Download Lecture: Condensation and precipitation

Condensation and Precipitation
Growth of Cloud Drops
Atmospheric
vertical winds
and eddies can
keep small and
light cloud
droplets and
condensation
nuclei aloft.
Figure 8.1
As the cloud
droplets knock
and join together,
they grow larger
and their weight
increases, causing
them to fall as
precipitation.
Evaporation & Droplet Size
Figure 8.2
Water molecules more easily evaporate from a curved surface than a
flat surface, which creates a greater concentration of molecules
above the droplet, known as increased equilibrium vapor pressure.
Vapor Pressure & Saturation
Smaller drops have
greater curvature and
require greater vapor
pressure to keep water
molecules from
evaporating away.
As the drop size
increases, the required
relative humidity
(RH) for equilibrium
decreases.
If the required RH is
exceeded, the drop
will grow.
Figure 8.3
Collision & Coalescence Process
Both a) collisions
that join together
small cloud droplets
b) coalescence that
attaches faster and
larger droplets with
smaller slower
droplets work
together to
assemble nearly 1
million cloud
droplets into a
raindrop large
enough to fall to
earth.
Figure 8.4A
Collision-Coalescence
Warm Cloud Processes
Collision and
coalescence
operates in
warm clouds (>
15° C) to
produce rain,
and is affected
by the clouds
liquid water
content, droplet
sizes, cloud
thickness,
updrafts, and
drop electrical
charges.
Figure 8.5
Evaporating Rain
Rain falling
into low
humidity air
below will
cause the
drops to
decrease in
size, possibly
evaporating
into streaks of
dry air as in
this virga.
Fall streaks & Sublimation
Ice crystals in
cirrus clouds
that fall and
sublimate into
drier air, fall
streaks are
produced that
indicate wind
speed and
direction.
Figure 8.15
This is similar
to rain
evaporating in
a virga.
Bergeron Process
Ice Crystal Process
Cold clouds may drop
below –40° C before
small droplets freeze into
ice embryos that can
serve as condensation
nuclei.
Figure 8.6
At very low
temperatures, vapor can
also condense as ice onto
nuclei formed by:
a) deposition
b) freezing
c) contact,
primarily in the glaciated
region of the cloud.
Molecules from Water to Ice
Figure 8.8
Ice crystals have lower saturation
vapor pressures than liquid droplets,
creating a gradient of high to low
water molecules from liquid to ice
that encourages ice growth.
This growth is critical to the icecrystal precipitation process.
Figure 8.9
Ice Particle Changes
As ice crystals fall and collide with super
cooled drops, they get bigger by accretion.
Falling icy matter is called graupel, and
aggregation describes the joining of two
ice crystals into snowflakes.
Figure 8.10A
Ice Crystal Growth
Ice crystal growth
is the dominant
cause of
precipitation in
nimobstratus and
cumulonimbus
clouds, both of
which have lower
liquid content than
warm-layered
clouds such as
stratus.
Figure 8.12
Snowflakes & Snowfall
Figure 8.17
Figure 8.18
Snowflakes are crystalline structures that can have plate,
column, dendrite, or needle forms.
Air temperature and humidity determine crystal form, and
dendrite is the most common habit.
Sleet & Freezing Rain
Environmental
temperatures may
reveal a warm zone
between two
freezing layers.
Snow falling into
the warm zone will
melt and either
a) fall as rain and
refreeze on contact
with the ground, or
b) refreeze and fall
as sleet.
Figure 8.19
Sleet & Freezing Rain - II
Figure 8.22A
Figure 8.22C
Figure 8.22B
Figure 8.22D
Four vertical temperature profiles are shown to illustrate the phase change
that a snowflake may experience in its path toward earth's surface.
Rime & Freezing Rain
Rime, a granular ice,
accumulates when super cooled
fog droplets touch a frozen
surface.
Figure 8.20
Freezing rain creates incredible
strain on branches and other
structures, resulting in costly
damages.
Figure 8.21
Snow Grain & Pellet
Figure 8.23
Flat and long snow grains fall as frozen drizzle, too small to bounce
or shatter.
Snow pellets are larger, and have a rounded layer of rimed ice that
creates air bubbles and a bounce.
Formation of Precipitation Types
Formation of Sleet
Formation of Hail
Hailstones & Damage
Figure 8.25
Figure 8.24
Updrafts in a towering cumulus cloud recirculate graupel
through an accretion and freezing process that produces large
heavy hailstones.
Such storms cause regular property damage, but only 2 U.S.
deaths in the 20th Century.
Coffeyville Hailstone
Figure 8.26A
Figure 8.26B
Regular and polarized light images of the 14 cm diameter
hailstone that fell in Kansas in 1970.
Hailstorms cause severe damage to crops and other structures.
Measuring Rain w/ Standard Gauge
Standard rain gauge uses a funnel
to collect rain and then stores it in
a narrower tube, so that the gauge
detection is amplified 10-fold.
The 50 cm long tube, when filled,
represents only 5 cm of total
rainfall.
Figure 8.27
Measuring Rain w/ Recording Gauge
Figure 8.28
Tipping bucket and weighing rain gauges record precipitation rate
at shorter time intervals, providing rain intensity data.
Snow intensity can be measured with depth recorders, or
accumulated totals with measuring sticks.
Radar Rainfall
Figure 8.29A
Figure 8.29B
Doppler radar uses microwave transmission and reception to
measure backscatter intensity, or reflectivity for large geographic
areas.
This signal is converted into maps of precipitation intensity, while
phase shift data provide information on storm movement.
Cloud Seeding
Figure 8.11
Artificial seeding, such as Silver Iodide, and natural seeding, such as
cirriform ice crystals, are available to increase the number of
condensation nuclei and encourage precipitation.