If you're working on your Private Pilot certificate, fall and winter will teach you something summer never did: the weather that cancels the most flights isn't always the dramatic stuff. Often, it's fog. It shows up quietly, it can take an airport from great VFR to below minimums in a few hours, and it's fair game on your checkride.
Let's look at the two types you'll run into most, radiation fog and advection fog, and how to see each one coming.
A Clear, Calm Evening... and a Trap
Imagine it's a Saturday evening in November, and you've got an 8 a.m. departure for your long solo cross-country. It rained all afternoon, but now the sky has cleared and the wind has gone dead calm. You check the weather at your home field before bed:
Wind calm, visibility 10 miles, sky clear, temperature 9°C, dew point 7°C.
Looks perfect, right? An experienced pilot sees wet ground, clear skies, no wind, a 2-degree spread, and a long night ahead. That's a recipe for fog by morning.
Fog Is Just a Cloud That Touches the Ground
Fog forms when air near the surface reaches its dew point and water vapor condenses into visible droplets, either because the air cooled or because moisture was added.
That's why the temperature/dew point spread is your early warning system. The FAA's Aviation Weather Handbook notes that fog seldom forms when the spread is greater than 2°C (4°F). What separates radiation fog from advection fog is how the air gets cooled, and that tells you how each one behaves.
Radiation Fog: The Overnight Fog
Radiation fog forms over land on clear nights. With no clouds acting as a blanket, the ground radiates its heat away and chills the air resting on it. Once that air hits its dew point, fog forms.
The classic ingredients:
- Clear skies, so the ground can cool efficiently
- Light or calm winds
- Moist air near the surface (rain-soaked ground followed by clearing skies is a textbook setup)
Wind matters here. Calm air gives you shallow fog, but a light breeze up to about 5 knots can actually thicken it by mixing the cooling through a deeper layer. Stronger winds break it up or lift it into low stratus.
It favors low spots like river valleys, and it only forms over land, since water barely cools overnight.
How it leaves: The sun warms the ground and the fog burns off. Shallow ground fog usually clears fairly quickly after sunrise, and deeper radiation fog generally clears before noon, unless a cloud layer moves in over the top and blocks the sun.
The trap: "It'll burn off by 10." Maybe. But fog is often at its thickest around dawn, right when you're preflighting, and "it'll burn off" is a forecast you're making. If you plan around that hope, have a plan for when it doesn't.
Advection Fog: The Fog That Travels
Advection simply means horizontal movement. Advection fog forms when moist air moves over a colder surface, like cold ocean water or cold ground, and gets cooled to its dew point from below.
Unlike radiation fog, advection fog needs wind. It deepens as the wind increases up to about 15 knots. Stronger than that, it usually lifts into a layer of low stratus or stratocumulus.
It's most common along coastlines. On the West Coast, fog forms over cold offshore water and the wind carries it inland, often pushing over land at night and pulling back in the morning. In winter, moist Gulf of Mexico air flowing north over cold ground can produce advection fog across the central and eastern U.S.
How it leaves: Often, not on your schedule. Advection fog is usually more extensive and more persistent than radiation fog, and it doesn't depend on the sun. It can roll in at any time of day or night.
The trap: You depart an inland airport in solid VFR, and your coastal destination looked fine an hour ago. But an onshore wind is pushing the marine layer inland, and by the time you arrive, the field is socked in. Clear skies where you are tell you very little about advection fog somewhere else.
You'll also hear about upslope, steam, and ice fog, but radiation and advection fog are the two you'll deal with most.
Reading the Clues in METARs and TAFs
Don't judge fog risk from a single METAR. Watch the trend. Here's a made-up but realistic overnight sequence at a valley airport:
- 7 p.m.: wind calm, 10SM, CLR, 09/07
- 10 p.m.: wind calm, 10SM, CLR, 07/06
- 1 a.m.: wind calm, 6SM BR, CLR, 06/06
- 4 a.m.: wind calm, 1/4SM FG, VV002, 05/05
The spread closed, the wind never picked up, and the visibility fell off a cliff. The codes to know:
- BR (mist): visibility from 5/8 up to 6 statute miles
- FG (fog): visibility less than 5/8 statute mile
- MIFG, BCFG, PRFG: shallow fog, patches of fog, and partial fog
- VV (vertical visibility): the sky is totally obscured; VV002 means you can see only 200 feet straight up
That VV matters. Under the definition of ceiling in 14 CFR 1.1, an obscuration counts as a ceiling, so VV002 isn't "no clouds." It's a 200-foot ceiling.
Check the TAF too. If it shows BR or FG developing overnight, take it seriously. Headed toward the coast? Look at the wind direction: an onshore flow is how advection fog shows up.
Making the Go/No-Go Call
Under 14 CFR 91.103, before any flight that leaves the vicinity of an airport, you must become familiar with weather reports and forecasts and with the alternatives available if the flight can't be completed. Fog is exactly why that second part matters.
It's the same go/no-go process you'd use for any weather, plus a few fog-specific habits:
- Check the spread the night before. A 2-degree spread with a calm, clear night ahead is a fog forecast in disguise.
- Know your fog-free alternate. Fog is often local; an airport on higher ground or farther inland may be wide open.
- Don't count on "it's lifting" at your destination. If it doesn't, you're a VFR pilot over a cloud deck with nowhere to land.
- Carry extra fuel whenever fog is in the picture, so diverting is an easy decision.
What Your Examiner Wants to Hear
Fog is listed right in the Private Pilot ACS under Weather Information (PA.I.C.K3j, Fog/mist). Expect questions like: What conditions favor radiation fog? Why does advection fog need wind?
Your examiner wants to know you'd recognize the setup the night before a flight, not that you memorized a definition. Chapter 18 of the Aviation Weather Handbook covers fog in depth, and the ASA Private Pilot Oral Exam Guide and Checkride Ready are great ways to practice explaining it out loud.
The Takeaway
Radiation fog is a right here, overnight problem: clear skies, calm wind, moist ground, and a closing spread. Advection fog is a what the wind is bringing problem, and it doesn't care what time it is. Learn to read the setup before the fog arrives, and you'll make the right call long before you're on the ramp staring at a gray wall.
For more resources to support your training, visit NorthstarVFR.com.