What do the minus sign, plus sign, and no prefix mean in a METAR present-weather group?

A minus sign (−) before a weather code means light intensity, a plus sign (+) means heavy intensity, and no prefix means moderate intensity. These intensity qualifiers only apply to precipitation codes such as RA (rain) or SN (snow); they are never used with obstructions to vision like fog (FG) or haze (HZ).

What is the difference between FG, BR, and HZ in a METAR?

All three indicate reduced visibility, but the specific code depends on visibility distance and the temperature–dewpoint spread. FG (fog) is reported when visibility is less than 5/8 statute mile. For visibilities from 5/8 sm to less than 7 sm, ASOS reports BR (mist) if the temperature–dewpoint spread is 4 °F or less, and HZ (haze) if the spread is greater than 4 °F. FZFG (freezing fog) is used when fog occurs with a temperature below 0 °C.

What does UP mean in a METAR and why does it appear?

UP stands for unknown precipitation and is generated by automated ASOS stations when sensors detect precipitation falling but cannot determine its type. It often appears during transitional weather when rain may be mixed with snow or ice pellets. When a certified human weather observer augments the report, UP is replaced with the correct precipitation code.

METAR Present-Weather Codes: Intensity, Descriptor, and Phenomena

Every METAR contains a present-weather group that tells a pilot, at a glance, exactly what kind of weather is occurring at or near the reporting station. Rather than writing out sentences, the code packs intensity, a qualifying descriptor, and the phenomenon itself into a compact string such as +TSRA (heavy thunderstorm with rain) or -FZFG (light freezing fog). Understanding how those three building blocks fit together—and how automated systems like ASOS and AWOS generate them—is essential for accurate preflight weather decoding.

The present-weather group appears in the body of a METAR between the visibility element and the sky-condition element. It may be omitted entirely when conditions are clear, but when any qualifying weather is occurring, the code follows a strict left-to-right order: intensity modifier → descriptor → phenomena. Multiple simultaneous weather types can appear in a single group or as adjacent groups separated by a space.

The Three-Part Structure Explained

1. Intensity Prefix

Intensity is indicated by a single character placed immediately before the rest of the group. A minus sign (−) means light, a plus sign (+) means heavy, and no prefix at all means moderate. Intensity qualifiers apply only to precipitation; they are never used with purely non-precipitation phenomena such as mist (BR) or haze (HZ). For example, RA alone means moderate rain, −RA means light rain, and +RA means heavy rain.

2. Descriptor

The descriptor is a two-letter qualifier that further characterizes the precipitation or phenomenon. The descriptors used in U.S. METARs include:

MI — Shallow (e.g., shallow fog, MIFG)
BC — Patches (e.g., patchy fog, BCFG)
PR — Partial (covering part of the airport, PRFG)
DR — Low Drifting (e.g., drifting snow, DRSA or DRSN)
BL — Blowing (e.g., blowing snow, BLSN; blowing dust, BLDU)
SH — Showers (e.g., rain showers, SHRA)
TS — Thunderstorm (e.g., thunderstorm with rain, TSRA)
FZ — Freezing (e.g., freezing rain, FZRA; freezing fog, FZFG)

Not every descriptor pairs with every phenomenon; the combination must describe a meteorologically possible condition. TS is one of the most operationally significant descriptors—it can appear alone as TS to indicate a thunderstorm without precipitation at the station, or it can prefix a precipitation type such as TSRA when rain accompanies the storm.

3. Phenomena

Phenomena codes describe the actual weather event and fall into three broad categories:

Precipitation: RA (rain), SN (snow), DZ (drizzle), GR (hail, diameter ≥ 1/4 in), GS (small hail or snow pellets), IC (ice crystals), PL (ice pellets), SG (snow grains), UP (unknown precipitation — used by automated stations when the sensor detects precipitation but cannot determine the type)
Obscurations / Obstructions to Vision: FG (fog), BR (mist), HZ (haze), FU (smoke), DU (widespread dust), SA (sand), VA (volcanic ash), PY (spray)
Other: SQ (squall), PO (dust/sand whirls), DS (duststorm), SS (sandstorm), FC (funnel cloud/tornado/waterspout — a plus sign, +FC, indicates a tornado or waterspout, meaning the funnel cloud has reached the surface)

How ASOS and AWOS Generate Present-Weather Codes

The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 24, explains that the Automated Surface Observing System (ASOS)—a joint program of the NWS, FAA, and DOD—serves as the nation's primary surface weather observing network. ASOS automatically observes, formats, and transmits METARs and SPECIs. Its present-weather capability covers rain (RA), snow (SN), freezing rain (FZRA), and unknown precipitation (UP) for precipitation type and intensity, plus thunderstorms on site (TS) and in the vicinity (VCTS), and the key obstructions fog (FG), mist (BR), and haze (HZ).

ASOS does not automatically report every possible present-weather code. Types like drizzle (DZ), ice pellets (PL), or blowing snow (BLSN) typically require a human augmenter—a certified weather observer—to add them to the report. When a human augments an automated observation, the report will show the station identifier without the AUTO tag; AUTO appears only in fully unaugmented reports.

For obstructions to vision, ASOS applies specific thresholds. FG is reported when visibility falls below 5/8 statute mile. FZFG (freezing fog) is reported when temperature is below 0 °C and visibility is below 5/8 sm. For visibilities between 5/8 sm and less than 7 sm, the station reports either BR (mist) or HZ (haze) depending on the temperature–dewpoint spread: if the spread is 4 °F (approximately 2 °C) or less, BR is used; a larger spread results in HZ.

The Automated Weather Observing System (AWOS) is similar to ASOS but generally reports fewer elements. Present-weather capability varies by system type. AWOS-3P adds a precipitation type and intensity sensor; AWOS-3PT adds a thunderstorm/lightning sensor on top of that; and AWOS-4 further includes freezing rain detection. Basic AWOS-1 and AWOS-2 units report no present-weather phenomena codes at all. AWOS generates a METAR every 20 minutes and does not issue SPECIs.

Both ASOS and AWOS produce One-Minute Observations (OMOs) available by radio and telephone. However, OMOs are time-averaged: sky condition uses a 30-minute rolling average, while visibility and present weather use a 10-minute average. The algorithms are intentionally asymmetric—deteriorating conditions are reflected much faster than improving conditions, meaning a station may continue to report fog for up to 10 minutes after it has physically dissipated.

Vicinity Weather: VC Qualifier

When a phenomenon is occurring within 5 to 10 statute miles of the airport but not at the airport itself, the qualifier VC (vicinity) is prepended. The most common examples are VCTS (thunderstorm in the vicinity) and VCFG (fog in the vicinity). ASOS can automatically generate VCTS; other VC codes generally require human augmentation.

Key Numbers and Rules

FG threshold: Visibility less than 5/8 statute mile — fog (FG) is reported.
BR vs. HZ: Visibility 5/8 sm to less than 7 sm; temperature–dewpoint spread ≤ 4 °F → BR; spread > 4 °F → HZ.
FZFG: Fog with temperature below 0 °C (32 °F).
+FC: Plus sign before FC specifically indicates a tornado or waterspout — a funnel cloud that has reached the surface; FC alone indicates a funnel cloud not reaching the surface.
+GR: Hail (GR) with diameter ≥ 1/4 inch. Smaller hail or snow pellets use GS.
TS alone: Thunderstorm at the station with no precipitation reaching the ground.
TSRA: Thunderstorm with rain; the TS descriptor always precedes the precipitation code.
UP: Unknown precipitation — used by automated stations when the sensor detects precipitation but cannot classify it; a human augmenter will replace UP with the correct code when possible.
AWOS-3 tiers: AWOS-3 adds cloud height and precipitation accumulation; AWOS-3P adds present weather; AWOS-3PT adds thunderstorm/lightning; AWOS-4 adds freezing rain.
ASOS SPECI threshold example: A special report is automatically triggered when visibility decreases to less than 3 statute miles.

Common Test Traps

Confusing BR and FG: Many students assume any low-visibility condition is fog. BR (mist) is specifically for visibilities between 5/8 sm and 7 sm with a tight temperature–dewpoint spread; FG requires visibility below 5/8 sm.
Intensity applies only to precipitation: You will never see −FG or +HZ. Intensity prefixes (− and +) are only valid with precipitation phenomena codes.
TS vs. TSRA: TS alone means a thunderstorm is occurring but no precipitation is reaching the surface at the station. TSRA means there is both a thunderstorm and rain at the station.
VCTS is not the same as TS: VCTS means the thunderstorm is 5–10 sm away, not overhead. Flight operations under VFR may still be legal, but the hazard is real and nearby.
AUTO limitations: ASOS/AWOS AUTO reports do not cover the full range of present-weather codes. UP appearing in a METAR is a red flag that an automated sensor detected precipitation of an unknown type — treat it with caution, especially in icing-prone conditions where FZRA might be undetected.