The Temperature-Dewpoint Spread, Radiation Fog, and Reading a METAR to Know When the Airport Is About to Go Down at Dawn

The temperature-dewpoint spread is the single most important number for predicting fog. When the temperature and dewpoint in a METAR are within about 2°C of each other and closing on a clear, calm night, expect radiation fog to form right around sunrise. Do the subtraction on every weather report - temperature minus dewpoint - and you can see the fog coming hours before it swallows the runway.

What Is the Temperature-Dewpoint Spread?

The dewpoint is the temperature the air must cool to before it can no longer hold its moisture. When air cools to its dewpoint, invisible water vapor condenses into something you can see - cloud, dew on your windscreen, or fog on the runway.

Think of the air as a sponge. Warm air is a big, dry sponge that soaks up water and still feels dry. As it cools, the sponge shrinks and holds less. Cool it enough and it’s squeezed so tight the water has nowhere to go but out. That squeezing-point temperature is the dewpoint.

The spread is simply temperature minus dewpoint. A wide spread - say a temperature of 28°C and a dewpoint of 10°C - means the air is thirsty and fog isn’t coming anytime soon. A narrow spread means the sponge is nearly full, and it won’t take much cooling to wring it out.

How Do I Read Temperature and Dewpoint in a METAR?

A METAR (Meteorological Aerodrome Report) is the current, observed weather at an airport - not a forecast. It’s issued roughly once an hour in a compressed string of code.

Here’s a real-world example from a Midwest field, KGSO:

0653Z, wind calm, visibility 10 miles, sky clear, temperature 19, dewpoint 17, altimeter 30.01

Most pilots read that and their eye jumps straight to “visibility 10, sky clear” and they think severe clear, let’s go. But look at the two numbers that matter for fog: temperature 19°C, dewpoint 17°C. Both are in degrees Celsius, and they are only 2 degrees apart.

That small spread is a loaded gun sitting on the table.

What Spread Should Worry Me?

Use this rule of thumb:

• Within 2°C and closing - expect fog or low clouds.
• 3°C - pay attention.
• 4 to 5°C and dropping fast overnight - keep watching it.

The smaller the spread gets, the closer the airport is to going down.

Why Does Fog Form at Dawn on a Clear Night?

It seems backwards - you’d think a clear sky is the safe sky. Here’s what’s actually happening.

During the day, the sun pumps energy into the ground and warms it. Overnight, that heat radiates back up into space. On a clear night, there’s no cloud layer acting like a blanket to trap it, so the heat escapes fast and the ground cools hard.

The cold ground chills the layer of air sitting on top of it, and that air temperature slides down through the night - coldest right around sunrise.

Now add the third ingredient: calm or light wind. Strong wind mixes the air and tends to produce low stratus instead, or nothing. But a light wind of 3 to 5 knots is the sweet spot - just enough to stir a slightly deeper layer of moist air without breaking it up. Dead calm can produce fog right at the surface too.

Put the three together - a clear night, moist air (a small spread), and light wind - and you have the exact recipe for radiation fog, forming at the worst possible time for a morning departure.

How Do I Spot Fog Coming Before It Forms?

One METAR is a photograph. What you want is the movie. Pull the last three or four reports in a row and watch what the spread is doing.

Here’s the trend for KGSO overnight:

• Midnight: temp 24, dewpoint 17 - spread 7
• 0200: temp 22, dewpoint 17 - spread 5
• 0400: temp 20, dewpoint 17 - spread 3
• 0653: temp 19, dewpoint 17 - spread 2

The dewpoint holds steady at 17°C while the temperature falls toward it, one step at a time. The gap closes like a drawbridge coming down - 7, 5, 3, 2 - and the sun isn’t even fully up, so the temperature has more room to fall. When that spread hits zero, the air is saturated and you get fog. You didn’t need a meteorology degree to see it coming - just four lines of subtraction.

What Should I Actually Do About It?

Predicting fog is only half the skill. The other half is the decision.

Bring in the forecast side of the briefing. The METAR is the observation; the TAF (Terminal Aerodrome Forecast) is the airport’s best guess for the next day or so. A good forecaster will call for fog or low visibility as a temporary condition around sunrise. When the TAF and the METAR trend tell the same story, believe them.

The practical move: do not plan your departure for first light when you see a closing spread on a clear, calm night. That’s precisely when the fog forms.

The good news is that radiation fog usually solves itself. As the sun climbs, it heats the ground, the ground heats the air, the temperature climbs away from the dewpoint, the spread opens, and the fog lifts. Sleep in an extra two hours and launch at 9 or 10 instead of 6. Same flight, minus the wall of gray.

And don’t trust your eyeball over the number. Standing on the ramp, your slant visibility looking up might seem fine, but that same fog looking horizontally down a mile of runway is a wall. Reported visibility can be a quarter mile while you’re convinced you can see fine. Trust the number.

Where Else Does the Spread Trick Pay Off?

1. Your destination. Landing in the evening near a river, lake, or low-lying wet ground? Check the spread there for your arrival time. Moisture near water plus cooling evening air is the same setup - a gorgeous afternoon can sock in right as you arrive.

2. Frost and freezing fog. Same physics, colder. If the dewpoint is near or below freezing and the spread closes, you get frost on the wings and possibly freezing fog. Frost is a serious hit to lift and stall margin and must be removed completely before flight - the clean aircraft concept isn’t optional.

3. A rising dewpoint. Watch the dewpoint by itself, not just the spread. If it climbs over the course of a day, more moisture is moving in and tonight’s setup is getting more dangerous - even if the midday spread still looks fine.

Why the Examiner Cares About This

The Airman Certification Standards care about exactly this skill. On the oral, the examiner won’t just check whether you can recite METAR code out loud - anybody can do that. They want to see whether you can turn temperature, dewpoint, wind, sky, and trend into a go / no-go decision with a reason behind it.

If you can point at a 2-degree spread on a calm, clear morning and say, “That airport is likely to have radiation fog around sunrise, so I’d delay departure until it burns off,” you’ve connected the data to a decision. That’s the whole game.

Key Takeaways

• The spread is temperature minus dewpoint. Calculate it on every METAR you read.
• A spread within 2°C and closing, on a clear, calm night, means radiation fog is likely around sunrise.
• Read the trend, not just one report - watch the last three or four METARs to see the gap closing.
• Radiation fog burns off as the sun climbs, so delaying departure a couple of hours often solves the problem entirely.
• Trust the reported visibility over what your eye tells you from the ramp.