FORMATION OF CLOUDS, FOG, DEW & FROST

Fall 1997

To explain how the water vapor content of air is quantified and define the following moisture parameters
- dewpoint temperature
- wet bulb temperature
- vapor pressure
- mixing ratio
- relative humidity
To describe the relationship between temperature and the capacity of air to hold water vapor.
To explain the temperature dependency of relative humidity.
To perform simple calculations involving moisture parameters.
To list the mechanisms involved in cooling air to saturation.
To identify the various lifting processes operating with the atmosphere.
To explain the types of cloud formation processes.
To describe the condensation processes within a cloud.
To distinguish between cloud condensation nuclei and ice forming (or freezing) nuclei.
To distinguish between warm clouds and cold clouds in both composition and appearance.
To compare and contrast the physical characteristics of fog and clouds.
To describe the various processes that result in fog development.
To explain how dew and frost form
To compare and contrast the physical characteristics of dew and frost.

Formation Options
Evaporation Mechanisms
Cooling Mechanisms
- Expansional Cooling
- Evaporative Cooling
- Advection Cooling - Horizontal Transport
- Radiative Cooling
Expansional Cooling Specifics
- Review of adiabatic processes
- Dry adiabatic lapse rate
  - No phase change of water or R.H. less than 100%
  - Rate is 10 Celsius degrees per 1000 m (or 5.5 Fahrenheit degrees per 1000 ft)
- Saturation (moist) adiabatic lapse rate
  - Phase change of water or R.H. = 100%
  - Rate is 6 to 7 Celsius degrees per 1000 m (or 3 Fahrenheit degrees per 1000 ft)
- Ways to lift for Expansional Cooling
  - Dynamic Lifting --
    - In surface low pressure systems
    - Along fronts
  - Lifting by Convection -- Localized heating
  - Mechanical Lifting -- Over mountain barriers (orographic lifting)

Fogs
- Definition
- Comparison between clouds and fog
- Fog Formation Mechanisms
  - Cooling to Saturation
  - Addition of moisture
- Significance
- Types of Fog
  - Fog Types Classification by temperature:
    - Warm fog
    - Cold fog
    - Ice fog (or ice crystal fog)
  - Fog Type Classification by formation process:
    - Radiation fog
    - Upslope fog (or mountain fog)
    - Frontal fog
    - Advection fog
    - Steam fog (also known as sea smoke)
    - Rain fog
Dew and Frost
- Definitions
- Formation
- Comparison and Contrasts
- Significance

Last revision 30 September 1997

Produced by Edward J. Hopkins, Ph.D.

Department of Atmospheric and Oceanic Sciences

University of Wisconsin-Madison Madison, WI 53706

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