diff --git a/src/CSET/operators/ensembles.py b/src/CSET/operators/ensembles.py
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+# © Crown copyright, Met Office (2022-2024) and CSET contributors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+Age of air operator.
+
+The age of air diagnostic provides a qualtitative view of how old air is within
+the domain, by calculating a back trajectory at each grid point at each lead time
+to determine when air entered through the lateral boundary. This is useful for
+diagnosing how quickly air ventilates the domain, depending on its size and the
+prevailing meteorology.
+
+The diagnostic uses the u, v and w components of wind, along with geopotential height to
+perform the back trajectory. Data is first regridded to 0.5 degrees.
+
+Note: the code here does not consider sub-grid transport, and only uses the postprocessed
+velocity fields and geopotential height. Its applicability is for large-scale flow O(1000 km),
+and not small scale flow where mixing is likely to play a larger role.
+"""