Dew Point Spread and Fog: Reading the Warning Signs
Updated: 2026-09-27 · First published: 2026-09-27 · About this site
The temperature group in a METAR — 17/16 — is the most under-read pair of numbers in aviation. The first is the air temperature, the second the dew point, and their difference, the spread, is a direct measurement of how close the air is to saturation. A small spread at the right time of day, with the right wind, is fog in the making, and you can watch it happen across successive reports before a single droplet forms.
What the dew point is
The dew point is the temperature to which the air must be cooled, at constant pressure, for the water vapour in it to start condensing. It is a measure of absolute moisture content: air with a dew point of 16 °C holds the same amount of water whether the air temperature is 17 °C or 30 °C. When the air temperature falls to the dew point, relative humidity is 100% and condensation begins — as dew on the grass, as fog in the air, as cloud aloft. Every airport page on this site shows the spread and the derived relative humidity next to the raw report, and the meteogram plots temperature and dew point as two lines so you can see them converge.
The spread as a fog predictor
| Spread | Meaning at night, light wind, clear sky |
|---|---|
| > 5 °C | Fog very unlikely in the next few hours. |
| 3–5 °C | Watch the trend. If the temperature is falling 1 °C per hour, fog by dawn is possible. |
| 2 °C | Classic pre-fog signature. Mist (BR) likely; fog probable at the coldest point of the night. |
| ≤ 1 °C | Saturation. Fog or mist is forming or present somewhere on the aerodrome. |
The table applies to radiation fog, the kind that forms on clear, calm nights when the ground cools by radiating heat to space and chills the air in contact with it. It needs three things: clear skies (cloud acts as a blanket), light wind (2–8 kt is ideal — enough to mix the cooled air a few metres deep, not enough to stir in warm air from above), and moist air near the surface (recent rain, wet ground, a river valley). Take away any one of them and the fog does not form.
Other fogs, other rules
- Advection fog — warm moist air flowing over a colder surface, typically the sea. It forms with wind, sometimes 15 kt, and it does not clear when the sun comes up because the sun does not warm the sea. Coastal airports such as LTBS Dalaman in spring, or the Californian coast in summer, get it in the daytime. Watch the wind direction and the sea temperature rather than the spread at the field.
- Frontal fog — rain falling from a warm front into cold air below saturates it. Look for a spread closing while the temperature is not falling and rain is in the report.
- Upslope fog — air lifted along rising terrain cools adiabatically to its dew point. It is stratus on the ground, common on windward slopes.
- Freezing fog (FZFG) — fog at temperatures below 0 °C, depositing rime ice on everything it touches, including your wings and the pitot tube.
Reading the trend across reports
A single METAR gives you one spread. The last five or six reports, which the station page lists under raw reports, give you the rate. Consider this evening at a valley airport:
1750Z 24/14 spread 10 wind 240/06 1850Z 21/14 spread 7 wind 250/04 1950Z 18/14 spread 4 wind VRB/02 2050Z 16/14 spread 2 wind 00000KT BR
The dew point is holding at 14 °C, the temperature is falling 2–3 °C per hour and the wind is dying. The spread will reach zero around 22:00 UTC and, with sunrise eight hours away, that fog will be deep and persistent by morning. Nothing in the last report says FG yet; everything in the sequence does. This is why the site keeps the recent history beside the current report rather than showing only the latest line.
When it clears
Radiation fog is destroyed by heating from below or by wind. The sun must warm the surface enough to open the spread by 2–3 °C, which depends on the season, the latitude and whether there is cloud above the fog. A fog layer beneath a stratus deck in a winter anticyclone can persist for days, because the stratus blocks the sun and the anticyclone blocks the wind. Wind above roughly 10 kt lifts the fog into low stratus — an improvement in visibility but often a ceiling of 200–400 ft, which is LIFR by another name. The TAF expresses the forecaster's expectation of clearance as a BECMG group; compare it with the model temperature forecast on the meteogram, which will show when the surface is expected to warm.
Checklist before a dawn departure
- Spread in the latest METAR and its trend over the previous three reports.
- Wind: calm to light favours radiation fog; onshore moderate wind favours advection fog at coastal fields.
- Sky: clear at night raises the risk; a cloud deck lowers it.
- Recent rain, wet ground, nearby water.
- The TAF: any PROB or TEMPO with FG, BR or BKN002 in the departure window.
- An alternate whose spread is comfortably open, found via the airport directory.
Frequently asked questions
What dew point spread means fog is likely?
A spread of 2 °C or less at night with light wind and clear skies is the classic warning; 1 °C or less means fog or mist is probably already forming somewhere on the field. Above about 4 °C radiation fog is unlikely within the next few hours.
What is the difference between BR and FG in a METAR?
Both are water droplets in the air. BR (mist) is reported when visibility is 1,000 m or more but below 5,000 m; FG (fog) when visibility is below 1,000 m. Both may carry prefixes: MIFG (shallow fog, below eye level), BCFG (patches), PRFG (partial, covering part of the aerodrome), VCFG (in the vicinity).
Why does fog form just after sunrise?
Because the sun's first heating lifts a little more moisture from the wet ground into air that is still at its coldest, and light mixing brings the saturated layer up to eye level. The minimum temperature — and therefore the smallest spread — often occurs 30 to 60 minutes after sunrise, not before it.
How quickly does radiation fog clear?
Once the sun heats the surface enough to raise the temperature 2–3 °C above the dew point. In summer that can be an hour after sunrise; in a December anticyclone with a stratus deck above the fog it may not clear at all. Wind above about 10 kt lifts it into low stratus rather than clearing it.
Does a high dew point mean fog?
Not by itself. A dew point of 22 °C with a temperature of 32 °C is a spread of 10 °C — humid, but far from saturated. Fog needs the spread to close, either because the air cools to the dew point or because moisture is added.
Read next
- How to Read METAR Reports: A Pilot's Guide — Learn to decode a METAR line by line: station, time, wind, visibility, weather, clouds, temperature, QNH and trend. Worked example plus the mistakes that catch pilots out.
- VFR vs IFR Flight Categories Explained — What the four flight categories mean, the exact ceiling and visibility thresholds, how to derive them from a METAR or TAF, and how they differ from flight rules and European colour states.
- How to Read a TAF Forecast — Decode a TAF: validity period, FM, BECMG, TEMPO and PROB groups, what each change group promises, and how to pick the worst period for alternate planning. Worked example included.
- Crosswind Component and Runway Selection — The sine rule, the clock-face shortcut, gust handling and a worked example. How crosswind, headwind and tailwind components decide which runway to use and when to divert.
- QNH, QFE and QNE: Altimeter Settings Explained — What each altimeter setting measures, when to use which, how to convert hPa and inHg, what a wrong setting does to your altitude, and how transition altitude and level work.
- All pilot guides — the full library, 15 articles.
- Airports by country — live METAR and TAF for every reporting station
- World weather rankings from the observation archive