Fog, mist and the dewpoint spread

The two numbers at the end of a METAR, the temperature and the dewpoint, quietly tell you whether fog is on the way. The closer they are, the nearer the air is to saturation, and saturation near the ground means reduced visibility.

This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.

Temperature, dewpoint, and saturation

The dewpoint is the temperature to which air must be cooled, at constant pressure and moisture, for it to become saturated and start condensing into water droplets. The dewpoint spread is simply the air temperature minus the dewpoint. It is a direct readout of how close the air is to forming cloud or fog.

• A large spread means dry air and clear conditions.
• A small spread, within roughly 2 to 3 degrees Celsius, means the air is close to saturation, and fog or low cloud becomes likely, particularly overnight when the temperature falls towards the dewpoint.
• A zero spread, where temperature and dewpoint are equal, means the air is saturated, which is what you see in a fog report.

This is why an evening METAR reading 09/08 (temperature 9, dewpoint 8) on a clear, light-wind night is a fog warning even though the visibility is still good: there is only one degree to go. The FAA Aviation Weather Handbook (FAA-H-8083-28) and the UK Met Office aviation guidance both treat the temperature-dewpoint relationship as the core of fog forecasting.

Mist and fog in the code

A METAR distinguishes the two by visibility:

• Fog (FG) is reported when the visibility is below 1000 metres.
• Mist (BR) is reported when the visibility is between 1000 and 5000 metres.

So the same suspended droplets are coded BR while you can still see a couple of kilometres, and become FG once they thicken and the visibility drops below 1000 metres. When the sky is hidden by fog, the cloud groups are replaced by a vertical visibility group such as VV002, meaning the observer can see 200 feet straight up into the obscuration but no defined cloud base. Watch, too, for FZFG, freezing fog, where the droplets are supercooled and freeze on contact, adding an icing hazard to the poor visibility.

A worked example

EGGD 0520Z 00000KT 0300 FZFG VV001 M01/M01 Q1029

Reading it: calm wind; visibility 300 metres; freezing fog; vertical visibility 100 feet; temperature minus 1, dewpoint minus 1, so the spread is zero and the air is fully saturated; pressure 1029. This is a textbook radiation-fog morning: calm, clear-cooled, saturated, and below freezing, so the fog is freezing fog. It will not lift until the ground warms enough to raise the temperature above the dewpoint and evaporate the droplets.

The main types of fog

Fog is cloud at the surface, and it forms in a few distinct ways, set out in the Aviation Weather Handbook:

• Radiation fog forms overnight under clear skies and light wind, when the ground radiates its heat away, cools the air above it to the dewpoint, and a thin layer of fog settles in low ground and valleys. It usually thins and lifts within a few hours of sunrise.
• Advection fog forms when warm, moist air moves over a cooler surface and is chilled to its dewpoint. It needs some wind to keep the air moving, can be deep, and can persist for a long time. Sea fog is the classic example, drifting onto coasts and coastal airfields.
• Upslope fog forms when moist air is forced up rising terrain, cooling as it climbs.
• Evaporation or steam fog forms when cold air moves over warmer water, and the water evaporates into the cold air and recondenses.
• Precipitation fog forms when rain falls into cooler air beneath a front, saturating it.

The practical point is that radiation fog tends to clear on a predictable schedule, while advection and sea fog do not, so the type matters as much as the presence.

Forecasting fog from the evening trend

You can often see radiation fog coming hours ahead, from the evening conditions, and the UK Met Office aviation guidance and the FAA Aviation Weather Handbook set out the ingredients. Radiation fog needs:

• Clear skies, so the ground can radiate its heat away to space and cool quickly.
• Light wind, ideally a slight breeze. Dead calm tends to form dew or a very shallow ground fog, while too much wind mixes the air and forms low stratus instead of fog.
• Moist air near the surface, which a small dewpoint spread tells you about.
• A long night for the cooling to do its work, which is why radiation fog is a problem of autumn and winter as much as of any particular airfield.

Watch the trend through the evening. If at 1800 the temperature is 12 and the dewpoint 9, a spread of three, and the sky is clearing with the wind dropping, then as the temperature falls overnight towards the dewpoint the spread closes, and somewhere in the early hours it reaches zero and fog forms. Radiation fog typically thickens after midnight, is at its worst around and just after dawn, and lifts a few hours into the morning as the sun warms the ground and raises the temperature back above the dewpoint.

A forecaster encodes this expectation in the TAF. Look for fog or mist in a change group: a BECMG group bringing BR or FG in the evening, a TEMPO group for patchy fog, or a PROB30 or PROB40 group attaching a probability to it. A worked reading: TEMPO 2200/0700 0400 FG in a TAF says that, temporarily between 2200 and 0700, expect visibility down to 400 metres in fog, exactly the radiation-fog window. The decoded spread on the evening METAR and the fog groups in the TAF are telling you the same story from two directions.

The contrast to hold onto is that this predictability belongs to radiation fog. Advection and sea fog arrive on the wind from elsewhere and do not follow the local cooling clock, so they can form in the middle of the day and sit for hours, which is why identifying the type is the first step in judging how long it will last.

Estimating a cloud base from the spread

The temperature-dewpoint spread also gives a rough estimate of the base of convective cloud on a day of surface heating, a different use from fog but resting on the same physics. As the sun warms the ground, rising air cools at a steady rate until it reaches its dewpoint and condenses into the base of a cumulus cloud. A common rule of thumb puts that base, in feet above the surface, at roughly the surface spread in degrees Celsius multiplied by about 400.

So on an afternoon with a surface temperature of 24 and a dewpoint of 12, a spread of 12, the estimated cumulus base is about 12 times 400, or roughly 4800 feet above the airfield. A small spread means a low base; a large spread means high-based cloud or none.

Two cautions keep this honest. It is a rough rule for the base of heating-driven cumulus, not for fog, stratus, or layered cloud, which form by different processes and do not follow it. And it is an estimate from surface values, so it gives an order of magnitude, not a measured ceiling. For the actual cloud base you read the METAR and the TAF; the spread rule is a sense-check, useful for anticipating how high convective cloud will sit on a warm day. The same closeness of temperature and dewpoint that warns of fog overnight, then, tells a slightly different story under an afternoon sun.

Common pitfalls

• Assuming all fog burns off. Only radiation fog is reliable about lifting after sunrise. Sea fog can sit all day.
• Ignoring a small evening spread. A one or two degree spread on a calm, clear night is a strong fog signal even while the visibility is still good.
• Missing freezing fog. FZFG adds icing to the visibility problem; the spread will be at or near zero below freezing.
• Forgetting the units. Visibility in the ICAO code is in metres, so 0800 is 800 metres, well into fog.
• Reading vertical visibility as a cloud base. VV002 is how far you can see up into the obscuration, not a defined ceiling.

In Pilot EFB

Pilot EFB decodes the METAR and TAF and shows the temperature and dewpoint together with the rest of the briefing, so the spread that warns of fog is right there, and it keeps the raw report so you can read groups such as FG, BR, FZFG or a VV group in the original. It reports what has been observed and forecast; it does not predict fog for you, so apply your own reading of the spread and the conditions. Briefings you have pulled stay readable offline, while fetching a fresh report needs a connection. Pilot EFB is offline-first and is not a certified Electronic Flight Bag.