface
The terminology associated with the collision process is as follows:
- Coalescence: the collision of liquid drops to form larger particles.
- Aggregation: the collision of solid particles to form larger particles.
- Accretion: the collision of solid and liquid particles to form larger particles.
- Fragmentation: the breaking up of an ice crystal or large water droplet
into smaller droplets. At times, the collision process causes fragmentation
which can lead to the newly created smaller particles re-entering the
collision process
DESCRIBE THE IMPACT OF STABILITY ON THE CHARACTER OF
PRECIPITATION
Unstable air creates conditions favourable to the formation of convective
clouds, which contains strong updrafts. The stronger updrafts and greater
vertical extent of the clouds create showery precipitation.
Stable air leads to the formation of stratiform clouds, which have weaker
updrafts and very little vertical extent. This type of cloud produces
continuous and intermittent precipitation.
DESCRIBE THE IMPACT OF STABILITY ON THE TYPE OF CLOUD
Stability is a measure of the tendency of air that has been lifted or lowered
to return to its original level, or equilibrium position.
Stability is determined by comparing the temperature of a rising parcel of air
to the temperature of its surroundings. Because of the weak upward currents
found in stratiform clouds, the precipitation falling from these will be
intermittent or continuous. But with cumuliform clouds the strong vertical
currents present will result in showery precipitation
LIST THE THREE FACTORS THAT EXPLAIN THE INTENSITY OF
PRECIPITATION
Updraft strength
- Assists in growing particles in cloud.
Cloud water content
- Higher temperatures increase potential for water content.
Cloud vertical thickness
- The increased thickness could favour greater intensities.
DESCRIBE THE THREE FACTORS THAT EXPLAIN INTENSITY OF
PRECIPITATION
Updraft strength
Due to the weak upward currents in stratiform clouds, the precipitation will
be intermittent or continuous.
Cumuliform clouds have strong vertical currents that result in showery
precipitation types. Strong updrafts are needed to produce large raindrops
(collision process). The Bergeron and collision processes are supported by
the strong updrafts that can be present in cumuliform cloud.
The greater the updrafts are the greater the collision and fragmentation
processes will be, which leads to greater precipitation intensity. For this
reason, the heaviest rainfall occurs with the largest convective type cloud.
Cloud water content
The water content of the cloud is related to both the temperature of the
cloud and the moisture source available for evaporation of water into the air.
Tropical air that has originated over the oceans produces the heaviest
precipitation. It is in this air mass that monsoons and hurricanes, which
cause devastating floods, occur.
Cloud vertical thickness
The vertical thickness of cloud is important for two reasons. First, the air
must be warm in order to hold a lot of water vapour. This is only possible
with a low cloud base. Second, ice crystals are required to produce
precipitation. The cloud top must extend above the freezing level (Bergeron
effect). These two statements imply that a thick cloud is necessary for a
heavy rainfall
