lecture recording on 12 March 2025 at 10.20.08 AM

Homogeneous Invasion and Droplet Sizes

• Homogeneous invasion results in droplets smaller than 0.01 micrometers.

• Small droplets require very high humidities to survive, typically above 12%, which is rare in clouds.

• Droplets measuring between 0.1 and 0.01 micrometers may survive under certain conditions, while those smaller than 0.01 micrometers typically do not.

Evaporation Factors

• Smaller droplets have greater curvature, increasing evaporation likelihood.

• Once droplets reach a size of less than 0.01 micrometers, their chances of survival are slim.

• Homogeneous droplets do not typically form clouds; they evaporate almost immediately due to high curvature effects.

Heterogeneous Nucleation

• Heterogeneous nucleation requires condensation nuclei (cloud condensation nuclei).

• Two types of condensation nuclei:

  • Hydrophilic (Guenopole): Attract water vapor.

  • Hygroscopic: Soluble in the resulting droplet, dissolves water, leading to lower saturation vapor pressure, enabling droplet formation at lower humidities (around 70%).

Growth Mechanisms

• Curvature and solute effects impact droplet growth differently:

  • Curvature Effect: Can inhibit droplet survival due to increased evaporation.

  • Solute Effect: Promotes droplet growth through hygroscopic nuclei.

• As droplets grow larger, their solute concentration decreases, and they behave more like pure water.

Nucleation Below Freezing

• Homogeneous droplets: Freeze at temperatures below -4°C; can exist as supercooled water droplets down to -40°C.

• Heterogeneous nucleation: Requires ice nuclei; droplets freeze between 0°C and -40°C, highlighting the necessity for both liquid water and ice crystals in cloud formation.

Vapor Pressure Differences

• Vapor pressure over ice is lower than over water; it requires additional energy for sublimation from ice to water vapor vs. evaporation from liquid water.

• This energy difference affects processes within clouds, such as the Bergeron model, where ice crystals grow at the expense of surrounding supercooled droplets.

Cloud Formation Processes

• Formation of clouds requires reaching the lifting condensation level by cooling air parcels to the dew point temperature.

• Mechanisms for cooling include:

  • Convection: Air rises due to different heating surfaces.

  • Orographic Lifting: Air forced over mountains.

  • Frontal Lifting: Low-pressure areas cause air to rise as warm and cold fronts meet.

  • Divergence Aloft: Increased wind speeds in the upper atmosphere leading to low pressure and upward air movement.

  • Convergence at Surface: Air converging and ascending, resulting in low surface pressure.

Fog Types and Formation

• Fog is essentially a cloud in contact with the ground, formed through:

  • Cooling: Example: Radiation fog forms from overnight surface cooling without rising air.

  • Mixing: Evaporation from rain can saturate the air and create fog.

• Types of fog:

  • Upslope Fog: Caused by air moving up a topographic barrier.

  • Radiation Fog: Caused by surface cooling overnight, clearer skies favor formation.

  • Advection Fog: Warm, moist air moves over a cooler surface, leading to cooling.

Growth of Precipitation

• Precipitation occurs when droplets or ice crystals grow large enough to overcome cloud updrafts and fall to the surface.

• Rain is defined as any form reaching the ground; can include drizzle, sleet, freezing rain, and hail.

• The size of droplets influences whether precipitation makes it to the ground without evaporating.

• Distinction between cloud droplets (lower case, small) and raindrops (larger, several mm).

Mechanisms for Precipitation Growth

• Collision and Coalescence:

  • Collisions between droplets can lead to larger droplet formation.

  • Efficiency for collisions and coalescence increases with larger collector droplets (over 20 micrometers).

  • Smaller droplets tend to be deflected rather than collide with larger ones.

Importance of Different Sizes in Clouds

• Diverse droplet sizes within clouds increase the likelihood of collisions leading to coalescence.

• In cold clouds containing ice crystals, precipitation mechanisms differ compared to warm clouds.

• It is crucial for droplets to be large enough to fall to the ground and avoid evaporation on their descent.