Temperature, Dew Point and Fog
Temperature and dew point are two of the plainest numbers on a METAR, but together they predict one of the fastest visibility killers a remote pilot will face: fog. A perfectly clear, calm forecast can still turn into a zero-visibility soup within an hour if the temperature/dew point spread is narrow and still closing. This lesson explains what temperature and dew point actually measure, how to read the spread, how fog physically forms, and how to build fog risk into your pre-flight and in-flight go/no-go decisions.
What each number actually means
Temperature is straightforward — it's how warm or cold the air is right now. Dew point is less intuitive: it's the temperature the air would have to cool down to (at constant pressure and moisture content) for it to become fully saturated — that is, for water vapor already in the air to start condensing out as liquid water. A high dew point means the air is already holding a lot of moisture. A low dew point means the air is dry, even if the temperature itself is warm.
The dew point spread
The "spread" is simply temperature minus dew point. A wide spread (10°C/18°F or more) means the air is relatively dry compared to how much moisture it could hold at that temperature — fog is unlikely. A narrow spread (2°C/4°F or less) means the air is already close to saturation — only a small additional drop in temperature, or a small increase in moisture, pushes it over the edge into fog or low cloud.
What matters even more than the spread's current value is its trend. A 6°C spread that is steadily narrowing as the sun sets is a much stronger warning sign than a steady 4°C spread that isn't moving. Pull two or three consecutive METARs before a dawn or dusk flight and watch which direction the spread is heading.
Why the air cools toward its dew point overnight
On clear nights with light wind, the ground radiates heat away into space faster than it can be replaced, cooling both the surface and the air layer just above it. As that thin surface layer of air cools, its dew point stays roughly constant while its temperature falls — so the spread narrows through the night. If the temperature reaches the dew point before sunrise, the moisture condenses into fog right at ground level, exactly where a sUAS launches and lands.
- Clear skies: no cloud layer to trap outgoing heat, so the ground cools faster.
- Light wind (calm to about 5 kt): too little mixing to stir warmer air down from above, so cooling stays concentrated near the surface.
- Moist air near the surface: recent rain, nearby water, or high humidity supplies the moisture that condenses once the air saturates.
Fog isn't one thing — common types a remote pilot will encounter
Different fog types form differently, and knowing which one you're looking at helps you judge how fast it will burn off (or won't).
- Radiation fog: the classic overnight/early-morning fog described above — forms in low-lying areas and valleys on clear, calm nights, and typically burns off within a few hours after sunrise as the sun reheats the ground. The most common fog a sUAS pilot flying dawn inspection or mapping missions will encounter.
- Advection fog: forms when warm, moist air moves horizontally over a colder surface (common near coastlines, lakes, and large rivers). Unlike radiation fog it doesn't depend on clear skies or overnight cooling, and it can persist for a full day or longer since it isn't tied to the sun's heating cycle.
- Upslope fog: forms when moist air is pushed up rising terrain, cooling adiabatically as it climbs until it saturates. Relevant for pilots operating in hilly or mountainous terrain.
- Freezing fog (FZFG): radiation or advection fog where the temperature is at or below 0°C — droplets freeze on contact with surfaces, including a drone's frame, props, and camera lens.
Radiation fog forms when the dew point spread narrows to zero overnight under clear skies and light wind — exactly the conditions common on calm mapping-mission mornings.
View LargerIt directly threatens your VLOS compliance
14 CFR 107.51 requires flight visibility of at least 3 statute miles from the remote pilot's control station, and fog is one of the fastest ways that number can drop below minimums. Unlike a stable overcast layer, radiation fog can go from "clear and a million" to sub-mile visibility over ground obstacles in well under 30 minutes as the spread closes the last degree or two.
Ground fog hides exactly what you need to see
Even a shallow fog layer only a few hundred feet thick can obscure your visual line of sight to the aircraft, hide other traffic and people on the ground near your operating area, and fog your camera or gimbal lens directly. For low-altitude Part 107 operations (at or below 400 ft AGL for most operations), a thin ground fog layer can affect the entire operating envelope, not just a slice of it the way a high cloud deck would.
Dawn and dusk missions carry the highest risk
Radiation fog is a sunrise problem by nature — the spread is narrowest right before dawn, after a full night of radiational cooling. Pilots flying early-morning inspection, agriculture, or mapping missions in calm, clear conditions are exactly the population most likely to launch into a location that fogs in while they're airborne, or that looks clear at the control station but is fogged over the actual flight area in a low spot.
Fog can form faster than you can react
Because radiation fog forms in place rather than moving in from a distance, there's often no visual warning before it happens — unlike a rain shower or an approaching cloud bank you can watch develop. That's why the dew point spread and its trend, checked before launch, matter more here than almost anywhere else in this course: it's a predictive number for a hazard with very little visual lead time.
| Code | Meaning | Typical visibility | Notes |
|---|---|---|---|
| BR | Mist | ≥ 5/8 SM but ≤ 6 SM (reduced but not severe) | A lighter version of the same saturated-air process as fog; often the stage right before fog thickens further. |
| FG | Fog | Below 5/8 SM (under 1 km) | Reported only when visibility drops below 5/8 SM; between 5/8 and 6 SM it's reported as mist (BR) instead. |
| FZFG | Freezing fog | Below 5/8 SM | Fog with droplets at or below freezing — ice can accrete on any exposed surface, including your aircraft. |
| VV/// | Vertical visibility, indefinite ceiling | Reported instead of a normal sky-condition layer | Common in dense fog — there's no distinct cloud base, just an obscured column of low visibility straight up. |
| BECMG ...FG | TAF forecast group | Gradual change to fog conditions | Signals the forecaster expects fog to develop gradually within the stated time window — a strong pre-flight planning cue. |
Dew Point Spread / Fog Risk Indicator
Building Fog Risk Into a Pre-Flight Check
Use this sequence for any flight planned near dawn, dusk, or over/near water, valleys, or recently rained-on ground.
Pull the current METAR and note temperature and dew point
Find the temp/dew point group (e.g. "14/12") from the nearest reporting station and calculate the spread. Also scan the remarks and present-weather group for BR, FG, or FZFG already being reported.
Pull one or two older METARs and check the trend
Compare the spread from the last hour or two against right now. A spread that's actively closing is a stronger warning than a narrow-but-stable spread.
Check the TAF for BECMG or TEMPO fog groups
The forecast may already flag an expected transition to fog or mist within your planned flight window, even if current conditions look clear.
Weigh the environmental setup
Clear skies, light wind, and a location near water, a valley floor, or recently saturated ground all raise the odds of radiation or advection fog independent of what the spread number alone shows.
Build in a buffer, not just a minimum
If the spread is narrow and closing before a dawn launch, consider delaying until after sunrise solar heating typically starts widening the spread again, rather than launching into a visibility margin that's actively shrinking.
| Mistake | Why it happens | Correct understanding | Regulation / source |
|---|---|---|---|
| Assuming a wide spread now means fog is off the table | Only checking a single current observation | A wide but rapidly narrowing spread late in the day can still close to zero before a dawn launch — trend matters as much as the current value. | FAA Aviation Weather Handbook (FAA-H-8083-28) |
| Confusing BR (mist) with FG (fog) as interchangeable | Both describe the same saturated-air process | They're distinguished strictly by visibility: BR is reported at 5/8 SM up to 6 SM, FG only below 5/8 SM. | FAA/NWS METAR coding standards |
| Trusting the control-station view over the reported spread | "I can see clearly from here" feels more reliable than a number | Fog forms first in low spots, over water, and in valleys — your control station's line of sight may not represent the whole operating area. | 14 CFR 107.51 |
| Ignoring dew point because temperature alone "feels" fine | Temperature is the more familiar, everyday number | A comfortable temperature says nothing about fog risk on its own — only the spread between temperature and dew point does. | FAA Aviation Weather Handbook (FAA-H-8083-28) |
Can dew point ever be higher than temperature?
No. Physically, dew point cannot exceed the actual air temperature. If you ever see a dew point reported higher than the temperature, that's a data or transcription error, not a real observation — and it's a good sanity check when reading any weather source.
How fast can fog actually form?
Under the right conditions — clear skies, calm wind, and a spread already inside a couple of degrees — radiation fog can form and drop visibility below VLOS minimums in well under an hour, sometimes in as little as 15-20 minutes right around sunrise. That's why checking the trend, not just the current spread, is critical for early-morning missions.
Does wind always prevent fog?
Light wind (roughly up to 5 kt) can actually help radiation fog form by gently stirring the cooling surface layer without mixing in enough warmer air from aloft to stop it. It's only wind above about 8-10 kt that typically prevents or breaks up radiation fog by mixing the cooled layer with drier, warmer air above it.
Why does fog usually burn off by mid-morning?
As the sun rises, it reheats the ground, which reheats the air just above it. That widens the temperature/dew point spread again, and the fog evaporates from the ground up. Advection fog is the exception — because it isn't tied to overnight radiational cooling, it can persist well into or through the day.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.