7.3 In-Flight Weather Advisories: AIRMETs, SIGMETs & PIREPs

In-Flight Weather Advisories: AIRMETs, SIGMETs & PIREPs

Quick Answer: AIRMETs (WA) are issued every 6 hours (valid for 6 hours) for weather hazardous to light aircraft: Sierra (IFR / mountain obscuration), Tango (moderate turbulence, winds $\ge 30\text{ kt}$, LLWS), and Zulu (moderate icing, freezing levels). SIGMETs (WS) cover non-convective hazards to all aircraft (severe icing, severe/extreme turbulence, duststorms $< 3\text{ SM}$, volcanic ash) and are valid for 4 hours. Convective SIGMETs (WST) are issued hourly at H+55 (valid for 2 hours) for severe thunderstorms (winds $\ge 50\text{ kt}$, hail $\ge 3/4\text{ inch}$, tornadoes), embedded storms, or squall lines $\ge 60\text{ NM}$ affecting $\ge 40%$ of their length. PIREPs (UA/UUA) provide in-flight ground truth.

Preflight weather briefings and en route updates rely on a layered system of in-flight weather advisories. These advisories are categorized according to the severity of the hazard and the category of aircraft affected.

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The In-Flight Weather Advisory Hierarchy

+---------------------------------------------------------------------------------------------------------+
|                                 IN-FLIGHT ADVISORY COMPARISON MATRIX                                    |
|                                                                                                         |
|  Advisory Type          Issuance / Validity   Target Aircraft        Covered Meteorological Hazards     |
|  ----------------------------------------------------------------------------------------------------- |
|  AIRMET (WA)            Every 6 hours         Light / Single-Engine  Moderate Icing, Moderate Turb,     |
|  (Sierra, Tango, Zulu)  Valid 6 hours         Aircraft (GA)          IFR (>50% area), Winds >= 30 kt    |
|                                                                                                         |
|  SIGMET (WS)            Unscheduled           ALL Aircraft           Severe Icing, Severe Turb / CAT,   |
|  (Non-Convective)       Valid 4 hours         (Airliners & GA)       Duststorms < 3 SM, Volcanic Ash    |
|                                                                                                         |
|  Convective SIGMET      Hourly at H+55        ALL Aircraft           Severe TS (Winds>=50kt, Hail>=3/4")|
|  (WST)                  Valid 2 hours         (Airliners & GA)       Embedded TS, Squall Lines >= 60 NM |
+---------------------------------------------------------------------------------------------------------+

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AIRMETs (WA / Graphical G-AIRMET)

An AIRMET (Airmen's Meteorological Information, bulletin identifier WA) advises of weather phenomena that are operational hazards to light aircraft and pilots with limited instrument proficiency, but which are generally manageable by larger commercial transport category aircraft.

• Issuance: Routinely issued every 6 hours with unscheduled amendments as required. Graphical G-AIRMETs are updated every 3 hours.
• Validity: Valid for a maximum of 6 hours from the time of issuance.

The Three AIRMET Classifications

+---------------------------------------------------------------------------------------------+
|                              AIRMET CLASSIFICATION CRITERIA                                 |
|                                                                                             |
|  1. AIRMET SIERRA (IFR & Mountain Obscuration):                                             |
|     - Widespread IFR conditions: Ceilings < 1,000 ft AGL and/or visibility < 3 SM           |
|       affecting more than 50% of the defined geographic area at any one time.               |
|     - Extensive mountain obscuration across ridgelines and mountain passes.                 |
|                                                                                             |
|  2. AIRMET TANGO (Turbulence & Surface Winds):                                              |
|     - Moderate turbulence (non-convective).                                                 |
|     - Sustained surface winds of 30 KNOTS OR GREATER.                                       |
|     - Non-convective Low-Level Wind Shear (LLWS) below 2,000 feet AGL.                     |
|                                                                                             |
|  3. AIRMET ZULU (Icing & Freezing Levels):                                                  |
|     - Moderate structural icing (non-convective).                                           |
|     - Freezing level heights and multiple freezing level contours across the region.        |
+---------------------------------------------------------------------------------------------+

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SIGMETs (WS — Non-Convective)

A SIGMET (Significant Meteorological Information, bulletin identifier WS) advises of non-convective weather phenomena that are hazardous to ALL aircraft, including transport category jets and heavy airliners.

• Issuance: Unscheduled, issued as meteorological events develop.
• Validity: Valid for 4 hours (SIGMETs associated with hurricanes/tropical cyclones or volcanic ash clouds are valid for 6 hours).
• Hazard Criteria:
  1. Severe Icing not associated with thunderstorms.
  2. Severe or Extreme Turbulence or Clear Air Turbulence (CAT) not associated with thunderstorms.
  3. Widespread Duststorms or Sandstorms lowering surface or flight visibility to less than 3 statute miles.
  4. Volcanic Ash Clouds from active eruptions (posing catastrophic engine failure and windscreen pitting risks).

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Convective SIGMETs (WST)

A Convective SIGMET (WST) is issued for convective weather phenomena presenting severe hazards to ALL aircraft.

• Issuance Schedule: Issued hourly at 55 minutes past each hour (H+55). If no convective activity meeting criteria is occurring or anticipated, a negative bulletin is transmitted ("CONVECTIVE SIGMET...NONE").
• Validity: Valid for 2 hours from the time of issuance.

+---------------------------------------------------------------------------------------------+
|                         CONVECTIVE SIGMET (WST) ISSUANCE CRITERIA                           |
|                                                                                             |
|  1. SEVERE THUNDERSTORMS occurring or forecast, evidenced by:                               |
|     - Surface winds >= 50 KNOTS.                                                            |
|     - Hail at the surface >= 3/4 INCH in diameter.                                          |
|     - Tornadoes, funnel clouds, or waterspouts.                                             |
|                                                                                             |
|  2. EMBEDDED THUNDERSTORMS:                                                                 |
|     - Thunderstorms obscured by haze, fog, or stratiform cloud decks (invisible to eye).    |
|                                                                                             |
|  3. LINE OF THUNDERSTORMS (Squall Line):                                                    |
|     - A line of convective activity at least 60 MILES LONG with thunderstorms               |
|       affecting at least 40% OF ITS LENGTH.                                                 |
|                                                                                             |
|  4. THUNDERSTORM CLUSTERS / EXPANSIVE CONVECTION:                                            |
|     - An area of active thunderstorms affecting AT LEAST 40% OF AN AREA covering            |
|       a minimum of 3,000 SQUARE MILES.                                                      |
+---------------------------------------------------------------------------------------------+

Universal Inherent Hazards: Every Convective SIGMET automatically implies severe or greater turbulence, severe structural icing, and low-level wind shear, even if these individual words do not appear in the advisory text.

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Pilot Weather Reports (PIREPs: UA & UUA)

PIREPs provide actual real-time "ground truth" weather observations reported by pilots in flight. They are the only observational source for in-flight cloud bases/tops, icing intensity, and turbulence aloft.

Report Classification

• UA (Routine PIREP): Standard in-flight report of normal weather conditions, cloud layers, or light turbulence/icing.
• UUA (Urgent PIREP): Issued immediately for critical hazards, including:
  1. Tornadoes, funnel clouds, or waterspouts.
  2. Severe or Extreme Turbulence (including CAT).
  3. Severe Icing.
  4. Hail.
  5. Low-Level Wind Shear (LLWS within 2,000 ft of surface with airspeed loss/gain $\ge 10\text{ kt}$).
  6. Volcanic ash clouds.

PIREP Header Decoding Table