ATM OCN (Meteorology) 100

FORMATION OF CLOUDS, FOG, DEW & FROST

Summer 2000

Lecture #8 Scheduled for:

22 JUN 2000 (R)

Recommended Readings from Moran and Morgan (1997):

pages 136-146; 297-299; 153-164.

Today's Lecture Objectives:

• To list the mechanisms involved in cooling air to saturation.
• To compare and contrast the physical characteristics of dew and frost.
• To explain how dew and frost form.
• To compare and contrast the physical characteristics of fog and clouds.
• To describe the various processes that result in fog development.
• To explain the types of cloud formation processes.
• To describe the condensation processes within a cloud, explaining the role of condensation nuclei.
• To distinguish between cloud condensation nuclei and ice forming (or freezing) nuclei.

Outline:

A. INTRODUCTION

• What is a cloud?
• Why are clouds important
• Background: Cloud, Fog, Dew and Frost

B. REQUIREMENTS FOR FORMING DROPLETS

• Statement of the Practical Problems
• The Need for Saturation Conditions
• The Role of the Condensation Process at the Earth's Surface
• The Role of the Condensation in the Free Atmosphere

C. LOW LEVEL SATURATION PROCESSES & PHENOMENA

• Dew and Frost
• Definitions
• Formation Mechanisms
• Comparison and Contrasts
• Significance

D. CLOUD (FOG) FORMATION REQUIREMENTS

• Special Problems
• Supersaturation
• Cloud Condensation Nuclei

E. MECHANISMS FOR CLOUD (FOG) FORMATION

• 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

Links to Other References:

• The cloud formation module from Weather World 2010 (Univ. of Illinois-Urbana-Champaign)