diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -1,12 +1,42 @@
 # Advisory Circular
 
-This is a twitter bot that alerts you when any aircraft is circling.
+This is a twitter bot that alerts you when aircraft fly in circles.
 You can see it in action at
 [@SkyCirclesLA](https://twitter.com/SkyCirclesLA).
 
 ![Example tweet](Screen Shot 2019-12-13.png?raw=true "Example tweet")
 
 
+## Criteria for tweeting
+
+The bot will consider an aircraft to be "circling" if:
+
+1. In the past 25 minutes, the sum of the aircraft's bearing changes
+   is more than 1440 degrees. Port turns cancel out starboard turns
+   and vice versa--there needs to be a net change of at least 1440
+   degrees.
+
+2. The centroid of the last 3 minutes worth of positions must be more
+   than 2.5 miles away from all known airports.
+
+3. The centroid must be located within the county of Los Angeles.
+   (This is just because I don't yet have airport data outside Los
+   Angeles County.)
+
+4. The aircraft must, at the moment of decision, be at an altitude of
+   300 feet or more.
+
+5. If an aircraft was previously circling, it must be considered
+   not-circling for at least 20 minutes before it is again eligible to
+   be considered circling.
+
+There are a lot of weird flight paths that can meet criterion #1, and
+I kind of like it that way. For example, see
+https://twitter.com/lemonodor/status/1203033917483175936 (consider
+whether we also want to detect something like
+https://twitter.com/lemonodor/status/1204550831103590400).
+
+
 ## Prerequisites to running the bot
 
 **Set up a Pelias instance containing information for the geographical