AIM Text
- Rain
- Light. From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.
- Moderate. Individual drops are not clearly identifiable; spray is observable just above pavements and other hard surfaces.
- Heavy. Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to height of several inches is observed over hard surfaces.
- Ice Pellets
- Light. Scattered pellets that do not completely cover an exposed surface regardless of duration. Visibility is not affected.
- Moderate. Slow accumulation on ground. Visibility reduced by ice pellets to less than 7 statute miles.
- Heavy. Rapid accumulation on ground. Visibility reduced by ice pellets to less than 3 statute miles.
Source: FAA Aeronautical Information Manual · current edition · paragraph 7-1-16.
Research Notes
AIM 7-1-16 covers Inflight Weather Advisories — SIGMETs, AIRMETs, Convective SIGMETs, and Center Weather Advisories.
SIGMET (Significant Meteorological Information): Weather hazardous to all aircraft. Categories:
- Severe icing (not associated with thunderstorms)
- Severe or extreme turbulence (not associated with thunderstorms)
- Dust storms or sandstorms reducing visibility to less than 3 SM
- Volcanic ash
Convective SIGMET (WST): Issued for thunderstorm-related hazards:
- Severe thunderstorms (winds 50 kts+, hail ¾ inch+, tornadoes)
- Embedded thunderstorms (in cloud)
- A line of thunderstorms
- Thunderstorms producing precipitation that affects flight
AIRMET (Airman's Meteorological Information): Hazards to small aircraft and VFR operations:
- AIRMET Tango: Moderate turbulence, surface winds 30 kts+
- AIRMET Sierra: Mountain obscuration, IFR conditions
- AIRMET Zulu: Moderate icing, freezing levels
Center Weather Advisory (CWA): Issued by Center Weather Service Unit for conditions developing within a Center's airspace, valid for up to 2 hours.
Reference: AIM 7-1-16; AC 00-45; AC 00-24 (Thunderstorms).