diff --git a/lexicons/space.notjack.thunderstorm.catalogSnapshot.json b/lexicons/space.notjack.thunderstorm.catalogSnapshot.json
new file mode 100644
index 0000000..e30f0cb
--- /dev/null
+++ b/lexicons/space.notjack.thunderstorm.catalogSnapshot.json
@@ -0,0 +1,36 @@
+{
+  "lexicon": 1,
+  "id": "space.notjack.thunderstorm.catalogSnapshot",
+  "defs": {
+    "main": {
+      "type": "record",
+      "description": "An immutable snapshot of the Racket package catalog at one moment in time. Snapshots are currently always Racksnaps daily builds. Record keys are TIDs rather than a date-derived slug, so nothing here assumes at most one snapshot per day.",
+      "key": "tid",
+      "record": {
+        "type": "object",
+        "required": ["url", "publishedAt", "createdAt"],
+        "properties": {
+          "url": {
+            "type": "string",
+            "format": "uri",
+            "description": "Catalog root URL for this snapshot: the directory containing pkg/, pkgs, and pkgs-all. This identifies the snapshot, being unique per snapshot and, for Racksnaps, containing the build date."
+          },
+          "packageCount": {
+            "type": "integer",
+            "description": "Packages listed in the upstream catalog, which may exceed the number Thunderstorm has ingested. Optional because whatever creates this record need not have fetched the package list."
+          },
+          "publishedAt": {
+            "type": "string",
+            "format": "datetime",
+            "description": "When the upstream catalog finished building this snapshot. Distinct from createdAt so that a consumer sees when the snapshot really appeared rather than when Thunderstorm happened to poll for it. Must be UTC, written with a 'Z' suffix rather than a numeric offset."
+          },
+          "createdAt": {
+            "type": "string",
+            "format": "datetime",
+            "description": "When Thunderstorm created this record. Must be UTC, written with a 'Z' suffix rather than a numeric offset."
+          }
+        }
+      }
+    }
+  }
+}