Intro to Cloud Physics
1. Water Distribution on Earth
- 94.9% of Earth's water is found in the oceans.
- 0.001% of Earth's total water is in the atmosphere as water vapor.
- This amounts to several thousand tons per person.
- This atmospheric vapor has the potential to be harnessed.
2. Atmospheric Water and Cloud Formation
2.1 Nuclei and Cloud Formation
- Nuclei are crucial for cloud formation.
- Types of nuclei:
- Natural: sea salt, dust, and organic particles.
- Anthropogenic: industrial emissions, smoke.
- Nuclei provide surfaces for water vapor condensation in supersaturated conditions.
2.2 Supersaturation and Condensation
- Supersaturation occurs when relative humidity exceeds 100%.
- Condensation is the process of water vapor molecules aggregating to form liquid water.
- Relative humidity increases as air cools.
- At 100% relative humidity, saturation occurs, establishing an equilibrium between evaporation and condensation rates.
2.3 Vapor Pressure and Dew Point
- Saturation vapor pressure: The pressure at saturation, dependent only on air temperature.
- As temperature rises, saturation vapor pressure increases.
- Dew point: The point in the cooling process where condensation begins (at 100% relative humidity).
- Dew point temperature: The air temperature at which condensation begins.
2.4 Cloud Droplet Formation
- Atmospheric relative humidities rarely exceed 101%.
- Cloud droplets can form at lower relative humidities due to hygroscopic particles (e.g., sea salt).
- Relative humidity = (Actual water pressure / Saturation water pressure) * 100
3. Cloud Growth and Precipitation Processes
3.1 The Growth of a Cloud
- As air rises, pressure decreases and temperature reduces.
- Dry adiabatic lapse rate: 1°C cooling per 100 meters of ascent.
- As air rises and cools, relative humidity increases until saturation and condensation occur.
- Latent heat of vaporization: ~600 calories per gram, released during condensation.
3.2 Cloud Types and Vertical Motion
- Visible clouds indicate regions of rising air where condensation has occurred on condensation nuclei.
- Cloud forms depend on the character of the vertical motion field.
3.3 Cloud Growth Processes
3.3.1 Natural Precipitation Processes
- Collision-coalescence process
- Wegener-Bergeron-Findeisen process
3.3.2 Cloud Seeding
- Introduces nucleation agents (e.g., silver iodide) to increase precipitation efficiency.
3.3.3 Ice Crystal Process
graph TD
A[Mixed-Phase Cloud] --> B{Temperature < 0°C?}
B -->|No| C[Remain Liquid Droplets]
B -->|Yes| D[Supercooled Liquid Water]
D --> E{Ice Nuclei Present?}
E -->|No| F[Remain Supercooled]
E -->|Yes| G[Ice Crystal Formation]
G --> H[Water Vapor Deposits on Ice]
H --> I[Ice Crystals Grow]
I --> J[Supercooled Droplets Evaporate]
J --> K[Water Vapor Transfers to Ice]
K --> L{Ice Crystals Large Enough?}
L -->|No| H
L -->|Yes| M[Ice Crystals Fall as Precipitation]
subgraph Supersaturation
N[Air Saturated w.r.t Water]
O[Air Supersaturated w.r.t Ice]
end
N --> D
O --> H
3.3.4 Coalescence Process
- Cloud droplets grow by colliding and merging with other droplets.
- Larger drops fall faster, overtaking and colliding with smaller ones.
- Collision efficiency: The fraction of droplets in the vertical path that actually hit the large drop.
4. Atmospheric Processes
4.1 Fog Formation through Radiative Cooling
- Lower atmosphere radiates heat
- Earth's surface cools
- Moist air near surface cools to dew point
- Water vapor condenses into tiny droplets
- Suspended droplets form fog layer
-
Upper dry air limits vertical extent
-
Typically occurs on clear nights with calm winds, creating a shallow fog layer near the ground.
4.2 Evaporative Cooling
- Cooling effect caused by water evaporating from surfaces (e.g., skin after swimming).
5. Water States and Phase Changes
5.1 Supercooled Water
- Pure water can remain liquid below 32°F, especially in small quantities like cloud droplets.
- Cloud droplets can remain liquid down to -40°F before certainly freezing into ice.
5.2 Ice Crystal Formation
- Ice-crystal nuclei facilitate ice formation at modest degrees of supercooling.
- Without nuclei, ice forms through spontaneous arrangement of water molecules at very low temperatures (around -40°F).
5.3 Saturation Vapor Pressure Differences
- Saturation vapor pressure of water is higher than that of ice at the same subfreezing temperature.
- A cloud of supercooled water droplets may have air saturated with respect to water but supersaturated with respect to ice.
6. Hydrometeors
- Definition: Water or ice particles in the atmosphere.
- Examples: clouds, fog, rain, snow, hail, dew, rime, glaze, blowing snow, and blowing spray.
7. Droplet Growth and Raindrop Formation
- A raindrop's volume is approximately one million times larger than a cloud droplet.
- Condensation alone cannot lead to rainfall due to decreasing growth rate with droplet size.
- Higher supersaturation leads to a greater number of cloud droplets, not larger droplets.