Lecture on Cloud Formation and Cloud Types

Introduction to Clouds

• Clouds are visible forms of water vapor in the atmosphere.
• Formed when air cools to the dew point temperature, leading to condensation.

Cooling and Condensation

• Air cools most easily by rising in the atmosphere.
• 100% relative humidity occurs when air temperature equals dew point temperature.
• Clouds form at 100% relative humidity.

Condensation Nuclei

• Condensation requires surfaces like dust and pollen, known as condensation nuclei.
• Hygroscopic nuclei, like salts near oceans, can form clouds at less than 100% relative humidity.

Cloud and Raindrop Sizes

• Cloud droplets: 0.02 mm.
• Typical raindrop: ~2 mm, much larger than cloud droplets.

Frost and Dew Formation

• Frost: Forms when dew point temperature is below freezing (deposition).
• Dew: Forms when dew point temperature is above freezing.

Cloud Classification

Based on Shape

• Cirrus: Wispy clouds.
• Cumulus: Vertical or puffy clouds, formed by convection.
• Stratus: Flat clouds, formed by advection.

Based on Height

• Cirr: High clouds (~20,000 feet).
• Alto: Mid-level clouds (6,000 – 20,000 feet).
• Strato: Low clouds (surface to 6,000 feet).

Based on Precipitation

• Nimbo/Nimbus: Indicates precipitation.
• Virga: Precipitation that evaporates before reaching the surface.

Specific Cloud Types

• High Clouds: Cirrus, cirrostratus, cirrocumulus.
• Middle Clouds: Altostratus, altocumulus.
• Low Clouds: Cumulonimbus (precipitation), nimbostratus (precipitation).

Fog Formation

• Type of stratus cloud forming at the surface.
• Radiation Fog: Due to radiative cooling.
• Advection Fog: Moist air over cool land.
• Upslope Fog: Air forced up terrain.
• Evaporation Fog (Steam Fog): Water vapor added to air.

Fog as a Weather Hazard

• Fog causes significant vehicle crashes and fatalities.
• Specific types: Valley fog (gravity drainage) and mixing cloud.

Locations and Conditions

• Radiation fog common in Phoenix, causing occasional foggy nights.
• Advection fog prevalent in coastal areas like San Francisco.
• Upslope fog along Rockies and Appalachian Mountains.
• Evaporation fog over lakes and from breath in cold weather.

Conclusion

• Understanding cloud types and formation aids in weather prediction and safety planning, especially with fog as a major weather hazard.