Types of Fog for Pilots

Introduction

• Fog is a low-lying cloud, made up of tiny water droplets at Earth's surface, reducing visibility.
• Fog forms when air temperature reaches the dew point.
• Difference between fog and mist:
  • Fog: Visibility < 5/8 statute mile.
  • Mist: Visibility > 5/8 statute mile.

Types of Fog

Fog is classified based on how it forms. There are seven primary types:

1. Radiation Fog

• Typically forms at night when the ground cools, cooling the air above it.
• Conditions for formation:
  • Clear skies: Allow maximum ground cooling.
  • Calm winds: Promote uninterrupted cooling.
  • Long nights: Provide extended cooling periods.
• Also known as ground fog if less than 20 ft thick.
• Common in early mornings, dissipates after sunrise.

2. Advection Fog

• Forms when warm, moist air moves over cooler surfaces (land or water).
• Can occur with strong winds and cloud cover.
• Associated with a temperature inversion.

3. Upslope Fog

• Occurs when moist air is lifted over rising terrain, cooling adiabatically.
• More common at higher elevations, can extend into valleys.
• Can form in high wind conditions.

4. Steam Fog (Evaporation Fog)

• Forms when cold air moves over a warm body of water.
• Warm water evaporates into the cooler air, increasing moisture content.
• Occurs in early fall or late spring.
• Appears as wispy tendrils resembling steam over water bodies.

5. Freezing Fog

• Forms in saturated air when surface temperatures are below freezing.
• Contains supercooled water droplets that freeze on contact with surfaces, forming rime ice.
• Common in cold regions and valleys.

6. Ice Fog

• Consists of tiny ice crystals, forms in extremely cold conditions.
• Occurs well below 32°F (0°C).
• Differentiates from freezing fog which contains liquid water droplets.

7. Frontal Fog

• Forms during warm or cold front passage.
• Warm rain falls into colder air, evaporating and increasing dew point.
• Types:
  • Prefrontal: Associated with warm fronts.
  • Postfrontal: Associated with cold fronts.
• Dissipates after front passes.

8. Precipitation Fog

• Forms when rain falls through cold, dry air.
• Increased moisture raises dew point, leading to saturation and fog.
• Associated with slow-moving cold fronts.

Conclusion

• Fog poses challenges for VFR flights; less so for IFR flights.
• Understanding the types and formation of fog can help pilots avoid visibility issues.