# Chapter 5 - Title by c1z1Y38

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```									     Chapter 5 - Title

Chapter 5 –
Cloud Development and
Precipitation
(pp. 75-107)
Contents

• stability
• cloud development
• precipitation
Clouds

What do they do for us?

•   Formation releases heat
•   Reflect, scatter, absorb IR radiation
•   Produce precipitation
•   Visually indicate the stability of the atmosphere
Questions

1.    How and why do clouds form on some days and
not on others?

2.    Why does the atmosphere sometimes produce
stratus clouds (thin layered) while other times
we get cumulus, or cumulonimbus clouds to
form?

The answer is largely related to the concept of
atmospheric stability...
Cloud Development - Stable Environment
• Consider this simple situation of a marble in the
bottom of a bowl
• If you push the marble up the side of the bowl, what
happens?
• Stable air (parcel) - vertical motion is inhibited
– if clouds form, they will be shallow, layered
clouds e.g. stratus
Cloud Development - Unstable Environment

• If the marble is on the top of the bowl and you give
it a little push, what happens?
• This is an unstable situation
• Unstable air (parcel) - vertical motion occurs
– commonly produces cummulus, cumulonimbus
clouds
• So, the question becomes,
how does one determine
the stability of the atmosphere?
Determining Stability
• Compare temperature (Tp) of a rising air parcel to
that of its surroundings (Te)

– Tp > Te : If rising air is warmer and less dense than
the surrounding air, it will _____ until it reaches
the same T as its surroundings.

– Tp < Te : If rising air is colder it will be more dense
than the surrounding air and _____ back to the
original level.

– Tp = Te : What happens?
Rising and Sinking Air

Expands
Lower P
and cools

Surrounding or
Environmental air

Air Parcel

without heat exchange
to surrounding air
Higher P           Compressed
and warms
See CW5 Q.A1
• As a parcel of air rises, it cools, but at what rate?

• rate of temperature change with height is called the
lapse rate. Units of lapse rate are °C /km

• Let's first consider an unsaturated parcel of air

• unsaturated parcels cool at a rate of 10°C /km - this is
called the dry adiabatic lapse rate

– If the surface temperature is 40°C What will be a
parcel's temperature at 1 km?

– What will be a parcel's temperature at 2 km?

• Moist Adiabatic Lapse Rate (MALR) = less

• If air cools to T = TDP, RH = 100%
– Cloud forms and heat is released

• Latent heat added due to condensation offsets
cooling due to expansion

• Air cools at a lesser rate (6°C / km)
– What will be a parcel's temperature at 3 km?
(note: its now cooling at MALR)

– What will be a parcel's temperature at 4 km?
Absolutely Stable Air

• Normally, air T decreases with height

• Environmental Lapse Rate (ELR)
– Radiosonde ELR = 4 °C/km (example)

• Stable if rising parcel cools more rapidly with height than
surrounding environmental T.
– Always colder than surroundings
MALR =             Absolutely Stable Air
6 °C/km                                                        Small
DALR =
ELR
10 °C/km
ELR = 5 °C/km

Fig. 5.3: An absolutely stable atmosphere. Rising air parcel is always colder
and heavier than surrounding air (DALR > MALR > ELR).
See CW5 Q.A2
Question

How would you characterize the stability of an
inversion layer?
Inversion Layer

• They are absolutely stable
DALR > MALR > ELR                                  ELR

• Q: How do you form stable
layers in the atmosphere?

How do we make the ELR
turn to the right?

ELR temp. is now dec. with
Ht. = inversion
Formation of Stable Layers
• Air aloft warms
– Sinking air (subsidence)

• surface cools
cooling
– Cold air moving in
at low levels
– Warm air moving over
a cold surface
(cold frontal passage)
Formation of Stable Layers

• Stable air strongly resists               Air is most stable
upward motion                             at sunrise.

• If clouds form in stable air

• e.g. cirrostratus, altostratus,
nimbostratus, stratus have
flat tops and bases

Fig. 5.4: Cold surface air. A stable
atmosphere that inhibits vertical air
movement
Unstable Air

• Unstable if environmental air Temperature
decreases with height more rapidly than a
rising air parcel cools

• The rising parcel will become warmer than
its environment and will rise
Unstable Air
ELR = 12 °C/km

MALR =
6 °C/km
DALR =
10 °C/km

Fig. 5.5: An absolutely unstable atmosphere. Rising air parcel is always
warmer and lighter than surrounding air (ELR > DALR > MALR).

See CW5 Q.A3
Conditional Stability
• The unsaturated parcel will be cooler than then environment
and will sink back to the ground
• The saturated parcel will be warmer than the environment
and will continue to rise
• This is an example of conditional instability
– What is the condition?

ELR = 8 ° C/km          MALR = 6 °C/km

DALR =
DALR > ELR > MALR
10 °C/km

See CW5 Q.A4
Conditionally Unstable Air

• Conditionally unstable if:
– Unstable when saturated (condensation releases heat,
making it more buoyant)
– Stable if unsaturated
– i.e. DALR > ELR > MALR

• Common situation
– Since average ELR is 6.5 which lies between MALR and
DALR
– (compare with average lapse rate)
Questions
What causes the atmosphere to become more unstable?

What causes air to rise so that clouds can form?
Formation of Unstable Layers
Destabilizing the Atmosphere

• Air T drops rapidly with
height
• ELR steepens

1. Cooling of air aloft
• Winds bring in cold air
• Clouds emit IR to space

2. Warming of the surface
• Daytime surface heating
• Influx of warm air by wind
• Air moving over a warm
surface
Formation of Unstable Layers

3. Warm air moving in at low levels
• This often occurs ahead of a cold
front

4. Cold air moving over warm
surface
• What is an example of this?
• Example over Lake Michigan
Stability Summary

• Clouds with rapidly rising air, especially cumulus
congestus and cumulonimbus, indicate unstable
conditions

• Stratus clouds indicate stable conditions
Review
•   The air temperature in a rising parcel of unsaturated air decreases at
the dry adiabatic rate, whereas the air temperature in a rising parcel of
saturated air decreases at the moist adiabatic rate
•   The dry adiabatic and moist adiabatic rate of cooling are different due
to the fact that _______ ______ is released in a rising parcel of
saturated air
•   In a stable atmosphere, a lifted parcel of air will be colder (heavier) than
the air surrounding it. Because of this fact, the lifted parcel will tend to
sink back to its original position
•   In an unstable atmosphere, a lifted parcel of air will be warmer (lighter)
than the air surrounding it, and thus will continue to rise upward, away
from its original position
•   The atmosphere becomes more stable (stabilizes) as the surface air
_______, the air aloft _______, or a layer of air sinks (subsides) over a
vast area
•   The atmosphere becomes more unstable (destabilizes) as the surface
air ______, the air aloft ______, or a layer of air is lifted
•   Layered clouds tend to form in a stable atmosphere whereas
cumuliform clouds tend to form in a conditionally unstable atmosphere
Cloud Development and Stability

• Stages:
– Thermals breaks from surface and ‘lifts’
– Expands and cools on lifting
– Cools to saturation point
– Moisture condenses
– Visible as cloud

• Can occur in 4 different ways:
Cloud Formation Mechanisms

Fig. 5.8: Cloud formation. (a) surface heating and convection, (b) forced
lifting by topography, (c) convergence, (d) frontal lifting
(a) Convection
Fig. 5.9: Cumulus cloud
formation. Rising and falling
currents of air sustain the cloud.

Fig. 5.10: Rising air in the cloud and
sinking air around it inhibits growth of
thermals around the Cu.

Growth shades the ground cutting off
• The vertical extent of the cloud is   surface heating. Leads to erosion by
largely determined by the             evaporation (fractus).

stability of the environment...
Stability and Cumulus Clouds

• In an absolutely stable, environment, no clouds will
likely form

• In a shallow conditionally unstable or absolutely
unstable environment, one may expect clouds to
develop, but their vertical growth will be limited...
Stability and Cumulus Clouds
• Stable above the cumulus
– Does not rise ‘fair weather Cu’ or cumulus humilis
• Unstable above the cumulus
– Towering cumulus congestus, then cumulonimbus

Fig. 5.10: Cb reaches stable part
of atmosphere forms an ‘anvil’ top
Questions

1.   Are the bases on convective clouds generally higher
during the day or the night? Explain.

2.   For least polluted conditions, what would be the best
time of day for a farmer to burn agricultural debris?
(b) Topography
• Orographic uplift
– forced lifting
along a barrier
• Forced lifting
produces cooling
• Cloud on
windward side
• Downwind side is
warmer and drier
why?

Windward side              Leeward side
Fig. 5.13: Clouds
also form on the
leeward (downwind)   Topography
side.

- Moist air crossing a mountain
barrier at altitude forms into waves
- Clouds form in wave crest
- ‘Lenticular’
(c) Convergence

• If air converges to a given
location near the surface:
– it can't "pile up" at that point
– it can't go downward, the
ground is there
– it must go up!

• Common at the center of an
extra-tropical cyclone
(L pressure system)
(d) Frontal Lifting

• If air is lifted into a stable layer:
– stratus or nimbostratus clouds
are often the result
(common along warm fronts)

• If air is lifted into a conditionally
unstable layer:
– cumulus or cumulonimbus are
often the result
(common along cold fronts)
Precipitation

• Q: How does precipitation form?
– not all clouds produce precipitation, why?

• Q: What determines the type of precipitation?
Too small
Rain Drops, Cloud Droplets, and CCN                 to fall

• Formation of raindrops from
cloud droplets is complex

– Cloud droplet: 0.02 mm
– Rain droplet: 2 mm

• Radius is 100 x greater,
so volume is 100 x 100 x 100
= 1x 106 times greater

• Problem is to explain how a cloud droplet can increase
in size a million-fold in 10’s of minutes
Precipitation Processes

• Condensation nuclei play a role

far too slow

• 1 million cloud drops to 1 raindrop must be
another process…
Rain in Clouds Without Ice (Warm Clouds)
• Collision and coalescence (sticky water)
–   Most important factor is liquid water content
–   Thickness of cloud
–   Updrafts
–   Electrical charge (+/-) (Smaller drops tend to bounce off one
another)
Rain in Clouds Without Ice

• Explains why thin stratus only produces
drizzle
• Towering cumulus with rapidly rising air can
produce heavy showers
Questions

1. Why is a warm, tropical cumulus cloud more likely to
produce precipitation than a cold, stratus cloud?

2. Clouds that form over water are usually more efficient in
producing precipitation than clouds that form over land.
Why?
Rain in Clouds With Ice (Cold Clouds)

• At temperatures < freezing ice crystals coexist with liquid drops

• No. cloud droplets > No. ice crystals

• Liquid water exists as so called ‘supercooled’ water
(-40°C < Temp < 0°C)

• only ice is found at altitudes above -40°C

Why?
• Smaller the amount of pure water, lower the freezing point
(less than 0 ºC)

• Few ice-forming nuclei (minerals, bacteria, leaf, ice crystals)
Supercooled Water Exp.

Time = 0 mins
17 droplets of water
placed in freezer at
11 ºC

Time = 10 mins
8 frozen droplets of
water

Bohren (1987)
Results: Homogeneous Freezing

• Pure water drops do NOT freeze at 0°C
– it needs to be colder
• Larger water drops will freeze at warmer
temperatures than smaller drops
• Smaller water drops require colder temperatures to
freeze
• Hence, you will find more smaller drops than larger
drops higher in the cloud
Rain in Clouds With Ice (Cold Clouds)

• Neither the liquid cloud drops or solid ice particles are
large enough to fall as precipitation

• Given a mixed cloud containing water and ice, which
type of particle (ice v.s. water) will grow more quickly
and why?
Bergeron Process (the 3 Phase process)
Bergeron or Ice-Crystal Process
• Supercooled liquid droplets surround each ice crystal
– Air is saturated, both are in equilibrium
• More water vapor molecules around liquid
– Escape from liquid surface easier
• Saturation V.P. above water droplet is greater than the ice
• Water vapor molecules diffuse from droplet to ice crystal
• Ice crystals grow larger at the expense of the water droplets

• Also
– Accretion
– Aggregation
Accretion, Fracturing and Aggregation

Accretion, fracturing and aggregation may contribute to production of rain
Rain in Clouds With Ice (Cold Clouds)
(homogeneous+
heterogeneous
nucleation)

Bergeron Process

(Heterogeneous
nucleation)

Collision and coalescence
Cloud Seeding

• Man-made: Inject nuclei e.g. silver iodide or dry ice
• Cloud particles become large enough to fall as rain
• Not very effective!

Fig. 5.22: Natural seeding by cirrus clouds
Questions

1. Suppose that a thick nimbostratus cloud contains ice crystals and
cloud droplets all about the same size. Which precipitation process
will be most important in producing rain from this cloud? Why?

2. When cirrus clouds are above a deck of altocumulus clouds,
occasionally a clear area, or "hole," will appear in the altocumulus
cloud layer. What do you suppose could cause this to happen?
Review

•   Cloud droplets are very small, much too small to fall as rain
•   Cloud droplets form on cloud condensation _____. Hygroscopic nuclei,
such as salt, allow condensation to begin when RH is less than 100 %
•   Cloud droplets, in above freezing air, can grow larger as faster-falling,
bigger droplets collide and coalesce with smaller droplets in their path
•   In the ice-crystal (Bergeron) process of rain formation, both ice crystals
and liquid cloud droplets must coexist at below-freezing temperatures.
The difference in saturation vapor pressure between the liquid and ice
causes water vapor to diffuse from the _________ _________ (which
shrink) toward the ________ ________ (which grow)
•   Most of the rain that falls over middle latitudes results from melted
snow that formed from the Bergeron process
•   Cloud seeding with _________ __________ can only be effective in
coaxing precipitation from clouds if the cloud is supercooled and the
proper ratio of cloud droplets to ice crystals exists
Precipitation Types Summary

• Much rain starts as snow through the ‘ice crystal
process’ and melts as it falls
• Rain drops are not tear shaped. They are round
or flattened
• Water freezes into different shapes at different
temperatures
• A warm layer above a cold surface can result in
sleet or freezing rain
• Hail requires strong updrafts to be supported
Precipitation Types - Rain
•   Rain – drop diameter > 0.5 mm (0.02 in)

•   Drizzle – drop diameter < 0.5 mm (most from stratus)

•   Virga – rain leaving cloud base and evaporates before hitting the ground
– often visible as evaporating streaks of precipitation

Rain Events:

•   Showers - localized, sometime heavy rain events
– usually associated with cumulonimbus
– sometimes called a "cloud burst“

•   Continuous rain - from nimbostratus
Are Raindrops Tear-Shaped?

• Which shape more accurately depicts a rain drop?

• Surface tension tends to squeeze drop to a shape
that has the smallest surface area for its volume
Precipitation Types - Snow
•   Snow - Often visible as fall streaks associated with high cirrus
– More likely to reach surface when cold

Snow Events:

•   Flurries - weak, intermittent - produced from developing Cu
•   Snow squalls - brief, heavy snow fall - produced from Cu
•   Steady Snow - continuous for hours - produced from Nb
•   Blizzard - low temperatures, strong winds, blowing snow
Snow

• Warm, moist air, thin film of water acts as glue
= giant snowflakes

• Cold, dry air = powdery flakes

• Branching ‘dendrite’ pattern

• Aggregates lead to many complex patterns
Other Types of Frozen Precipitation

• The vertical variation of temperature near the ground can
have a dramatic influence on the type of precipitation that
is observed at the ground

SNOW

Snow                                 Rain
Precipitation Types - Sleet
•   If a deep freezing layer exists at
low levels, sleet may form
•   Snow will melt in warmer air
layers
•   Surface layer will re-freeze into
ice pellet known as sleet

•   Layer may be too shallow to
refreeze, liquid may refreeze on
contact with ground
•   Thickness of cold layer distinguishes
between sleet and freezing rain
(glaze)
Meteograms
A more complicated version of the graphs shown in these slides.

Which shows Sleet and which shows Freezing Rain?

A                                                     B
Precipitation Types - Freezing Rain
•   If a shallow freezing layer exists at low levels, freezing rain,
or glaze may form

•   Supercooled fog or
cloud freezing = rime
Precipitation Types - Snow Grains and Pellets

• Snow grains – small opaque
grains of ice (drizzle sized
from St cloud)
• Snow pellets (graupel, soft hail)
– white opaque grains of ice
size of a rain drop
(break/bounce on impact)

Fig. 5.29 Accumulation of small hail after a thunderstorm
Precipitation Types - Hail

• Graupel, frozen rain,
insects act as
accumulation points
‘embryos’
• Grow by accretion in Cb
clouds

Largest ever reported in US. Fell in
Aurora, NE in 2003, 7 inches across.
Measuring Precipitation
• Rain gauge contains a funnel to
magnify the effective rain depth

• EXAMPLE: 0.1 inches of rain would
fill the inside tube with 1 whole inch of
water, and a scaled ruler would
measure this as 0.1 inches of actual
rainfall.

*Imagine how hard it would be to measure to
the nearest .01 of an inch in a regular bucket of
water with a standard ruler!
Measuring Precipitation

• A tipping bucket gauge –
a pair of buckets seesaw,
alternately fill and dump rain
every 0.01”

• The amount of precipitation
can be recorded along with
the time of occurrence and
intensity (amount of rain per
unit of time).
• ra(dio) d(etecting) a(nd) r(anging)
– Radars transmit electromagnetic waves that bounce off of
precipitation particles
• Radar waves are scattered by rain or snow
– Almost no scattering by cloud droplets (too small)
– Strength of radar echo is related to rain or snowfall rate

• Doppler radar measures everything a non-Doppler measures,
plus speed of approaching or receding objects)

Frequency/
wavelength
change of echo
‘Doppler shift’

(change in frequency (or λ) of
waves caused by object
approaching or receding)

•   Many TV stations disguise the
fact that they use NWS radar

•   Some have their own radar,