The 7 Types of Fog Every Pilot Needs to Know

December 2025

By Jordan Ash, CFI/I

As a flight instructor who’s spent more chilly pre-dawn weather briefings than I care to count, I’ve seen firsthand just how dangerous—and surprisingly sneaky—fog can be for student pilots. Correction, make that all pilots. Understanding how fog forms, the different flavors it comes in, and when to expect it isn’t just academic: it’s critical for safety, go/no-go decisions, and avoiding a nasty “VFR into IMC” surprise on a clear preflight. In this post I’ll walk you through how fog forms in general, then dive into the seven types of fog every pilot should understand, and explain why this knowledge pays off.

☁️ How Fog Forms — The Basics

Fog is basically a cloud that forms at the surface. For fog to occur, air near the ground must become saturated, meaning its temperature drops enough (or moisture is added) such that water vapor condenses into tiny liquid-water droplets — or sometimes ice crystals — suspended in the air.

That saturation typically occurs when one of two things happen:

Cooling: The air temperature drops until it reaches its dew point.
Moisture Addition: Water vapor is added to the air (for example, from evaporation or precipitation) until it saturates.

Then, if there are enough tiny particles in the air (condensation nuclei — dust, pollen, salt, etc.), the water vapor condenses, forming fog.

From a pilot’s perspective, a helpful red flag is when the temperature–dew point spread shrinks to ~5 °F (or ~2–3 °C) or less — especially if it’s trending down. That’s when you start thinking “Could fog form soon?”

✈️ The 7 Types of Fog Student Pilots Should Know

Meteorologists and aviation weather guides don’t always agree on a strict “seven types,” but these are the most commonly taught in pilot training — and certainly the ones you’ll need to recognize on a checkride or preflight briefing.

1. Radiation Fog

This is ground fog that forms at night or early morning over land — especially in valleys, low-lying fields, and near rivers or lakes. When the sky is clear and winds are calm or light, the ground cools rapidly (radiating its heat), chilling the air above. If that air is moist and the temperature drops to dew point, fog will form.

When: Late night to early morning, often following a warm, humid day.

Where: Valleys, flatlands, near water or moist ground.
Why it matters to pilots: It can form after you’ve parked the plane — and fade quickly after sunrise. IFR or low-visibility conditions may greet you just before or during your preflight walk-around or taxi.

2. Advection Fog

Advection fog occurs when warm, moist air moves horizontally (advects) over a colder surface — often a cold body of water, snow-covered ground, or a cold land mass. As the warm air cools to its dew point, fog forms. This type of fog doesn’t require nighttime cooling and can happen at any time of day or night.

Where: Coastal areas, over cold ocean currents, or over snow-covered ground.
When: With onshore breezes, seasonal shifts — often winter or spring.
Why it matters: Advection fog can be persistent and widespread, keeping airports IFR for hours or even days. It can also advect inland from water into areas you think are “safe.”

3. Steam Fog (Evaporation Fog / “Sea Smoke”)

Steam fog — also known as evaporation fog or sea smoke — happens when cold, dry air moves over warmer water (lakes, rivers, coastal water). The warmer water evaporates moisture into the dry air, the added moisture saturates the air, and fog rapidly forms. Visually, it can look like wisps or smoke rising from the water.

Where: Over lakes, rivers, or coastal waters — especially when air temperatures drop suddenly but water remains relatively warm.
When: Often in colder seasons when early morning or evening air is significantly cooler than water.
Pilot hazard: Steam fog can form quickly and may coincide with low-level icing conditions if temps are near or below freezing — an insidious hazard for small aircraft early in the morning.

4. Upslope Fog

Upslope fog forms when moist, stable air is forced up a slope or mountain. As the air rises, it expands and cools adiabatically until it reaches dew point — then fog (or low clouds) forms along the terrain slope. Adiabatically means without any heat being added to or removed from the air parcel. In aviation and meteorology, when we talk about air rising or sinking adiabatically, we mean the temperature changes only because of pressure changes, not because the air is exchanging heat with its surroundings.

Where: Mountainous or hilly terrain, especially windward slopes.
When: Whenever steady winds push moist air uphill.
For pilots: If you’re flying in or near mountainous areas — especially in VFR terrain flight or pattern work — upslope fog can balloon up slopes or ridges and hide terrain or runway until just before you’re on final.

5. Frontal Fog (Precipitation-Induced Fog)

Frontal fog, this type occurs when rain or drizzle falls through a layer of cooler air near the ground. As precipitation evaporates, it adds moisture to the air. If enough moisture is added and the air saturates, fog forms.

When/Where: Near fronts, during warm front passages, or whenever rain falls through a cooler air mass.
Hazard for flight: Because precipitation often coincides with other IFR conditions (low ceilings, low clouds, moisture), precipitation-induced fog can be part of a larger weather threat — requiring serious go/no-go thinking.

6. Ice Fog / Freezing Fog

When temperatures are well below freezing, water vapor may condense directly as ice crystals in the air — forming ice fog. Alternatively, supercooled water droplets may remain liquid (freezing fog) and can freeze on surfaces. Conditions are often similar to radiation fog (clear, calm), except the freezing temps make ice formation possible.

Where/When: Cold-weather environments (especially arctic or sub-freezing nights), often over land.
Flight risk: Ice fog significantly reduces visibility and the presence of supercooled droplets can pose serious icing hazards — on wings, prop, or windshield.

7. Freezing / Mixed (Frontal) Fog (sometimes combined with Ice Fog)

Some folks treat “freezing fog” or “frontal fog” separately, others combine them. This fog forms when moist air is cooled below freezing — often post-precipitation or frontal passage — leading to supercooled droplets or ice crystals near the ground.

Because classification varies between meteorology sources— and sometimes local weather services — it’s useful for pilots to think of this category broadly as fog in sub-freezing conditions, whether liquid (freezing fog) or ice (ice fog).

Why You, As a Pilot, Should Know This

Fog can appear or disappear quickly: Radiation fog can burn off fast after sunrise. Steam fog can form suddenly when cold air moves over warm water. Precipitation-induced fog may pop up after a rain shower. If you don’t recognize the conditions, you may get caught on the ground or in the air unexpectedly.
Fog comes in more flavors than “just fog”: Each type has different triggers, time-of-day/weather associations, and hazards (e.g., icing, low-level turbulence, or terrain hiding fog). Treating all fog as identical is a recipe for complacency.
It affects go/no-go decisions, planning, and risk management: As a pilot — especially as a VFR-rated student or VFR flyer — knowing fog types helps you interpret METARs, TAFs, surface observations, and weather briefings more intelligently. It influences preflight planning, alternates, and minimums.
It builds situational awareness and weather literacy: Recognizing fog triggers reinforces a core principle of aviation weather: think in terms of temperature, dew point, moisture, wind, terrain, and time — not just “clouds” or “visibility.”

A Tip for Student Pilots: Use Good Tools

While understanding theory is crucial, having the right planning tools helps cement that knowledge and makes your preflight work more efficient. For example, the NorthstarVFR 2026 FAR/AIM Tabs are a great choice to keep your FARAIM organized, especially when you’re trying to quickly look up weather-related regulations or limitations before a flight.

And if you want to practice partial-panel, hood work, or simulate low-visibility conditions safely on the ground before you attempt real foggy flights, the NorthstarVFR View Limiting Device (Foggles) remains a staple in many CFI toolkits.

Final Thoughts

As you go from student pilot to confident aviator, weather knowledge grows from “just memorize how to read METARs” to a full mental model of how the atmosphere behaves — how temperature, moisture, terrain, and time interact to shape what you see (or don’t see) from the cockpit. Fog may look simple and passive, but it’s often hiding more danger than a thunderstorm, especially for small-plane pilots flying VFR.

Take the time to understand all seven types of fog above. Ask yourself during preflight: “Could any of these fog types form given today’s dew point, winds, terrain, and forecast?” If the answer is “yes,” give yourself a bigger margin — or better yet, postpone the flight until you’re sure.

Fly safe, stay sharp — and respect the fog.

By Jordan Ash, CFI/I

If you’re a student pilot looking for reliable study tools and solid flight gear, NorthstarVFR.com has you covered. We offer a curated selection of pilot essentials—like our Pre-Tabbed FAR/AIM, Oral Exam Guides, kneeboards, logbooks, and more—made to simplify your training and keep you organized. Everything is geared toward helping you pass your checkrides and feel confident in the cockpit. Take a look and get set up for success.

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