4.1 Cloud formation processes

Cloud formation is a complex process involving atmospheric moisture, nucleation, and thermodynamics. Water vapor condenses on tiny particles called aerosols, forming cloud droplets. This process is influenced by factors like temperature, humidity, and air movement.

Understanding cloud formation is crucial for weather prediction and climate studies. Different types of clouds form under various atmospheric conditions, impacting precipitation, radiation balance, and overall weather patterns. Cloud microphysics plays a key role in these processes.

Atmospheric moisture

• Atmospheric moisture plays a crucial role in cloud formation processes and weather patterns
• Understanding atmospheric moisture is fundamental to studying cloud physics and precipitation in Atmospheric Physics
• Water vapor, the gaseous form of water, is a key component of atmospheric composition and energy transfer

Water vapor in atmosphere

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• Water vapor constitutes approximately 0-4% of the atmosphere by volume
• Unevenly distributed both vertically and horizontally in the atmosphere
• Serves as the primary source for cloud formation and precipitation
• Plays a significant role in atmospheric energy balance and heat transfer
  • Absorbs and emits infrared radiation
  • Releases latent heat during condensation processes

Humidity measurements

• Relative humidity measures the amount of water vapor in the air compared to the maximum amount possible at a given temperature
• Specific humidity represents the mass of water vapor per unit mass of air
• Dew point temperature indicates the temperature at which air becomes saturated with water vapor
• Mixing ratio defines the mass of water vapor per unit mass of dry air
• Instruments used for humidity measurements
  • Hygrometers (electronic and mechanical)
  • Psychrometers (wet and dry bulb thermometers)

Saturation vapor pressure

• Maximum amount of water vapor that can be held in the air at a given temperature
• Increases exponentially with temperature according to the Clausius-Clapeyron equation
• Saturation vapor pressure over water differs from that over ice at the same temperature
• Relative humidity reaches 100% when actual vapor pressure equals saturation vapor pressure
• Supersaturation occurs when relative humidity exceeds 100%, often leading to cloud formation

Cloud nucleation

• Cloud nucleation marks the initial stage of cloud formation in the atmosphere
• This process involves the condensation of water vapor around tiny particles suspended in the air
• Understanding cloud nucleation is essential for predicting cloud formation and studying their effects on climate and weather

Aerosols as cloud condensation nuclei

• Aerosols serve as surfaces for water vapor condensation in the atmosphere
• Natural sources include sea spray, dust, and volcanic emissions
• Anthropogenic sources encompass industrial emissions and biomass burning
• Size and chemical composition of aerosols affect their efficiency as cloud condensation nuclei (CCN)
• Hygroscopic particles (salt, sulfates) make particularly effective CCN
• Concentration and type of aerosols influence cloud droplet size and number

Homogeneous vs heterogeneous nucleation

• Homogeneous nucleation occurs when water vapor molecules cluster together without a surface
• Requires extremely high supersaturation levels (>400%) rarely found in the atmosphere
• Heterogeneous nucleation involves water vapor condensing on aerosol particles
• Dominates cloud formation processes in the atmosphere due to lower energy barrier
• Occurs at much lower supersaturation levels compared to homogeneous nucleation
• Aerosol properties significantly influence the efficiency of heterogeneous nucleation

Köhler theory

• Describes the equilibrium vapor pressure over a solution droplet
• Combines the effects of surface curvature (Kelvin effect) and solute (Raoult's law)
• Köhler curve shows the relationship between droplet size and supersaturation
• Critical supersaturation represents the peak of the Köhler curve
  • Determines the activation of cloud condensation nuclei
  • Varies with aerosol size and composition
• Activation diameter defines the size at which a particle becomes a cloud droplet
• Provides a theoretical framework for understanding cloud droplet formation and growth

Condensation and evaporation

• Condensation and evaporation processes drive the formation, growth, and dissipation of clouds
• These phase changes play a crucial role in atmospheric energy transfer and water cycle
• Understanding these processes is fundamental to predicting cloud behavior and precipitation

Latent heat release

• Latent heat represents the energy absorbed or released during phase changes of water
• Condensation releases latent heat, warming the surrounding air
  • Contributes to atmospheric instability and convection
  • Fuels the development of thunderstorms and tropical cyclones
• Evaporation absorbs latent heat, cooling the surrounding environment
  • Influences surface temperature and atmospheric boundary layer dynamics
  • Plays a role in the formation of sea breezes and cold pools
• Latent heat flux significantly impacts global energy balance and atmospheric circulation patterns

Droplet growth mechanisms

• Diffusion growth occurs through the direct condensation of water vapor onto droplets
  • Dominates initial stages of cloud droplet formation
  • Rate depends on supersaturation and droplet size
• Collision-coalescence involves larger droplets colliding and merging with smaller ones
  • Becomes important for droplets larger than about 20 micrometers in radius
  • Leads to the formation of precipitation-sized droplets in warm clouds
• Bergeron process describes ice crystal growth at the expense of supercooled water droplets
  • Operates in mixed-phase clouds (containing both liquid water and ice)
  • Crucial for precipitation formation in many mid-latitude and high-latitude clouds

Evaporation processes

• Occurs when the environment is subsaturated with respect to the droplet's surface
• Rate of evaporation depends on relative humidity, temperature, and air movement
• Plays a crucial role in cloud dissipation and precipitation
  • Virga forms when precipitation evaporates before reaching the ground
  • Evaporative cooling can lead to downdrafts and gust fronts in thunderstorms
• Influences cloud lifetime and extent
  • Entrainment of dry air can lead to partial or complete cloud evaporation
  • Shapes the vertical structure of clouds through differential evaporation rates

Cloud microphysics

• Cloud microphysics focuses on the small-scale processes within clouds
• This field examines the formation, growth, and interactions of cloud particles
• Understanding cloud microphysics is crucial for accurate weather prediction and climate modeling

Droplet size distribution

• Describes the range and frequency of droplet sizes within a cloud
• Typically follows a gamma or lognormal distribution
• Influenced by factors such as
  • Aerosol concentration and composition
  • Updraft velocity and supersaturation
  • Cloud age and type
• Impacts cloud optical properties and precipitation efficiency
• Measured using instruments like cloud droplet probes and disdrometers
• Evolution of size distribution over time provides insights into cloud processes

Collision and coalescence

• Primary mechanism for raindrop formation in warm clouds
• Collision efficiency depends on droplet sizes and relative velocities
• Coalescence efficiency influenced by surface tension and electrical charges
• Gravitational settling leads to differential fall speeds, promoting collisions
• Turbulence enhances collision rates, especially for smaller droplets
• Collision-coalescence becomes significant for droplets larger than 20 micrometers
• Leads to the broadening of the droplet size distribution over time

Ice crystal formation

• Occurs through various nucleation processes in clouds colder than 0°C
• Homogeneous freezing of pure water droplets requires temperatures below -38°C
• Heterogeneous ice nucleation involves ice nucleating particles (INPs)
  • Deposition nucleation: direct deposition of water vapor onto INPs
  • Immersion freezing: freezing of a droplet containing an immersed INP
  • Contact freezing: freezing initiated by an INP contacting a supercooled droplet
• Ice crystal habits (shapes) depend on temperature and supersaturation
  • Includes plates, columns, dendrites, and needles
• Secondary ice production mechanisms
  • Rime splintering (Hallett-Mossop process)
  • Fragmentation during collisions
  • Droplet shattering during freezing

Cloud types and classification

• Cloud classification systems help meteorologists and atmospheric scientists describe and analyze cloud formations
• Understanding different cloud types aids in weather forecasting and climate studies
• Cloud types reflect atmospheric conditions, stability, and ongoing physical processes

Low vs middle vs high clouds

• Low clouds form below 2 km (6,500 ft) altitude
  • Stratus: flat, featureless layers often producing drizzle
  • Stratocumulus: patches or rolls of clouds with some vertical development
  • Cumulus: puffy, cotton-like clouds with flat bases and rounded tops
• Middle clouds occur between 2-7 km (6,500-23,000 ft)
  • Altostratus: gray or bluish cloud sheets, often thin enough to reveal the sun
  • Altocumulus: white or gray patches, often in rows or waves
  • Nimbostratus: thick, dark gray cloud layers associated with continuous precipitation
• High clouds form above 7 km (23,000 ft)
  • Cirrus: thin, wispy clouds composed of ice crystals
  • Cirrostratus: transparent veil-like clouds often producing halos
  • Cirrocumulus: small, round white puffs arranged in patterns

Stratiform vs cumuliform clouds

• Stratiform clouds develop horizontally in stable atmospheric conditions
  • Characterized by uniform appearance and extensive coverage
  • Typically produce light, steady precipitation (stratus, altostratus)
  • Form through large-scale lifting or radiative cooling
• Cumuliform clouds grow vertically in unstable atmospheric conditions
  • Exhibit distinct, often towering shapes with sharp outlines
  • Associated with convection and localized, intense precipitation (cumulus, cumulonimbus)
  • Develop due to surface heating, frontal lifting, or orographic effects

Special cloud formations

• Lenticular clouds: lens-shaped clouds formed in stable air over mountains
• Mammatus clouds: pouch-like structures hanging beneath the base of a cloud
  • Often associated with severe thunderstorms
• Noctilucent clouds: high-altitude clouds visible in twilight at polar latitudes
• Kelvin-Helmholtz clouds: wave-like formations caused by wind shear
• Pyrocumulus: formed by intense heat from wildfires or volcanic eruptions
• Contrails: linear clouds produced by aircraft exhaust in the upper troposphere

Atmospheric stability

• Atmospheric stability determines the potential for vertical motion and cloud development
• Plays a crucial role in weather forecasting and understanding atmospheric dynamics
• Stability conditions influence cloud types, precipitation patterns, and severe weather potential

Dry vs moist adiabatic lapse rates

• Dry adiabatic lapse rate (DALR) describes temperature change of unsaturated air parcels
  • Approximately 9.8°C per kilometer of vertical displacement
  • Remains constant regardless of temperature or pressure
• Moist adiabatic lapse rate (MALR) applies to saturated air parcels
  • Variable, typically ranging from 4-9°C per kilometer
  • Depends on temperature and pressure due to latent heat release
  • Generally less steep than DALR, leading to potential instability
• Environmental lapse rate (ELR) represents the actual vertical temperature profile
  • Comparison with DALR and MALR determines atmospheric stability

Stability criteria for cloud formation

• Absolute stability occurs when ELR < MALR
  • Inhibits vertical motion and cloud development
  • Often associated with stratiform clouds or clear conditions
• Conditional instability exists when MALR < ELR < DALR
  • Stable for unsaturated air, but potentially unstable if saturation occurs
  • Can lead to cumulus cloud formation if lifting mechanism present
• Absolute instability happens when ELR > DALR
  • Promotes strong vertical motion and convective cloud development
  • Often results in cumulonimbus clouds and severe weather
• Potential instability involves a decrease in equivalent potential temperature with height
  • Can lead to instability if the entire layer is lifted
  • Important for assessing severe weather potential

Convective available potential energy

• Convective Available Potential Energy (CAPE) quantifies atmospheric instability
• Represents the amount of energy available for convection
• Calculated as the area between the environmental temperature profile and the parcel's path
• Higher CAPE values indicate greater potential for strong updrafts and severe weather
  • 0-1000 J/kg: weak instability
  • 1000-2500 J/kg: moderate instability

  • 2500 J/kg: strong instability

• Influenced by factors such as
  • Surface temperature and moisture
  • Mid-level temperatures
  • Presence of capping inversions
• Used in conjunction with other parameters for severe weather forecasting

Orographic cloud formation

• Orographic clouds form due to the interaction between air flow and topography
• Understanding these processes is crucial for local weather forecasting in mountainous regions
• Orographic effects significantly influence precipitation patterns and regional climate

Mountain wave clouds

• Form in the lee of mountains when stable air flows over a topographic barrier
• Characterized by stationary, lens-shaped clouds (lenticular clouds)
• Occur in a series of waves downwind of the mountain
• Formation process
  • Air is forced upward on the windward side, cooling adiabatically
  • As air descends on the lee side, it warms and clouds evaporate
  • Oscillations continue downwind, creating a wave pattern
• Associated with turbulence, important for aviation safety
• Can lead to localized areas of precipitation enhancement or suppression

Föhn effect

• Warm, dry wind occurring on the lee side of a mountain range
• Known by various names in different regions (Chinook, Santa Ana winds)
• Formation process
  • Moist air is forced upward on the windward side, cooling and condensing
  • Precipitation occurs on the windward slope, releasing latent heat
  • Air descends on the lee side, warming at the dry adiabatic lapse rate
  • Results in warmer, drier conditions on the lee side
• Impacts local climate and can lead to rapid temperature increases
• Associated with increased fire danger and potential health effects

Upslope fog

• Forms when moist air is forced up a topographic slope
• Common in coastal areas where moist air moves inland over rising terrain
• Formation process
  • Air cools adiabatically as it rises along the slope
  • Cooling leads to condensation if the air reaches its dew point
  • Fog forms near the ground, often extending up the slope
• Can persist for extended periods if wind direction remains constant
• Impacts visibility, affecting transportation and local activities
• May lead to drizzle or light precipitation in some cases

Cloud dynamics

• Cloud dynamics encompasses the physical processes governing cloud formation, evolution, and dissipation
• Understanding these processes is crucial for accurate weather prediction and climate modeling
• Cloud dynamics plays a key role in atmospheric energy transfer and precipitation formation

Entrainment and mixing

• Entrainment involves the incorporation of environmental air into a cloud
• Occurs at cloud edges and top due to turbulent motions
• Affects cloud properties and evolution
  • Dilutes cloud water content and temperature
  • Can lead to evaporation and cloud dissipation
  • Influences cloud droplet size distribution
• Mixing processes within clouds
  • Homogeneous mixing: rapid and complete mixing of entrained air
  • Inhomogeneous mixing: patchy mixing leading to diverse droplet populations
• Entrainment rates vary with cloud type and environmental conditions
  • Cumulus clouds experience more entrainment due to their turbulent nature
  • Stratiform clouds have lower entrainment rates

Updrafts and downdrafts

• Updrafts represent rising air motions within clouds
  • Drive cloud growth and development
  • Influenced by buoyancy, convergence, and orographic lifting
  • Strongest in convective clouds (cumulus, cumulonimbus)
  • Transport moisture, heat, and momentum vertically
• Downdrafts involve sinking air motions
  • Can be caused by precipitation loading and evaporative cooling
  • Important in mature and dissipating stages of convective clouds
  • Contribute to gust fronts and microbursts in severe thunderstorms
• Interaction between updrafts and downdrafts
  • Creates complex circulation patterns within clouds
  • Influences cloud lifetime and precipitation efficiency
  • Plays a role in the development of severe weather phenomena

Cloud lifecycle

• Cumulus stage: initial development characterized by rising air parcels
  • Dominated by updrafts
  • Cloud droplets grow primarily through condensation
• Mature stage: peak development with both updrafts and downdrafts present
  • Precipitation begins to fall
  • Maximum cloud height and horizontal extent reached
• Dissipating stage: weakening of convection and cloud breakup
  • Downdrafts become more prevalent
  • Evaporation leads to cloud erosion and eventual dissipation
• Factors influencing cloud lifecycle
  • Environmental stability and moisture content
  • Presence of wind shear or capping inversions
  • Diurnal heating cycle and surface conditions
• Understanding cloud lifecycle crucial for
  • Short-term weather forecasting
  • Predicting severe weather development
  • Estimating precipitation amounts and duration

Precipitation processes

• Precipitation processes involve the formation and growth of water droplets or ice particles to sizes large enough to fall from clouds
• Understanding these processes is crucial for accurate weather forecasting and water resource management
• Precipitation formation mechanisms vary depending on cloud type and temperature

Warm rain formation

• Occurs in clouds with temperatures above freezing throughout
• Common in tropical and subtropical regions
• Collision-coalescence process
  • Larger cloud droplets fall faster than smaller ones, colliding and merging
  • Initial size differences arise from variations in condensation nuclei
  • Process accelerates as droplets grow, leading to rapid raindrop formation
• Factors influencing warm rain efficiency
  • Cloud thickness and liquid water content
  • Droplet size distribution
  • Updraft strength and duration
• Warm rain process can produce precipitation in as little as 15-20 minutes
• Typically results in light to moderate rainfall intensities

Cold cloud processes

• Involve ice particles and occur in clouds with temperatures below freezing
• Dominant precipitation formation mechanism in mid-latitudes and polar regions
• Ice crystal growth mechanisms
  • Deposition: direct vapor deposition onto ice crystals
  • Riming: supercooled water droplets freezing upon contact with ice crystals
  • Aggregation: ice crystals colliding and sticking together
• Factors affecting cold cloud precipitation
  • Temperature and humidity profiles within the cloud
  • Concentration and types of ice nuclei
  • Presence and amount of supercooled liquid water
• Can produce a variety of precipitation types
  • Snow: when the entire atmospheric column is below freezing
  • Sleet: when ice pellets partially melt and refreeze
  • Freezing rain: when supercooled raindrops freeze on contact with surfaces

Bergeron process

• Also known as the ice crystal process or cold rain process
• Occurs in mixed-phase clouds containing both supercooled water droplets and ice crystals
• Based on the difference in saturation vapor pressure over ice and water
  • Ice has a lower saturation vapor pressure than water at the same temperature
  • Creates a vapor pressure gradient, causing water vapor to deposit on ice crystals
• Process steps
  • Ice crystals grow at the expense of surrounding water droplets
  • As ice crystals become larger, they begin to fall
  • May melt into raindrops if they pass through warmer layers
• Enhances precipitation efficiency in mid-latitude clouds
• Plays a crucial role in the formation of snow and other types of frozen precipitation
• Influenced by factors such as
  • Cloud temperature profile
  • Availability of ice nuclei
  • Updraft strength and cloud dynamics

Cloud radiative effects

• Cloud radiative effects describe how clouds interact with both solar and terrestrial radiation
• These interactions play a crucial role in Earth's energy balance and climate system
• Understanding cloud radiative effects is essential for accurate climate modeling and predictions

Albedo and cloud reflectivity

• Cloud albedo refers to the fraction of incoming solar radiation reflected back to space
• Varies depending on cloud properties
  • Optical thickness: thicker clouds generally have higher albedo
  • Droplet size distribution: smaller droplets increase reflectivity
  • Cloud height: higher clouds tend to be more reflective
• Low, thick clouds (stratus) have high albedo, cooling Earth's surface
• Thin, high clouds (cirrus) have lower albedo but trap outgoing longwave radiation
• Global cloud albedo effect estimated to cool Earth by about 50 W/m²
• Aerosol-cloud interactions can modify cloud albedo (first indirect effect)
  • Increased aerosols lead to more numerous, smaller cloud droplets
  • Results in higher albedo for the same liquid water content

Greenhouse effect of clouds

• Clouds absorb and re-emit longwave radiation, contributing to the greenhouse effect
• Trapping of outgoing terrestrial radiation warms the surface and lower atmosphere
• Effectiveness depends on cloud properties
  • Height: higher clouds have a stronger greenhouse effect
  • Optical thickness: thicker clouds trap more radiation
  • Temperature: colder cloud tops are more effective greenhouse agents
• Low clouds have a weak greenhouse effect due to their warm temperatures
• High clouds (cirrus) have a strong greenhouse effect, warming Earth's surface
• Net effect of clouds on Earth's energy budget
  • Cooling effect from albedo generally outweighs warming from greenhouse effect
  • Global average net cooling effect estimated at about 20 W/m²

Cloud feedback mechanisms

• Describe how clouds respond to and influence climate change
• Represent a major source of uncertainty in climate projections
• Positive feedbacks amplify warming, negative feedbacks dampen it
• Types of cloud feedbacks
  • Cloud amount feedback: changes in total cloud cover
  • Cloud altitude feedback: shifts in cloud height distribution
  • Cloud optical depth feedback: changes in cloud thickness or water content
• Low cloud amount feedback
  • Decrease in low cloud cover with warming (positive feedback)
  • Driven by increased surface evaporation and atmospheric stability
• High cloud altitude feedback
  • Rising of high clouds with warming (positive feedback)
  • Related to the rise in tropopause height
• Phase change feedback
  • Transition from ice to liquid in mixed-phase clouds (positive feedback)
  • Liquid clouds have higher albedo but persist longer
• Current understanding suggests an overall positive cloud feedback
  • Magnifies global warming by about 0.4-1.2°C for doubled CO2
  • Largest contribution from low cloud amount feedback
• Ongoing research focuses on reducing uncertainties in cloud feedback estimates