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.
The short answer
AIM 7-1-16 is FAA advisory guidance for estimating the intensity of rain and ice pellets. Rain intensity (light, moderate, heavy) is judged by how drops wet surfaces and form spray, while ice pellet intensity is judged by ground accumulation and the visibility reduction it causes, with heavy ice pellets cutting visibility to less than 3 statute miles.
Research Notes
Common Questions
How is heavy rain described in the AIM?
Per the AIM, heavy rain seemingly falls in sheets, individual drops are not identifiable, and heavy spray to a height of several inches is observed over hard surfaces. Moderate rain is when individual drops are not clearly identifiable and spray is observable just above pavements.
What visibility corresponds to moderate versus heavy ice pellets?
The AIM states moderate ice pellets cause slow accumulation on the ground with visibility reduced to less than 7 statute miles, while heavy ice pellets cause rapid accumulation with visibility reduced to less than 3 statute miles.
What counts as light rain?
According to the AIM, light rain is scattered drops that, regardless of duration, do not completely wet an exposed surface, up to a condition where individual drops are easily seen.