What is the difference between AO1 and AO2 in a METAR?

AO2 stations have a precipitation discriminator that can distinguish between rain, snow, and freezing rain. AO1 stations detect that precipitation is falling but cannot identify the type - a critical limitation when temperatures are near freezing.

Why does the METAR body show calm winds when the remarks show a 47-knot peak wind?

The standard body reports conditions at the moment of the observation. PK WND in the remarks captures the maximum gust recorded during the previous hour, so the field could have experienced severe conditions that have since calmed.

How do I decode the T group temperature in the remarks?

The T group is the letter T followed by eight digits - four for temperature, four for dewpoint, each in tenths of a degree Celsius. The first digit of each four-digit group is the sign: 0 for positive, 1 for negative. T02310211 = +2.3°C temperature and +2.1°C dewpoint.

What should I do if I see TSNO in the remarks?

TSNO means the lightning detection sensor is not operational. Do not treat the absence of TS in the present weather as confirmation that no convective activity exists nearby - the station simply cannot detect it. Seek current PIREPs or other nearby METARs to assess the convective picture.

How is PRESFR different from a normal low altimeter setting?

PRESFR indicates the pressure is actively falling rapidly - defined as a drop of at least 2.03 inches of mercury in under one hour. A low altimeter setting is a static snapshot; PRESFR is a trend code that tells you conditions are likely deteriorating fast and you should evaluate the broader weather picture before proceeding.

The METAR Remarks Section, the AO2 Precipitation Discriminator, and the Six Codes Every Student Pilot Skips That Carry the Most Useful Information in the Entire Observation

Most student pilots can work through the standard METAR body - wind, visibility, sky condition, temperature, altimeter - but stop cold at the RMK section. That section contains information the standard body cannot convey: how reliable the observation is, what changed in the past hour, and what weather threats are nearby. Skipping it means skipping some of the most actionable data in the entire preflight chain.

What Is the METAR Remarks Section?

A METAR (Aviation Routine Weather Report) is issued every hour at most certified observation stations, and more frequently when conditions change rapidly. The standard body captures a snapshot of current conditions at the moment of observation. The RMK section extends that snapshot - reporting on sensor capability, recent trends, and nearby hazards that fall outside the standard format’s scope.

Think of the body as what conditions are right now. Think of the remarks as what has been happening and what the observation itself is capable of telling you.

What Do AO1 and AO2 Mean?

AO1 and AO2 identify the type of automated observing station that produced the report. This is almost always the first code after RMK, and it directly affects how much you can trust what follows.

An AO2 station has a precipitation discriminator - a sensor that distinguishes between liquid and frozen precipitation. It can differentiate rain, snow, and freezing rain. An AO1 station knows precipitation is falling but cannot identify what kind.

That distinction is not a technicality. Flying into a small airport in late October with temperatures near freezing and light precipitation in the present weather, the difference between rain and freezing rain is the entire decision. If that METAR came from an AO1 station, the observation simply cannot make that call. Smaller, rural, and backcountry airports are more likely to be AO1 only. Check this code before trusting any precipitation report.

How Do I Read the Peak Wind Code (PK WND)?

The standard METAR wind reports sustained wind and current gusts at observation time. It does not capture how strong gusts peaked over the previous hour. PK WND fills that gap.

The format is: PK WND [direction][speed]/[time]. For example, PK WND 27048/1430 means the peak wind was from 270 degrees (due west) at 48 knots, recorded at 1430 Zulu.

Here is the practical scenario: current winds show 12 gusting 18 knots - manageable. But the remarks show PK WND with a 47-knot gust 25 minutes ago. Something moved through. It may have been thunderstorm outflow, a squall line, or a mountain rotor. Conditions have calmed, but the atmosphere at that location was severe recently and is not necessarily stable. The PK WND code remains in the remarks for the full hour, so it captures the worst of what happened even if the field looks benign by the time you read it.

What Does a Wind Shift (WSHFT) Mean?

WSHFT followed by a four-digit Zulu time means the wind direction shifted at that station. A wind shift at a surface observation often means a frontal passage or convective outflow has moved through the area.

At a towered airport, a shift triggers updated ATIS and likely a runway change. At a nontowered field, pilots may still be using the previously favored runway because nobody has updated the advisory frequency. Seeing WSHFT tells you to verify what traffic is actually doing before entering the pattern - don’t assume the calm-wind runway from an hour ago is still the active choice.

Wind shifts are also among the earliest observable surface signatures of approaching convective outflow. If WSHFT appears alongside any convective activity in the area, treat it as a data point worth investigating further.

How Do I Use the Sea Level Pressure (SLP) Code?

The altimeter setting in the METAR body is given in inches of mercury for your Kollsman window. SLP gives the same pressure in hectopascals, encoded in a compressed three-digit form.

Decoding the value: if the first digit is 9, prepend 9 to get a value below 1000 hPa. If the first digit is 0 or higher leading to a value of 1000+, prepend 10. So SLP137 = 1013.7 hPa; SLP982 = 998.2 hPa.

For cross-country planning, the absolute value matters less than the trend. Falling pressure indicates deteriorating conditions on the way. Rising pressure indicates improvement. The faster the change, the more significant the implication.

What Do PRESRR and PRESFR Mean?

These two codes take the SLP trend further by flagging dramatic shifts:

PRESRR = pressure rising rapidly
PRESFR = pressure falling rapidly

The FAA defines “rapidly” as a change of at least 2.03 inches of mercury (approximately 6.8 hectopascals) in under one hour. That is a significant atmospheric event.

PRESFR at your destination should stop your briefing. Look at what frontal system is approaching, review the prog charts, and check whether a low is tracking across your route. PRESRR following a front is a useful positive signal - if you are deciding whether to wait out a system, rapid pressure rise at your destination suggests conditions may improve sooner than expected.

How Do I Decode Lightning Codes in the Remarks?

The standard METAR body flags a thunderstorm with TS in the present weather group - but that only covers a five-mile radius from the station. Lightning is visible at much greater distances, and the remarks section captures it.

Lightning codes combine a frequency descriptor and a type descriptor:

Frequency:

OCNL - occasional (fewer than 1 flash per minute)
FRQ - frequent (more than 1 flash per minute)
CONS - continuous (more than 6 flashes per minute)

Type:

IC - in-cloud
CC - cloud to cloud
CG - cloud to ground
CA - cloud to air

So FRQ LTGCG means frequent cloud-to-ground lightning. That is specific, actionable information about a convective cell that may not yet be within five miles of the station. If you are watching a developing convective situation, check the lightning codes in the remarks before the body of the report even shows TS. They can give you early warning of an approaching cell.

What Is the T Group Temperature Code?

The standard METAR rounds temperature and dewpoint to the nearest whole degree Celsius. The remarks T group provides values to the nearest tenth - useful for icing analysis and density altitude calculations.

The format is the letter T followed by eight digits: four for temperature, four for dewpoint. The first digit of each four-digit group indicates sign: 0 = positive, 1 = negative. So T02310211 reads as +2.3°C temperature and +2.1°C dewpoint.

Notice what that example shows: a temperature-dewpoint spread of 0.2°C. Relative humidity is essentially 100 percent. Add any upslope flow, mechanical turbulence, or rising air along the route and you will immediately encounter cloud or freezing fog.

For icing specifically, the 0°C to -10°C band contains the most active supercooled liquid water and produces the most aggressive structural icing rates. Knowing you are at +2.3°C rather than +3°C tells you exactly how close you are sitting to that threshold.

What Do VIRGA, TSNO, FZRANO, and PWINO Mean?

These four codes each carry a distinct operational implication:

VIRGA - precipitation is falling from clouds but evaporating before reaching the surface. Visually, you see rain curtains hanging below the cloud base that never hit the ground. In dry, high-elevation environments - desert and mountain airports especially - virga is a documented microburst precursor. The evaporating precipitation cools the air column beneath the cloud, which can generate a dense, rapidly descending air mass. Virga does not always produce a clear radar return because the precipitation never reaches the scan level.

TSNO - the lightning detection sensor at the observing site is not installed or not functional. The absence of TS in the present weather does not mean there is no convective activity nearby. It means the station cannot detect it. If you see TSNO and there is any possibility of convection in the area, you need another source.

FZRANO - the freezing rain sensor is inoperative. If you are flying in conditions where freezing rain is possible and this code appears, verify through a current PIREP, an ASOS direct phone readout, or ground frequency reports before departing.

PWINO - the precipitation identifier is broadly inoperative. Same principle: if conditions could produce anything unusual and this code is present, seek confirmation before launching.

How Should I Use the Remarks Section in a Preflight Briefing?

Read every METAR in two passes.

First pass - standard body: wind, visibility, present weather, sky condition, temperature, dewpoint, altimeter. You already know how to do this.

Second pass - remarks section. Answer three questions:

How reliable is this data? Check AO1 vs. AO2. Look for any inoperative sensor codes (TSNO, FZRANO, PWINO).
What has changed recently? Look at PK WND, WSHFT, and pressure tendency (SLP, PRESRR, PRESFR).
What threats are nearby that the body doesn’t fully capture? Look for lightning codes, VIRGA, and any convective indicators.

For checkride preparation: the Airman Certification Standards for the private pilot certificate explicitly require you to decode and apply weather information. A briefing that stops at the altimeter setting is an incomplete briefing. Examiners expect you to demonstrate the full picture.

The fastest way to build fluency with these codes is to practice without a flight. Pull a raw METAR from ForeFlight, SkyVector, or aviationweather.gov, find the RMK section, and spend 30 seconds decoding what you see. After a few weeks of that, the codes become automatic. The FAA’s advisory circular on aviation weather services is the definitive free reference for every code in the preflight chain, available at faa.gov.

Key Takeaways

AO1 stations cannot distinguish precipitation type - if you are flying in marginal icing conditions, that limitation changes how much you can trust the observation.
PK WND captures the worst gust of the past hour, not just current conditions - a calm current wind with a recent 47-knot peak tells a different story.
PRESFR means pressure is dropping fast (2.03 in Hg in under one hour) - stop your briefing and look at the broader picture.
Lightning codes in the remarks can warn of convective cells before they appear in the standard body, which only covers a five-mile radius.
TSNO, FZRANO, and PWINO mean the sensor is blind - absence of a phenomenon code does not mean the phenomenon is absent.
Use a three-question framework on every remarks section: reliability, recent changes, nearby threats.