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Chapter 5: Cloud Chapter 5: Cloud Development and Development and Precipitation Precipitation Atmospheric Stability Atmospheric Stability Determining stability Determining stability Cloud development and stability Cloud development and stability Precipitation processes Precipitation processes Precipitation types Precipitation types Measuring precipitation Measuring precipitation 1

Chapter 5: Cloud Development and Precipitation Atmospheric Stability Atmospheric Stability Determining stability Determining stability Cloud development.

Dec 28, 2015



Hugo McCarthy
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  • Chapter 5: Cloud Development and PrecipitationAtmospheric StabilityDetermining stabilityCloud development and stabilityPrecipitation processesPrecipitation typesMeasuring precipitation


  • Atmospheric StabilityQ: Why does the air rise on some occasions and not on others? Why do the size and shape of clouds vary so much when the air does rise? A: because of different atmospheric stability conditions

    Stable and unstable equilibrium stability analysis: giving initial perturbations,

    Stable: parcel moves back

    Unstable: parcel moves away from the original position *

  • Determining StabilityStability analysis: assume a parcel rises following the dry or moist adiabatic process; then compare its Tp with the environmental Te (following environmental lapse rate); colder T means denser air.

    Stable condition: If a rising parcels Tp < Te, it is denser and would sink back.

    Unstable condition: If the rising parcels Tp > Te, it is less dense and will continue to rise

    Stability does not control whether air will rise or sink. Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.


  • Atmospheric Stabilityadiabatic process: no heat exchange of the air parcel with the environment so that rising air parcel expands and coolsdry adiabatic lapse rate for parcels (10 C/km)moist adiabatic lapse rate for parcels (taken as 6 C/km)environmental lapse rate for the atmosphere (~ 6.5 C/km)

    Q: if a rising parcels T decreases at 10 C/km in an adiabatic process, does its T increases at 10 C/km as the parcel descends? a) yes, b) noQ: why is the moist lapse rate lower than dry lapse rate?because condensation occurs in the moist adiabatic processbecause evaporation occurs in the dry adiabatic processbecause condensation occurs in the dry adiabatic process*

  • Q: why is the moist lapse rate lower over tropics (higher T) than over polar regions (lower T)?because warm saturated air contains more liquid water for condensation; because cold saturated air contains more liquid water for condensation

    Q: What does a radiosonde measure?dry adiabatic lapse rate; moist lapse rate; environmental lapse rate


  • Absolutely Stable Atmospherestabilizing processes nighttime surface radiational cooling; warm air advected to cold surface; air aloft warming (e.g., subsidence inversions)Stable air provides ideal conditions for high pollution levels.

    Q: For a rising surface dry parcel with dew point T of 20C, its temperature at 2000 m height is: a) 10C, b) 14C, c) 18 C, d) 20C*

  • Absolutely Unstable Atmospheredestabilizing processes daytime solar heating of surface air; cold air advected to warm surfacesuperadiabatic lapse rates (> 10 C/km)Unstable air tends to be well-mixed.

    Q: For a descending saturated parcel at 1000 m height (with T = 24C), its T at surface is: a) 28C, b) 30C, c) 32C, d) 34C*

  • Conditionally Unstable AirConditional instability: environmental lapse rate between dry and moist lapse ratesLifting condensation level is the cloud baseLevel of free convection

    Q: Between what heights in the figure would Tp = Te?a) 0-1 km, b) 1-2 km, c) 2-3 km*

  • Q: If environmental lapse is greater than dry lapse rate, the atmosphere is a) stable, b) unstable, c) conditionally unstable

    Q: If environmental lapse is less than moist lapse rate, the atmosphere is a) stable, b) unstable, c) conditionally unstable

    Q: The earths atmosphere is ordinarily a) stable, b) unstable, c) conditionally unstable

    Q: The air T in an unsaturated parcel follows the a) dry lapse rate, b) moist lapse rate, c) environmental lapse rate

    Q: If air T increases with height, the air is surely stable. If it decreases with height, the air is: a) stable, b), unstable, c) conditionally unstable, d) undecided*

  • Q: For stable condition, if clouds exist, they are usuallycumuliform clouds, b) stratiform clouds

    Q: For conditionally unstable conditions, if clouds exist, they are usuallycumuliform clouds, stratiform clouds

    Q: Do you usually expect to see layered clouds in Tucson during the day in summer? a) yes, b) no


  • Four Ways for Cumulus Development surface heating and free convectionuplift along topographyconvergence of airlifting along weather fronts*

  • Convection and Cloudsthermalsfair weather cumulusFair weather cumulus provide a visual marker of thermals.Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.

    Q: why are there large clear areas between cumulus clouds?*

  • Topography and Cloudsrain shadow: Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.*

  • Precipitation ProcessesThe diameter increases by 2 orders of magnitude from condensation nucleus to cloud droplet, and from cloud droplet to raindrop

    Growth of cloud droplets by condensation is too slow (a few days), but rain drops can develop in < 1 hr in nature

    Q: 0.0002 mm is: a) 20 m, b) 2 m, c) 0.2 m, d) 0.02 m *

  • Collision and Coalescence ProcessWarm clouds (with T above freezing)terminal velocity large drops fall faster than small dropscoalescence: the merging of a large cloud droplet with small droplets by collisionQ: Do larger drops fall faster in a vacuum? a) yes, b) no

    Q: Does coalescence exist for uniform droplets? a) yes, b) no *

  • Ice Crystal Processcold clouds (ice crystals and liquid droplets coexist) supercooled water droplets due to lack of ice nuclei

    Q: what are the two reasons for the existence of the anvil?*

  • Saturation vapor pressures over liquid water is higher than over iceThis causes water vapor molecules to diffuse from water droplet towards the ice crystalsIce crystals grow at the expense of water droplets. It is called ice-crystal (or Bergeron) process.


  • Snow pellets and snowflakesAccretion: ice crystals grow by colliding with supercooled water droplets to form snow pellets


  • Cloud Seeding and Precipitationcloud seeding: inject (or seed) a cloud with small particles that will act as nuclei so that the cloud particles will grow large enough to fall to the surface as precipitationsilver iodide (AgI): as cloud-seeding agent because it has a crystalline structure similar to an ice crystal; it acts as an effective ice nucleus at T = -4C and lower.Very popular in some countriesIt is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadnt been seeded?*Q: What are the preferable clouds for seeding?A: existence of supercooled liquid droplets; low ratio of ice crystals to droplets.

  • Q: In warm clouds (with T above freezing), cloud droplets grow to rain droplets primarily throughcollision/coalescence processice crystal process, c) accretion

    Q: Cloud liquid droplet collision is calledcoalescence, b) accretion

    Q: What is the purpose of using silver iodide for the seeding of supercooled clouds?Increase the number of ice nucleiIncrease the number of cloud nuclai

    Q: Cloud (liquid) droplet does not exist for temperature:warmer than 0C, b) warmer than 20C, c) warmer than 40C, d) colder than 40C


  • Precipitation TypesRainSnowSleet and freezing rainHail21

  • RainRain: drop diameter of 0.5 ~ 6mmDrizzle: drop diameter < 0.5mmVirga: rainfall not reaching surfaceRain drop shape#1: tear-shaped#2: spherical shape for small raindrops with diameter 2 mm21

  • SnowFallstreaks: ice crystals and snowflakes from high cirrus clouds that usually do not reach surfaceFlurries: light snow falling from cumulus cloudssnow storm: heavy snowfallBlizzard: low T and strong wind bearing large amounts of snow, reducing visibility to a few meters


  • SnowDendrite: Snowflake shape depends on both temperature and relative humidityAnnual snowfall in U.S. and Canada *Mt. Rainier in Washington receives an annual average snowfall of 17 m!

  • Sleet and Freezing RainSleet: frozen raindrop (called ice pellet); makes a tapping sound when falling on glass; bounce when striking the groundFreezing rain: supercooled liquid drops spread out and freeze on cold surface

    *Q: what is the necessary vertical temperature profile for sleet?A: a below-cloud (above freezing) layer to melt snowflake; deep freezing layer to freeze the raindrop

    Q: what is the necessary vertical temperature profile for sleet?A: shallow below-cloud layer (so that supercooled raindrop can not be frozen)

  • Sleet and Freezing RainIce storm: substantial accumulation of freezing rainRime: white granular ice, formed by freezing small, supercooled cloud or fog droplets*

  • Snow Grains and Snow Pelletssnow grains: snow equivalent of drizzle snow pellets: larger than snow grains and bounce on surface; formed as ice crystals collide with supercooled water droplets; usually from cumulus congestus cloudsGraupel: when snow pellets accumulate a heavy coating of rime, they are called graupel*

  • Hailhail is produced when graupel grows by accumulating supercooled liquid droplets, a process called accretion. Strong updrafts are needed. A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.*18.75 inch in diameter

  • Q: what is the shape of a small rain droplet? a) tear-shaped, b) spherical, c) mushroom

    Q: What is the difference between sleet and freezing rain?A: sleet refers to raindrops freezing through a deep cold layer below cloud and could bounce on the ground, while freezing rain refers to supercooled liquid drops spreading out and freezing on cold surface (ground, trees, ) and substantially affect driving conditions (both on the road and on the wind shield)

    Q: When ice crystals collide with supercooled water droplets, what would be formed? a) snow pellets, b) graupel, c) hail


  • Precipitation Measurement Instrumentsstandard rain gauge: 0.01 inch interval ; tracetipping bucket rain gauge: used in ASOSdifficult to capture rain in a bucket when wind blows strongly.Tipping bucket underestimates rainfall for heavy events*

  • Doppler Radar and PrecipitationRadar: radio detection and ranging Doppler radar: use Doppler shift (e.g., a higher-pitchedwhistle as a train approaches you); Provide precipitationarea and intensity;Provide horizontal speed of falling rainPolarimetric radar: identify rain from snow(by using horizontal and vertical pulses)*

  • *Measuring precipitation from space.

  • *Q: Can you claim from your rain gauge that precipitation rate is 0.001 in/hour? a) yes, b) no

    Q: Surface temperature is above 0C forsleet, b) frozen rain, c) hail

    Q: how to transform a stable atmosphere near surfaceto an unstable atmosphere?

    Q: for a thick nimbostratus cloud with ice crystals and supercooled cloud droplets of about the same size, which precipitation process would be most important in producing rain from the cloud? Why?