Fog in METAR: When visibility falls below 5/8 mile, it's reported.

Under what visibility conditions can fog (FG) be reported in a METAR?

• A. Less than 5/8 miles
• B. Less than 1 mile
• C. Only during rain
• D. Any visibility condition

Answer: (A)

The visibility conditions under which fog can be reported in a METAR are defined as when the visibility is less than 1 statute mile. Fog occurs when the water vapor in the air condenses into tiny water droplets, leading to reduced visibility. Typically, a visibility of less than 1 mile is classified as fog in aviation meteorology. In contrast, the other options do not correctly reflect the criteria for reporting fog. For instance, while visibility less than 5/8 miles may indicate dense fog, the more accurate threshold for reporting the presence of fog is less than 1 mile. Furthermore, fog can occur at any time, and is not solely dependent on precipitation conditions, which is why the choice of being only during rain is inaccurate. Thus, the selection that defines fog conditions by visibility being less than 1 statute mile is the correct one.

Fog has a way of leveling the sky, turning landmarks into silhouettes, and making flight planning feel a bit like navigating through a mystery. For pilots, air traffic controllers, and weather enthusiasts, understanding how fog is shown in METARs isn’t just trivia—it’s a practical skill that keeps people safe and flights on track. So let’s hone in on a specific question: when can fog, abbreviated FG, be reported in a METAR? What’s the threshold that triggers the FG tag?

Here’s the short, useful answer: fog is reported when the prevailing visibility is less than 1 mile. In other words, if the visibility in the METAR drops to under one statute mile, you’ll typically see FG in the weather section. Simple, right? But as with many weather topics, there are little nuances and a bit of confusion that can creep in, especially when textbooks throw around numbers like 5/8 mile (0.625 mile). Let me explain how it all fits together and why the one-mile rule matters day in and day out.

FG in the METAR: what it means in plain language

• FG stands for fog. It’s the phrase you’ll see in the METAR after the visibility portion, signaling that the air near the surface has enough water vapor to form tiny droplets. These droplets scatter light and reduce visibility, which changes the way you plan routes, approaches, and landings.

• The key cue is the visibility value. If the reported prevailing visibility is less than 1 mile, many observers will note FG as the weather phenomenon causing that reduced visibility. That “less than 1 mile” benchmark is the anchor you’ll hear in training, in NOAA materials, and in many flight operations contexts.

• You’ll often see FG paired with low ceilings as well. Fog tends to come with cloud bases very close to the surface, which means you’re looking at IFR conditions (or even worse, LIFR) when visibility is also restricted. That combo—fog plus low ceilings—shows up in METARs as FG along with cloud layers, giving you a fuller picture of what the sky and the runway look like.

A quick reality check: what about 5/8 mile?

• It’s true that some references and quizzes mention a threshold like 5/8 mile (0.625 mile) for dense fog and related descriptors. That number sits under 1 mile, so fog is still possible, but it’s not the official cutoff that starts the FG tag in standard METAR practice.

• In the real world, the METAR code uses the prevailing visibility value to decide the presence of FG. If that value drops below 1 mile, FG is typically in play. If you ever encounter a source saying “less than 5/8 mile,” treat it as a mnemonic or a specific context for a particular training scenario, not as the formal METAR rule.

• The important takeaway is: less than 1 mile is the reliable, widely used threshold for FG in aviation weather reporting. When you’re decoding METARs, that’s the line to memorize first.

Why fog forms and how it shows up in METARs

• Fog is basically water vapor that condenses into tiny droplets right at the surface. You’ve got to have enough moisture and a cooling air mass that’s near the dew point. If the air is cold and humid, you’re primed for fog; if it’s warm and dry, fog tends to stay away.

• In aviation terms, fog is a surface-based phenomenon. It makes the immediate environment hazy and reduces the distance a pilot can see. Because it affects landings, takeoffs, and taxiing, METARs use FG to warn crews to expect reduced visibility and adjust flight operations accordingly.

• Fog doesn’t rely on rain alone. It can form in calm, moist nights, over water, in valleys, or in urban areas where cool air settles in low-lying pockets. That’s why you’ll see fog reported in many different meteorological setups, not just during or after precipitation.

A peek under the hood: reading a METAR chunk that includes FG

If you’ve opened a METAR, you’ll see a string of elements. Here’s a simplified example to illustrate where FG fits:

• METAR KXYZ 051452 17012KT 3/4SM FG OVC005 09/07 A2992

Let’s break it down:

• KXYZ 051452: station and time

• 17012KT: wind coming from 170 degrees at 12 knots

• 3/4SM: visibility is 0.75 statute miles

• FG: fog is present

• OVC005: overcast cloud deck at 500 feet

• 09/07: temperature and dew point

• A2992: altimeter setting

In this example, the visibility is less than 1 mile, so FG is the logical tag describing what’s happening with the weather near the surface. If the numbers were something like 4SM with FG, that would be a mismatch for most METAR conventions, because FG implies fog and typically correlates with sub-1-mile visibility. That’s why pilots learn to cross-check the visibility value and the FG tag together for a coherent read.

Why this matters in practice

• Flight planning: If FG is reported, expect instrument flight rules (IFR) conditions at the least. Takeoff minimums, approach minima, and ground handling all shift with reduced visibility. You’ll want updated weather briefs before every leg of a flight and a clear plan for alternates.

• Ground operations: Ground vehicles, ramp operations, and airport personnel also adjust to FG. Ramps can become slick, taxiways less visible, and surface markings harder to discern. Communication between pilots and ground crews becomes extra important.

• Training and safety culture: Knowing that FG ties to a sub-1-mile visibility threshold helps crews anticipate the kinds of decisions they’ll face—from holding patterns to late braking on a landing attempt.

Where to verify and how to practice decoding

• Official sources: The National Weather Service (NOAA) and the Aviation Weather Center (aviationweather.gov) publish METAR formats, glossary terms, and example METARs. They’re the best places to confirm what FG means in the current reporting system and to see how it’s paired with visibility and ceiling values.

• Decoding tools: Many online METAR decoders let you paste a METAR line and see a plain-language interpretation. They’re handy for quick checks when you’re learning.

• Real-world context: Look at multiple METARs from different airports. You’ll notice that FG can occur in coastal areas, inland valleys, and mountain towns—each with its own flavor of how fog forms and sticks around.

A few practical tips to keep in mind

• Always compare visibility and any FG tag. If the visibility is reported as less than 1 mile, FG is likely present. If you see FG with a higher visibility value, double-check the time and the latest observation—weather can be a moving target.

• Watch for related indicators. Fog frequently accompanies low ceilings (cloud bases near the ground). If you see FG with BKN or OVC ceilings at 1000 feet or lower, you’re dealing with a tight IFR scenario, which has real implications for takeoff and landing decisions.

• Don’t rely on a single data point. Weather is a moving target. In the cockpit, you’ll combine METARs with TAFs, satellite data, and radar trends to build a reliable picture of what’s ahead.

A small digression that helps the intuition

Fog feels like a natural, almost mundane thing—until you need to fly through it. Think of fog as a curtain that lowers itself over the landscape. The curtain isn’t the same everywhere; it can thicken or thin within hours. That’s part of why aviation weather reporting is so dynamic. The instruments and the human observers work together to describe a shifting scene: the FG tag tells you “the curtain is down,” the visibility number tells you “how thick the curtain is,” and the ceilings tell you “how tall the stage is beneath it.”

Wrapping it up

To answer the question head-on: fog (FG) is reported in a METAR when the prevailing visibility is less than 1 statute mile. While you’ll encounter discussions of specific sub-threshold values like 5/8 mile in some study materials or mnemonics, the standard threshold used in aviation weather reporting centers on the one-mile mark. Fog forms when water vapor condenses near the surface, and its presence in a METAR signals pilots and crews to anticipate reduced visibility and potentially IFR conditions.

If you’re curious to deepen your understanding, start by checking a few METARs from airports you know. Notice how FG shows up alongside the visibility number and the ceiling line. Then explore a couple of glossaries from NOAA or FAA to see how the language around fog adjectives—like “dense fog” or “shallow fog”—is used in practice. The more you read, the more these weather snippets begin to feel like a real-time weather map in motion, guiding decisions rather than just telling you what the sky did yesterday.

And yes, fog is a natural part of weather. It’s also a reminder that the sky isn’t just about big storms and dramatic fronts. Sometimes, it’s the quiet, low-lying mist that quietly shapes the way we move through the air—and the way we read the words in a METAR.