AIM Text
- Light. Visibility more than 1/2 statute mile.
- Moderate. Visibility from more than 1/4 statute mile to 1/2 statute mile.
- Heavy. Visibility 1/4 statute mile or less.
Source: FAA Aeronautical Information Manual · current edition · paragraph 7-1-17.
The short answer
AIM 7-1-17 is FAA advisory guidance for estimating the intensity of snow or drizzle from visibility: light is visibility more than 1/2 statute mile, moderate is more than 1/4 up to 1/2 statute mile, and heavy is 1/4 statute mile or less. It standardizes how observers describe frozen and liquid precipitation intensity.
Research Notes
Common Questions
What visibility makes snow or drizzle “heavy”?
Per the AIM, snow or drizzle is reported as heavy when visibility is 1/4 statute mile or less. Moderate covers visibility from more than 1/4 statute mile up to 1/2 statute mile, and light is any visibility more than 1/2 statute mile.
How is the intensity of snow estimated without special equipment?
The AIM bases snow or drizzle intensity on prevailing visibility alone: more than 1/2 statute mile is light, more than 1/4 to 1/2 statute mile is moderate, and 1/4 statute mile or less is heavy.
Does this guidance use statute or nautical miles?
The AIM expresses all snow and drizzle intensity thresholds in statute miles — more than 1/2, more than 1/4 to 1/2, and 1/4 statute mile or less — consistent with how surface visibility is reported.