METEOROLOGY
GEL-1370
Chapter Five
Cloud Development & Precipitation
Goal for this Chapter
We are going to learn answers to the following questions:
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Why there any instabilities in the atmosphere?
How can we make the atmosphere more stable?
Why cloud droplets seldom reach the ground?
How rain drops are produced?
How does the ice crystal process forms precipitation?
What is cloud seeding?
Difference between freezing rain and sleet?
How does Doppler radar measure intensity of rain?
Why heavy showers fall from cumuliform while steady precipitation is derived from stratiform clouds?
Atmospheric stability
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A rising parcel of air expands and cools, while a sinking parcel is compressed and warms
When air is in stable equilibrium, after being moved up or down, tends to come back to its original position
Adiabatic Process: A process in which there is no transfer of heat between the air parcel and its surroundings
(compression --- warming & expansion --- cooling)
Dry adiabatic rate: Rate of change of temp in a rising or descending unsaturated air parcel; ~10°C/1000 m in
elevation
Moist adiabatic rate: Rate of change of temp in a rising or descending saturated air parcel; ~6°C/1000 m in
elevation
Concept of equilibrium
What happens to a rising air??
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Rising air----- cools ----- RH increases as the air temp approaches the dew-point temp-----if air cools to its dew
point temp, RH ~100%----- further air lifting leads to condensation ----- cloud forms -----latent heat is released ----Stable Air: If the rising air is colder than its surrounding air, then, it is heavier and will sink back to its original
position – stable air strongly resists upward vertical motion. If clouds form in rising air, cloud will spread
horizontally in relatively thin layers – cirrostratus, altostratus, nimbostratus or stratus clouds
Dry adiabatic rate; unsaturated air cools @10°C/1000m
Absolute stable atmosphere when rising air parcel is colder and heavier than surrounding air
Stable Air – contd.
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Atmosphere is stable when lapse rate is small
The cooling of surface air could be due to:
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Nighttime radiational cooling of the surface
Influx of cold air from other region brought by wind
Air moving over a colder surface
The air is generally most stable in the early morning around sunrise
Subsidence Inversion: Inversion produced by compressional warming – the adiabatic warming of a layer of sinking
air
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Presence of inversion near the ground fog, haze, & asso-ciated pollutants are kept close to the surface
Cold surface air produces a stable atmosphere that inhibits vertical motions – fog & haze are kept close to
the ground
Unstable Air
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When air temperature decreases rapidly as we move up, air becomes unstable
The warming of air may be due to:
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Daytime solar heating of the surface
An influx of warm air brought in by the wind
Air moving over a warm surface
As the surface air warms during the day, the air becomes more unstable – most unstable during summer months and
when there is much temp fluctuation in a day
Sinking air produces warming and a more stable atmosphere while rising air produces cooling and unstable
atmosphere
Unstable atmosphere – rising air parcel is warmer and lighter than the surrounding air
How stability of air affects the type of clouds formed
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Unsaturated Air parcel if forced to rise ---- expands and cools at the dry adiabatic rate --- cools until dew point –
now RH is 100% --- further lifting results in condensation and the formation of cloud --- The elevation above which
the cloud first forms is called condensation level
Conditionally unstable atmosphere (or conditional instability): When the environmental lapse rate is less than
the dry adiabatic rate but greater than the moist adiabatic rate, conditional instability exists.
Level of free convection: Level at which a lifted parcel of air becomes warmer than the surrounding in a
conditionally unstable atmosphere
Unstable Air. Warmth from the forest fire heats the air, causing insta. near the surface; warm, less dense air bubbles
upward, expanding & cooling as it rises – rises air cools to dew point, condensation begins & cumulus cloud forms
Conditionally unstable air – when unsaturated stable air is lifted to a level where it becomes saturated and
warmer than the air surrounding air
Cloud Development and stability
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Some surface heats up quickly --- air in contact warms --- hot ‘bubble’ of air (thermal) rises --- undergoes
expansion & cooling when it rises --- Two things can happen: i) thermal mixes with cooler air and looses its
identity and air vertical movement slows down; ii) air keeps cooling until it reaches to its saturation point --moisture will condense --- thermal becomes visible as a cumulus cloud
Outside of a cumulus cloud, there is downward movement of air because i) evaporation around the outer edge of the
cloud makes the air cooler and denser; & ii) completion of the convection current started by the thermal
How clouds form: a) surface heating & convection; b) forced lifting along topographic barriers; c) convergence of
surface air; d) forced lifting along weather fronts
Cumulus cloud formation from the hot air rising from earth’s surface – around the cloud, air is sinking
Why Cumulus clouds appear-disappear-reappear
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Cumulus clouds grow – shuts off surface heating and upward convection --- without continual supply of air, cloud
disappears --- heating and upward convection starts again
Topography and Clouds
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Large air masses rise when approaching a mountain chain --- this leads to cooling & if the air is cool,
clouds form --- Orographic clouds---during this condensation, latent heat is released
Temperature at the leeward side is higher (loss of heat in the upwind side); dew point temp on the
leeward side is lower than the windward side
Drier air in the leeward side; More rain in upwind side and rain shadow (low precipitation) in the
leeward side
Rain shadow, Orographic uplift & cloud development
Formation of lenticular clouds: Moist air rises in the upwind side of the wave, it cools and condenses,
producing cloud; in the downwind side, air sinks and warms – the cloud evaporates
Precipitation Processes
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Average diameter cloud droplets ~ 0.02 mm
Typical raindrop size ~ 2 mm
Growth of cloud droplets by condensation is slow to produce rain; clouds can develop and begin to rain
in less than an hour
1 million average size cloud droplets will make a average size raindrop – Other processes??
Two important processes on how rain is produced
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Collision-Coalescence Process
Ice-crystal (or Bergeron) process
Relative sizes of raindrops, cloud droplets, & condensation nuclei
Collision & Coalescence: a) warm cloud composed only of small cloud droplets of uniform size; b)
different size droplets
Collision & Coalescence – contd.
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In clouds warmer than -15°C(5 °F), collision between droplets play a significant role
Larger drops may form on larger condensation nuclei (salt particles or through random collision droplets; turbulent mixing
between cloud and drier environment)
Amount of air resistance depends on the size of the drop and its rate of fall --- speed of falls increases until the air resistance =
gravity – Terminal velocity – Larger drops means less evaporation also
Coalescence: Merging of droplets by collision
Forces that hold together tiny droplet together are so strong that if the droplets collide with another droplet, they would not stick
together
How surface area depends on the size
Collision & coalescence – contd.
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Rising air currents slow the rate at which drops fall --- thick cloud with strong updrafts will maximize the time
droplets spend in a cloud --- the bigger size droplets
When the fall velocity of the drop > updraft velocity, droplet slowly descends; when it reaches the bottom of the
cloud, size ~ 5 mm --- typically occur in a rain shower originating in the warm, convective cumulus clouds
Factors in the production of raindrops
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Cloud’s liquid water content (most important)
Range of droplet sizes, cloud thickness, updrafts of the cloud, electric charge of the droplets and the electric field in the cloud
Cloud droplet rising & then falling through a warm cumulus cloud by growth and coalescence
Ice crystal Process
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Bergeron process of rain formation: A process that produces precipitation; involves tiny ice crystals in a
supercooled cloud growing larger at the expense of the surrounding liquid droplets
Ice crystals and liquid cloud droplets must coexist in clouds at below freezing
Accretion or riming of ice crystals: Ice crystals grow larger by colliding with the supercooled liquid droplets; the
droplets freeze into ice and stick to the ice crystal
Distribution of ice and water in a cumulonimbus cloud
Water droplets and ice crystal are in equilibrium; water vapor molecules > liquid is saturation vapor pressure over
water is greater than it is over ice
Cloud Seeding & Precipitation
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Cloud Seeding: Inject a cloud with small particles that will act as nuclei, so that cloud particles will grow large
enough to fall to the surface as precipitation
Silver iodide is used: has a crystalline structure similar to ice crystal, as it acts as an effective ice nucleus at temp. of
-4°C (25 °F) and lower
Important factors in cloud-seeding experiment: Type of cloud, its temperature, moisture content, droplet size
distribution, and updraft velocities in the cloud
Cloud seeding in certain instances may lead to more precipitation; in others, to less precipitation, and in still others,
to no change in precipitation amounts;
Can avoid hail storms --- very important use
Natural seeding by cirrus clouds may lead to precipitation downwind
Precipitation Types
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Rain (Meteorology definition!): falling drop diameter  0.5 mm
Drizzle: Water drop diameter < 0.5 mm
Most drizzle falls from stratus clouds; also, rain passing through undersaturated zone and undergo evaporation
leading to smaller-sized droplets – drizzle
Virga: Precipitation that falls from a cloud but evaporates before reaching the ground
Raindrops that reach the earth’s surface are seldom larger than ~6mm as collision between raindrops tend to break
them apart into many smaller drops
Virga: Streaks of Falling precipitation evaporates before reaching the ground
Raindrops < 2mm nearly spherical; >2mm, elliptical
Precipitation Types – Contd.
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Snow: Much of the precipitation reaching the ground begins as snow
During summer, freezing level is usually high & snowflakes falling from a cloud melt before reaching the surface
During winter, freezing level is much lower, and falling snowflakes have a better chance of survival
Snowflakes can fall ~300 m below the freezing level before completely melting
Fallstreaks: Falling ice crystals that evaporate before reaching the ground
Ice crystals have been observed falling at temp ~-47°C
Ice crystals beneath cirrus clouds
Precipitation Types – contd.
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When snowflakes fall through very cold air with a low moisture content, they do not readily stick together &
powdery flakes of ‘dry’ snow accumulates on ground
Flurries: Light snow showers that fall intermittently for short duration; often from developing cumulus clouds
Snow Squall: A more intense snow showers (comparable to summer rain showers); usually form from cumuliform
clouds
Ground Blizzard: Drifting + Blowing snow after snow fall ended
Blizzard: Weather with low temp & >30 knot winds bearing large amounts of fine, dry, powdery snow
Sleet & Freezing Rain
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Sleet: Partially snowflake (or cold raindrop) passing through warmer air undergoes partial melting; when it again
goes through subfreezing surface layer of air, partially melted snowflake or cold raindrop turns back into a tiny
transparent ice pellet, called, sleet
Freezing Rain: Supercooled liquid drops upon striking a cold surface, form a thin veneer of ice – this form of
precipitation is called freezing rain
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Freezing drizzle: If the water droplets are small, then, it is called freezing drizzle
Rime: White/Milky granular deposit of ice formed by the rapid freezing of supercooled water drops when they
come in contact with an object in below-freezing air
Sleet – partially snowflake (cold droplet) freezes into a pellet of ice before reaching the ground
Accumulation of rime on tree branches
Ice storm caused tree limbs to break & Power lines to sag
Snow grains, pellets and hail
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Snow grains: Small, opaque grains of ice (equivalent of drizzle); fall from stratus clouds
Snow Pellet: White, opaque grains of ice of the size of rain drop
Hail: Pieces of ice either transparent or partially opaque, ranging in size from that of small peas to that of golf balls
or larger; biggest size in US 757 g & 14 cm diam.;
Single hailstorm can damage in minutes; annual loss hundreds of millions of $ in US;
Hail is produced in a cumulonimbus cloud when large frozen raindrops that grow by accumulating supercooled
liquid droplets
Hail – contd.
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Graupel: Ice particles between 2-5 mm in diameter that form in a cloud often by the process of accretion
For a hail to grow to the size of golf ball, it must remain for 5-10 minutes in the cloud
Ice crystals of appreciable size that can’t be supported by rising air, begin to fall – Hail
Largest form of precipitation occurs during the warmest time of the year (due to strong updraft that keeps the
crystal to become bigger)
Preventing hailstorm--- cloud seeding --- excessive nuclei prevents from growing
Accumulation of small hail after a thunderstorm
Coffeyville Hailstone (Sept. 3, 1970), Kansas: Layered structure indicates travel through a cloud of varying
water content and temp.
When updrafts are tilted, ice particles are swept horizontally through the cloud, producing the optimal
trajectory for hailstone growth
Measurement of Precipitation
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Rain Gauge: Instrument to collect & measure rainfall
Tipper Bucket rain gauge: Receiving funnel leading to two small metal collectors; bucket below the funnel collects
the rain water; each time a bucket tips (with 1/100”), an electric contact is made – recorded; each ‘tip; it loses some
rainfall – limitation; Automated weather stations use this technique
Weighing-type rain gauge: Precipitation is caught in a cylinder & accumulates in a bucket; special gears translate
weight of rain (or snow) into mm or inch of precipitation; info can be transmitted to satellites or land-based stations
Standard rain gauge – surface area = 10 x area of the cylinder
Tipping bucket rain gauge: 1/100” bucket tips
Rain/snow conversion & Doppler Radar
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10 cm of snow ~ 1 inch of water
Fresh snowpack: water equivalent 10:1
Useful about spring runoff and potential for flooding
Radar (RAdio Detection And Ranging): Gathers info about storms and precipitation in previously inaccessible
regions
A transmitter sending short, microwaver signals --- Fraction of the energy is scattered back by the ‘target’ to the
Transmitter & detected by a Receiver – Returning signal provides info about target’s distance & intensity of the
rainfall
Doppler Radar
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Doppler Radar: Provide information on: distance, amount of rainfall and whether the rain/cloud is stationary or
moving
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Concept of Doppler Shift
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Doppler Radar allows scientists to peer into a tornado-generating thunderstorms and observe its wind
Doppler radar display of precipitation intensity –Oklahoma, April 24, 1999
Doppler radar display of 1-hr rainfall amounts - Oklahoma, April 24, 1999
chapter –5- Summary
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Adiabatic process; dry adiabatic & moist adiabatic rate
Environmental lapse rate
Conditions for Stable and unable atmosphere
What cloud type is formed in stable air
Condensation nuclei, cloud seeding
Rain shadow, orographic uplift
Coalescence, accretion
Rain, drizzle, virga, shower, fallstreaks, flurries, snow squall, sleet, freezing rain
Blizzard, hailstone, standard rain gauge
Doppler radar
Water equivalent