diff --git a/lib/xmet/raob.py b/lib/xmet/raob.py
index fd21d1b..20f4759 100644
--- a/lib/xmet/raob.py
+++ b/lib/xmet/raob.py
@@ -319,6 +319,10 @@ class RAOBReader():
     A reader for the global `Current.rawins` file provided by UCAR:
 
         https://weather.rap.ucar.edu/data/upper/Current.rawins
+
+    The format is best documented here:
+
+        https://www.atmos.albany.edu/facstaff/ralazear/ATM211/Home_files/RAOB_Code_packet.pdf
     """
     def __init__(self, fh: io.TextIOBase):
         self.fh        = fh