Cloud Development and Stability

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Cloud Development and Stability

• Chapter 5

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Stability

• Probably the hardest chapter for students
• So read!!!
• Ask questions if anything is unclear
• In general, clouds form as a result of warm air
  rising, cooling, and expanding
• So the questions are
• Why do we have clouds sometimes and not others?
• Thunderstorms?
• Different cloud types (shapes/sizes)?

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Stability

• All of these questions can be answered by
  examining the concept of atmospheric stability
• Stable Equilibrium - Something (air) pushed away
  from its original position wants to return
• Unstable Equilibrium - Something (air) pushed
  away from its original position wants to keep
  moving away
• For our purposes, were talking about air being
  pushed up or down (vertical air motions)

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Equilibrium
A
Stable
Unstable
B
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Stability

• In the atmosphere, these pushes of air include
• Heating
• Fronts
• Terrain

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Air Parcel

• We used the term parcel when talking about
  moving air up or down in the atmosphere
• Just a balloon-like volume of air that does not
  mix with the surrounding air
• Defn. - Adiabatic - a process in which no heat
  is exchanged between an air parcel and the
  surrounding environment.
• If it rises, the air inside expands and cools
• If it sinks, the air inside compresses and warms
• Same amount of air, just at different pressures

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Adiabatic Process

• The rate at which a parcel cools as it rises or
  warms as it sinks depends on whether or not the
  air is saturated (Avg. 6.5º C per 1000 meters)
• If the air is unsaturated (RHlt100), this rate is
  10º C per 1000 meters is is called the dry
  adiabatic lapse rate

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Dry Adiabatic Lapse Rate
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Moist Adiabatic Lapse Rate

• If an unsaturated parcel of air rises and cools,
  it will eventually cool to its dew point where it
  will be saturated (RH100)
• Further cooling results in condensation
• This is when a cloud begins to form
• Also, condensation represents a phase change of
  water from a gas to a liquid. Latent heat is
  released
• So if the air still continues to rise, will it
  still cool at the dry adiabatic rate?

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Moist Adiabatic Lapse Rate

• No, the rate will be less due to the release of
  latent heat
• So, rising saturated air does not cool as quickly
  as rising unsaturated air
• In fact, it cools at an average rate of 6ºC per
  1000 meters which is called the moist adiabatic
  lapse rate
• Same rate for warming if air is sinking

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Lapse Rates
4º
RH 100
3000 m
10º
RH 100
2000 m
20º
RH lt 100
1000 m
30º
RH lt 100
Surface
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Determining Stability

• Now we know how air temperature will change as
  air rises or descends
• All we need to know is the air parcels
  temperature and moisture at some level (like at
  the surface)
• And we also know what mechanisms start air moving
  vertically (hot surface, fronts, terrain)
• We need to know one more thing in order to
  determine whether or not air will continue to
  rise on its own once started
• ????

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Determining Stability

• Need to know if the air parcel is warmer or
  cooler than its environment if lifted
• So we need to know the temperature of the
  environment
• Defn. - environmental lapse rate - the actual
  rate of temperature change w/ height
• How do we do this?
• Radiosonde
• Once we know this, we can figure out what the
  stability properties of the atmosphere are

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Determining Stability

• If a parcel rises and cools, and is then colder
  than the surrounding air, it will sink back to
  its original position - stable
• If the parcel is warmer than the surrounding air,
  it will continue to rise - unstable

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A Stable Atmosphere

• Example 1
• Environmental lapse rate is 4ºC/1000 m
• If air is unsaturated, it will cool at the dry
  adiabatic rate if lifted and will always be
  cooler than environment
• So, its tendency is to sink back to its original
  position

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A Stable Atmosphere

• Example 1
• If air is saturated, it will cool at the moist
  adiabatic rate if lifted and will always be
  cooler than environment
• So, its tendency is to sink back to its original
  position

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A Stable Atmosphere

• So in the previous example, it didnt matter
  whether or not the parcel was saturated
• It would always have been cooler than the
  surrounding environment if lifted
• This is called an absolutely stable atmosphere
• If air were to be forced to rise (mountains) in
  this type of situation, it would tend to spread
  our horizontally and result in thin, layered
  clouds - stratus, altostratus, etc.

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A Stable Atmosphere

• The atmosphere is stable when the environmental
  lapse rate is small (or negative)
• Absolutely stable if less than moist adiabatic
  rate
• So, the atmosphere becomes more stable as the air
  aloft warms and/or the surface air cools
• Surface air cools by radiation (night), cold
  fronts, air moving over cold surfaces
• This is why thunderstorms usually die at night -
  surface cools

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An Unstable Atmosphere

• Example 2
• Environmental lapse rate is 11ºC/1000 m
• If air is unsaturated, it will cool at the dry
  adiabatic rate if lifted and will always be
  warmer than environment
• So, its tendency is to keep rising on its own

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An Unstable Atmosphere

• Example 2
• If air is saturated, it will cool at the moist
  adiabatic rate if lifted and will always be
  warmer than environment
• So, its tendency is to keep rising on its own

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An Unstable Atmosphere

• So in example 2, again it didnt matter whether
  or not the parcel was saturated
• It would always have been warmer than the
  surrounding environment if lifted
• This is called an absolutely unstable atmosphere
• If air is forced to rise in this type of
  situation, it would tend to keep rising on its own

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An Unstable Atmosphere

• The atmosphere becomes unstable when the
  environmental lapse rate becomes large (cools
  quickly with height)
• Absolutely unstable if greater than dry adiabatic
  rate
• So, the atmosphere becomes more unstable when air
  aloft cools and/or air near the surface warms
• Surface air warms during the day, when a warm
  front passes, air moving over warm surfaces
• Almost never find absolutely unstable layers
  except near the surface on hot days

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A Conditionally Unstable Atmosphere

• Example 3
• Environmental lapse rate is 7ºC/1000 m
• Lies between the moist and dry lapse rates
• If an unsaturated parcel is lifted, it will
  always be cooler than the surrounding air and
  will tend to sink back to its original position
• Stable with respect to unsaturated air

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A Conditionally Unstable Atmosphere

• Example 3
• If a saturated parcel is lifted, it will always
  be warmer than the surrounding air and will tend
  to rise on its own
• Unstable with respect to saturated air

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A Conditionally Unstable Atmosphere

• In example 3, stability is dependent upon whether
  or not rising air is saturated or unsaturated
• If unsaturated - stable
• If saturated - unstable
• This is called a conditionally unstable
  atmosphere
• Condition??
• Whether or not the air becomes saturated

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Environmental Lapse Rate of 9ºC
T3º
4º
RH 100
3000 m
T12º
10º
RH 100
2000 m
(Condensation level)
T21º
20º
RH lt 100
1000 m
30º
RH lt 100
T30º, dew point 10º
Surface
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Stability

• Absolutely stable if the environmental lapse rate
  lt moist adiabatic rate
• Absolutely unstable if the environmental lapse
  rate gt dry adiabatic rate
• Conditionally unstable if the environmental lapse
  rate is between the dry and moist adiabatic rates

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Cloud Development and Stability

• Touched on this briefly already
• 4 major ways air is forced to rise and produce
  clouds
• 1) Heating at the surface (convection)
• 2) Topography (mountains, hills, etc.)
• 3) Convergence of surface air (air flows come
  together)
• 4) Uplift along fronts
• 1st two for now, 3 4 later on

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Cloud Development and Stability
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Convection

• Talked about this already
• Hot surface heats air
• Warm air rises
• Cooler air from above sinks to replace it
• If the condensation level is low
• One thermal may cause a cumulus cloud
• If high
• May take several thermals

Sinking air at sides causes lots of blue sky in
between clouds
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Convection on a Summer Day
Thermals do all of this!
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Cloud Streets
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Convection

• How much vertically a cumulus cloud grow will
  depend on stability
• If stable near cloud top - difficult time growing
• probably stay small
• If unstable or conditionally unstable through a
  deep layer - good chance for much vertical growth
• towering cumulus clouds
• If unstable over a very deep layer (several
  miles) - possible thunderstorm growth

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Convection in an Unstable Atmosphere
Thunderstorm
Towering cumulus clouds
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Topography

• Basically, surface air must rise over a mountain
  range if the wind is coming from a certain
  direction
• This forced ascent over mountains is called
  orographic uplift
• Has a MAJOR effect on precipitation and
  temperature
• Rain shadows

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Hawaii Rain Shadows
Wind
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Precipitation

• We know some clouds produce rain, right?
• But why do some clouds precipitate but not
  others?
• And how do cloud droplets grow large enough to
  fall to the earth?
• From earlier, what do all cloud droplets need in
  order to form?
• Condensation nuclei
• Water vapor condenses on them

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Precipitation

• Based on the sizes of CN, cloud droplets, and
  rain drops.do you think rain drops form just due
  to condensation???
• It would take about 3 days for that to happen
• How long does it take for storms to pop up in the
  summer?
• 1 hour or less

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Precipitation

• So, the process of condensation is not quick
  enough to produce raindrops
• How do they form then??
• 2 processes

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Collision and Coalescence

• 1) Collision and Coalescence
• Also called the warm rain process because it
  only involves liquid water
• Main thing needed for this process to work is
  droplets of different sizes
• How might that happen?
• 1) Simply random collisions
• 2) Different size CN
• Ex. Salt is larger than dust so its cloud drop
  will be larger too

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Collision and Coalescence

• Since larger drops are heavier, they fall faster
  than smaller drops
• As they fall they collide and merge with smaller
  drops - coalescence

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Collision and Coalescence

• The absolute most important factor in this
  process is liquid water content
• Must have water!!
• Other important factors include
• Range of droplet sizes
• some larger than others
• Cloud thickness
• Updrafts in a cloud
• Electric charge of droplets
• opposites attract

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Collision and Coalescence

• This cumulus cloud has a good chance of becoming
  a thunderstorm and producing rain
• It has lots of water
• Its thick
• It has updrafts
• What about nimbostratus clouds??
• Thin - means small if any updraft and not much
  water
• Drizzle at best

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Ice Crystal Process

• 2) Ice Crystal Process
• Occurs in clouds comprised of both liquid water
  and ice crystals - like thunderstorms

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Ice Crystal Process

• All water at low levels
• All ice at very high levels
• In between - both water and ice
• How does this happen if temp is below freezing?
• Two reasons
• 1) Small cloud droplets freeze at lower temps
• 2) Ice crystals need ice nuclei on which to
  form
• Not many of these things
• Clay, bacteria, etc.

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Ice Crystal Process

• Remember saturation?
• In the middle of the cloud (ice and water), the
  air is saturated
• But, since water is warmer, it takes more water
  vapor molecules to saturate the air around it
• So, is the saturation vapor pressure greater
  around the drop or the ice??

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Ice Crystal Process

• Around the water since there are more molecules
• Things are out of balance now and vapor molecules
  will move toward the ice to equalize the pressure
• But if they move into already saturated air over
  the ice, what happens???

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Ice Crystal Process

• Molecules must condense out of the air onto the
  ice
• Ice grows
• Also, now the air is unsaturated over the water
• So more molecules evaporate from the drop to
  replace those lost to the ice

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Ice Crystal Process

• Bottom line
• The ice crystals grow in size at the expense of
  the water drops
• Ice crystals get heavy and fall

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Ice Crystal Process

• Falling crystals collect super-cooled water
  droplets - accretion
• The icy product is called graupel
• melts to form rain

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Icy Crystal Process

• Falling crystals can also fracture as they
  collide with other crystals
• Happens in colder clouds
• If the pieces stick together..

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Ice Crystal Process

• Snowflakes
• This process of ice crystals forming snow is
  called aggregation
• So the thunderstorms we see around here have all
  kinds of things going on in them
• ice, water, snow, graupel, hail, etc
• Does a snowflake really look like this?

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By the way, what shape is the correct one for a
rain drop??
Except small drops
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Types of Precipitation

• Im not going into all of the types (maybe too
  boring)
• Well concentrate on rain, snow, sleet, and
  freezing rain
• How do these differ and why do we get each type?
• Has to do with the vertical profile of temperature

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Snow and Sleet

• Snow is easy - freezing from cloud down to the
  surface
• Sleet - happens when there is a deep freezing
  layer near the surface and an above-freezing
  layer below the cloud

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Freezing Rain and Rain

• Freezing rain - a lot like sleet except the
  sub-freezing layer near the surface is shallow -
  so rain stays in liquid form until it hits the
  surface
• Rain - occurs when there is a deep above-freezing
  layer from the surface upward

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More Frozen Precipitation

• Occurs mostly in the spring and summer??
• Hail
• Lethal stuff
• Can damage crops, cars, property
• 2K to my car a couple of years ago
• Even responsible for killing livestock
• Big hail is pretty heavy (1/2 lb or so) and falls
  quickly

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Hail

• Forms in T-storms w/ strong updrafts
• Begin as embryos
• usually ice particles but sometimes bugs too
• updrafts cause embryo to come in contact with
  super-cooled water which freezes on it
• 5-10 minutes to become golf ball size

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Measuring Precipitation

• 2 primary methods
• 1) Gauges
• 2) Radar

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Rain Gauge

• Old fashioned gauge
• Funnel on top channels water into a tube
• How can a person possible measure rain with this
  thing if only .05 of an inch fell?
• Top of funnel is 20 cm wide (10X width of tube)
• So rainfall is amplified 10X in the tube allowing
  for precision

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Tipping Bucket Rain Gauge

• Bucket holds .01 inches of rain before it tips
• Need to have at least .01 inches to use the term
  rainfall
• If less - trace

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RADAR

• Stands for RAdio Detection And Ranging
• How it works
• Transmitter sends out a microwave pulse
• If the pulse hits something (rain), some of it is
  reflected back toward the radar
• How much comes back indicates the intensity of
  the rain
• Advantage
• We can see and estimate rainfall in previously
  inaccessible regions

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Doppler Storm Total Rainfall

• Estimates total rainfall over a period of time
  based on intensity during that period