7 Types of Fog That Could Complicate Your Next Flight

Fog can be challenging to deal with, even for experienced IFR pilots. It can form quickly, change conditions along your route, and reduce visibility to the point where taxi, takeoff, or landing become unsafe. For VFR pilots, fog can turn a routine flight into a go or no-go decision. For IFR pilots, it can still introduce additional workload and risk.

While fog may seem harmless when you are standing on the ramp, its different forms each have their own mechanics and hazards. Learning how each type forms and where it appears helps you recognize trouble early and make better weather decisions.

In this guide, we will walk through the physics behind fog in simple terms, then break down the seven main types of fog that affect pilots and how to plan around them.

Table of Contents

1. Why Fog Matters to Pilots
2. How Fog Forms: Temperature, Dew Point, and Stability
3. The Seven Types of Aviation Fog
4. 1. Radiation Fog
5. 2. Advection Fog
6. 3. Steam Fog (Evaporation Fog)
7. 4. Upslope Fog
8. 5. Ice Fog and Freezing Fog
9. 6. Frontal Fog
10. 7. Precipitation Fog
11. Flying Smarter Around Fog
12. Frequently Asked Questions
13. Takeaway and Further Reading

Why Fog Matters to Pilots

Fog is essentially a cloud that has formed at the surface. It reduces horizontal visibility and can hide runways, terrain, and other traffic from view. From an aviation perspective, fog is important because:

• It can arrive and dissipate quickly, changing the usability of an airport in minutes.
• It is often shallow and localized, which means nearby METARs might not show the worst of it.
• It is closely tied to temperature and dew point trends, which pilots can monitor in advance.
• It is a common factor in VFR into IMC accidents and runway incursions during taxi.

Fog is also tied to a common pilot decision-making trap known as get-there-itis. Because fog often forms close to the destination, pilots may feel pressure to continue the flight even as visibility drops. Recognizing this human factor is just as important as reading the METAR, especially when conditions appear to be almost good enough.

How Fog Forms: Temperature, Dew Point, and Stability

Fog forms when the air near the surface becomes saturated. That happens in two main ways:

• The air cools down to its dew point.
• Moisture is added until the dew point rises to the current temperature.

A key number to watch is the temperature and dew point spread. When this spread gets within about 3 degrees Celsius (5 degrees Fahrenheit), fog or low clouds become more likely. The closer the spread and the faster it is closing, the more you should be thinking about fog risk.

Atmospheric stability also matters. A stable layer of air near the ground traps moisture and cooling, allowing fog to form and persist. In unstable air, rising motion tends to turn surface moisture into higher cloud layers instead of fog.

Each type of fog you will see on a weather test or in a METAR is just a specific combination of cooling, moisture, wind, and terrain.

The Seven Types of Aviation Fog

• Radiation fog - night cooling of moist air over land.
• Advection fog - warm moist air moving over a colder surface.
• Steam fog (evaporation fog) - cold air over warm water.
• Upslope fog - moist air forced up rising terrain.
• Ice fog and freezing fog - fog in very cold air.
• Frontal fog - warm rain falling through cooler air.
• Precipitation fog - evaporation from rain raising humidity to saturation.

1. Radiation Fog

Radiation fog is one of the most common types of fog pilots encounter over land. It forms when the ground cools at night and chills the air directly above it. If that air is moist enough, it cools to its dew point and fog forms near the surface.

Conditions that favor radiation fog:

• Clear skies at night allowing the ground to cool.
• Light winds that mix cooled air upward slightly.
• Moist ground and high humidity, especially after rainfall.
• Low lying terrain where cool, dense air can settle.

Key points for pilots:

• Where it forms: Valleys, low fields, and near rivers or lakes.
• When it forms: Late night and early morning, especially in fall and winter.
• Impact on aviation: Can reduce visibility to near zero during taxi and landing.
• How it clears: Usually dissipates after sunrise, but may thicken briefly at first.

2. Advection Fog

Advection fog forms when warm, moist air moves horizontally over a colder surface. The colder surface cools the air from below until it reaches its dew point and fog forms.

Unlike radiation fog, advection fog requires wind to move the air mass. Because of this, it often covers large areas and can push inland from coastal regions.

Key points for pilots:

• Where it forms: Along coastlines, over cold ocean currents, or above snow covered ground.
• When it forms: During seasonal transitions or with onshore breezes.
• Impact on aviation: Can be widespread and persistent, keeping airports IFR all day.
• How to prepare: Monitor surface temperatures, wind patterns, and marine air movement.

3. Steam Fog (Evaporation Fog)

Steam fog, also called evaporation fog or sea smoke, forms when cold air moves across a warmer water surface. The warm water evaporates into the colder air, adding moisture until the air reaches saturation.

This fog often has a dramatic, wispy appearance rising from the water surface.

Key points for pilots:

• Where it forms: Over lakes, rivers, bays, and coastal waters.
• When it forms: Most common in colder seasons when air temperatures suddenly drop.
• Impact on aviation: Can form quickly and affect approach paths along shorelines.
• Special consideration: May accompany low-level icing conditions when temperatures are below freezing.

4. Upslope Fog

Upslope fog forms when moist, stable air is pushed up sloping terrain by the wind. As the air is lifted and expands, it cools to its dew point and fog develops along the slope.

Key points for pilots:

• Where it forms: Along mountain ranges and rising terrain.
• When it forms: Anytime steady winds force moist air uphill.
• Impact on aviation: Can obscure mountain ridges and passes, increasing CFIT risk.
• Preparation tips: Study winds aloft and terrain orientation before cross mountainous areas.

5. Ice Fog and Freezing Fog

Ice fog and freezing fog are often grouped together because they occur in very cold conditions, but they are not the same thing.

Ice Fog

Ice fog is composed of tiny ice crystals suspended in the air. It typically requires extremely cold temperatures and is especially common in polar or high altitude regions.

Ice Fog Key Points:

• Where it forms: Arctic regions or very cold valleys.
• When it forms: During extremely cold, calm, clear conditions.
• Impact on aviation: Severe visibility reduction and impacts on engine operation.
• Mitigation: Follow cold-weather procedures and be cautious about taxiing in low visibility.

Freezing Fog

Freezing fog consists of supercooled liquid droplets. These droplets freeze instantly when they contact an aircraft, runway, or taxiway surface.

Freezing Fog Key Points:

• Where it forms: Cold winter climates, arctic or temperate regions.
• When it forms: When surface temperatures drop below freezing while fog is present.
• Impact on aviation: Dangerous airframe icing and icy runways.
• Preparation tips: Use proper de icing procedures and be conservative with takeoff and landing decisions.

In summary:

• Ice fog: Fog made of ice crystals.
• Freezing fog: Supercooled droplets that freeze on surfaces.

6. Frontal Fog

Frontal fog develops when warm rain falls into cooler air along a frontal boundary. As the rain evaporates in the cooler air, it adds moisture and raises the dew point until saturation occurs.

Key points for pilots:

• Where it forms: Along warm fronts or widespread precipitation zones.
• When it forms: During steady rain events.
• Impact on aviation: Often accompanies low ceilings, rain, and icing.
• How to anticipate: Monitor surface analyses and TAFs for frontal activity and precipitation.

7. Precipitation Fog

Precipitation fog forms when rain falls through cooler air and evaporates, adding moisture until saturation occurs. This type of fog is often associated with fronts but can occur without a defined boundary.

Key points for pilots:

• Where it forms: Under widespread light rain or drizzle.
• When it forms: During prolonged precipitation events.
• Impact on aviation: Significant visibility reduction combined with low ceilings and possible turbulence.
• Planning tip: Build alternates into your plan and expect extended IFR conditions.

Flying Smarter Around Fog

Every type of fog is driven by a combination of temperature, moisture, wind, and terrain. As a pilot, your job is not to memorize every meteorology term, but to recognize patterns that matter for your flight.

Here are simple checks and habits that help you stay ahead of fog risk:

• Watch temperature and dew point trends.
• Study local geography. Valleys, coastlines, and rising terrain are fog magnets.
• Check METAR and TAF trends. Not just the most recent, but several hours back.
• Use graphical tools. GFA, satellite imagery, and area forecasts provide the full picture.
• Choose smart alternates. Avoid terrain traps and valley airports in fog-prone conditions.

Using Personal Minimums and ADM to Stay Safe

Even when you understand how fog forms, good decision making is what ultimately keeps your flight safe. This is where personal minimums and Aeronautical Decision Making (ADM) come into play. Legal weather minimums are not always safe minimums, especially for newer pilots or those who have not flown in low visibility recently.

The PAVE checklist is one of the best tools for evaluating fog risk:

• P – Pilot: Are you current and proficient if you must enter IMC?
• A – Aircraft: Is your airplane properly equipped for possible low visibility, icing, or night operations?
• V – enVironment: What are the visibility and ceiling trends, temperature/dew point spread, and fog likelihood?
• E – External Pressures: Are schedules or expectations influencing your decision to continue?

Adding the PAVE checklist into your preflight and in flight evaluation process makes it far more likely you will recognize fog hazards early and act decisively.

Frequently Asked Questions

• What is the rarest type of fog?
  Ice fog is among the rarest types because it requires extremely cold temperatures, usually below about minus 20 degrees Fahrenheit, where water vapor freezes directly into ice crystals.
• What are the main types of aviation fog?
  The most important types are radiation fog, advection fog, upslope fog, steam fog, freezing fog, ice fog, frontal fog, and precipitation fog.
• What is a very thick fog called?
  Very thick fog is often referred to as dense fog. In METAR reporting, it typically means visibility has dropped below a quarter mile.
• What is frozen fog called?
  Frozen fog can mean either freezing fog (supercooled droplets that freeze on contact) or ice fog (fog made of ice crystals).
• What is the difference between fog and mist?
  In aviation weather reports, Mist (BR) is used when visibility is between 5/8 statute mile and 6 miles. Fog (FG) is reported when visibility drops below 5/8 statute mile. Both involve suspended water droplets, but fog is denser and reduces visibility far more severely.

Takeaway

Fog can sneak up on you quickly and change a workable plan into a no go situation or an unplanned instrument flight. Each type of fog forms under a specific set of conditions, which gives you an opportunity to see it coming.

By watching temperature and dew point trends, understanding local terrain, and using modern weather tools, you can recognize fog patterns early and make smart decisions about routing, timing, and alternates.

Stay curious about the weather, build conservative habits, and you will be far better prepared for the next time fog tries to complicate your flight.

Interested in Aviation Weather?

Our guides are designed to help you build real world weather confidence:

• Weather Fronts and Flight: What Every Pilot Should Know
• Winter Flying Tips for Cold Weather Operations
• AWOS vs ASOS: What You Should Know
• AIRMET vs SIGMET: Everything You Need to Know

Did you find this article helpful?

Do you think we missed an important fog scenario that pilots should know about? Let us know in the comments below.