Make cube_delta a lazy operation.

Author: trexfeathers

docs/src/common_links.inc

@@ -67,6 +67,7 @@
 .. _@HGWright: https://github.com/HGWright
 .. _@jamesp: https://github.com/jamesp
 .. _@jonseddon: https://github.com/jonseddon
+.. _@jrackham-mo: https://github.com/jrackham-mo
 .. _@jvegasbsc: https://github.com/jvegasbsc
 .. _@lbdreyer: https://github.com/lbdreyer
 .. _@marqh: https://github.com/marqh

docs/src/whatsnew/3.11.rst

@@ -192,8 +192,6 @@ v3.11.1 (19 Dec 2024)
     Whatsnew author names (@github name) in alphabetical order. Note that,
     core dev names are automatically included by the common_links.inc:
 
-.. _@jrackham-mo: https://github.com/jrackham-mo
-
 
 .. comment
     Whatsnew resources in alphabetical order:

docs/src/whatsnew/3.12.rst

@@ -288,7 +288,6 @@ v3.12.3 (22 Aug 2025)
 
 .. _@fnattino: https://github.com/fnattino
 .. _@jfrost-mo: https://github.com/jfrost-mo
-.. _@jrackham-mo: https://github.com/jrackham-mo
 .. _@stefsmeets: https://github.com/stefsmeets
 .. _@valeriupredoi: https://github.com/valeriupredoi
 

docs/src/whatsnew/latest.rst

@@ -59,6 +59,11 @@ This document explains the changes made to Iris for this release
    :func:`~iris.cube.Cube.slices` to work with dataless cubes.
    (:issue:`6725`, :pull:`6724`)
 
+#. `@trexfeathers`_ and `@jrackham-mo`_ added support for lazy calculation in
+   :func:`iris.analysis.calculus.cube_delta` (used in the
+   :func:`~iris.analysis.calculus.differentiate` function). (:issue:`6734`,
+   :pull:`6772`)
+
 
 🐛 Bugs Fixed
 =============

lib/iris/analysis/calculus.py

@@ -149,7 +149,7 @@ def cube_delta(cube, coord):
     .. note:: Missing data support not yet implemented.
 
     .. note::
-            This function does not maintain laziness when called; it realises data.
+            This function maintains laziness when called; it does not realise data.
             See more at :doc:`/userguide/real_and_lazy_data`.
 
     """
@@ -175,7 +175,7 @@ def cube_delta(cube, coord):
     # Calculate the actual delta, taking into account whether the given
     # coordinate is circular.
     delta_cube_data = delta(
-        cube.data, delta_dim, circular=getattr(coord, "circular", False)
+        cube.core_data(), delta_dim, circular=getattr(coord, "circular", False)
     )
 
     # If the coord/dim is circular there is no change in cube shape
@@ -260,7 +260,7 @@ def differentiate(cube, coord_to_differentiate):
     .. note:: Spherical differentiation does not occur in this routine.
 
     .. note::
-            This function does not maintain laziness when called; it realises data.
+            This function maintains laziness when called; it does not realise data.
             See more at :doc:`/userguide/real_and_lazy_data`.
 
     """