diff --git a/environment.yml b/environment.yml
index 7225490..550ca1a 100644
--- a/environment.yml
+++ b/environment.yml
@@ -7,6 +7,7 @@ dependencies:
 - numpy
 - pandas
 - earthkit-utils>=0.0.1
+- earthkit-transforms>=0.5.0
 - make
 - mypy
 - myst-parser
@@ -24,3 +25,4 @@ dependencies:
 - nbconvert
 - nbsphinx
 - ipykernel
+- xarray
diff --git a/pyproject.toml b/pyproject.toml
index 4e590a9..4f012a7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -27,8 +27,10 @@ classifiers = [
 ]
 dynamic = [ "version" ]
 dependencies = [
+  "earthkit-transforms>=0.5",
   "earthkit-utils>=0.0.1",
   "numpy",
+  "xarray",
 ]
 optional-dependencies.gpu = [
   "cupy",
diff --git a/src/earthkit/meteo/extreme/__init__.py b/src/earthkit/meteo/extreme/__init__.py
index d3ee0b6..aefbbee 100644
--- a/src/earthkit/meteo/extreme/__init__.py
+++ b/src/earthkit/meteo/extreme/__init__.py
@@ -15,6 +15,7 @@
 planned to work with objects like *earthkit.data FieldLists* or *xarray DataSets*.
 """
 
+from . import excess_heat  # noqa
 from .cpf import *  # noqa
 from .efi import *  # noqa
 from .sot import *  # noqa
diff --git a/src/earthkit/meteo/extreme/excess_heat.py b/src/earthkit/meteo/extreme/excess_heat.py
new file mode 100644
index 0000000..bbee843
--- /dev/null
+++ b/src/earthkit/meteo/extreme/excess_heat.py
@@ -0,0 +1,216 @@
+# (C) Copyright 2025 - ECMWF and individual contributors.
+
+# This software is licensed under the terms of the Apache Licence Version 2.0
+# which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
+# In applying this licence, ECMWF does not waive the privileges and immunities
+# granted to it by virtue of its status as an intergovernmental organisation nor
+# does it submit to any jurisdiction.
+
+import functools
+import numbers
+
+import earthkit.transforms._tools as _ekt_tools
+import earthkit.transforms.temporal
+import numpy as np
+import xarray as xr
+
+
+class _with_metadata:
+    """Decorator to attach metadata to an output DataArray"""
+
+    # TODO just a quick solution until something better is in place
+    # TODO input-dependent unit handling (take input unit and transform)
+
+    def __init__(self, name, **attrs):
+        self.name = name
+        self.attrs = attrs
+
+    def __call__(self, f):
+        @functools.wraps(f)
+        def wrapped(*args, **kwargs):
+            return f(*args, **kwargs).rename(self.name).assign_attrs(self.attrs)
+
+        return wrapped
+
+
+def _rolling_mean(da, n, shift_days=0):
+    return earthkit.transforms.temporal.rolling_reduce(
+        da, n, center=False, how_reduce="mean", time_shift={"days": shift_days}, how_dropna="any"
+    )
+
+
+__DMT_TIME_SHIFT_COORD = "__daily_mean_temperature_time_shift"
+
+
+@_with_metadata("dmt", long_name="Daily mean temperature")
+def daily_mean_temperature(t2m, day_start=9, time_shift=0, **kwargs):
+    """Daily mean temperature, computed from min and max.
+
+    It is recommended to install flox for efficient aggregations.
+
+    Supports custom definitions of "day". E.g., by defining a later start of
+    the day (positive `day_start`), the usual early morning temperature minimum
+    can be attributed to the previous day, so that the mean value is derived
+    from the daytime maximum and the minimum of the following night (rather
+    than the previous night). At the same time, local time zones can be
+    accounted with the `time_shift` parameter by specifying the time zone
+    offsets with respect to the time coordinate of the input data as a function
+    of the spatial coordinates.
+
+    Example
+    -------
+
+    Assume that the time coordinate of the data is given in UTC and we want to
+    define the day from 10:00 to 10:00 Atlantic Standard Time (UTC -4 hours),
+    then we need to call
+
+        daily_mean_temperature(..., day_start=10, time_shift=-4)
+
+    Note that while both offest parameters lead to a "later" start of the day
+    relative to the reference time coordinate of the data, the sign of their
+    value is different to match their use as outlined in the text above. Set
+    `day_start` to zero and integrate the start offset into `time_shift` if
+    you prefer to specify only a single offset for both settings. The above
+    call is equivalent to
+
+        daily_mean_temperature(..., day_start=0, time_shift=-14)
+
+    Parameters
+    ----------
+    t2m : xr.DataArray
+        2-metre temperature.
+    day_start : number | np.timedelta64
+        Constant offset for the start of the day in the aggregations. By
+        default, the day is defined from 09:00 to 09:00. Positive offsets
+        indicate a late start of the day (see default), negative an early
+        start. A numeric value is interpreted as hours.
+    time_shift : np.timedelta64 | number | str | xr.DataArray
+        Offset relative to time coordinate of input. Can be a function of
+        space to specify timezones. A numeric value is interpreted as hours.
+        Provide a string value to take a correspondingly named coordinate
+        from the input DataArray.
+    **kwargs
+        Keyword arguments for the daily_min and daily_max functions of
+        earthkit.transforms.temporal.
+    """
+    assert isinstance(t2m, xr.DataArray)
+
+    if isinstance(day_start, numbers.Number):
+        day_start = np.timedelta64(day_start, "h")
+    assert isinstance(day_start, np.timedelta64)
+
+    if isinstance(time_shift, numbers.Number):
+        time_shift = np.timedelta64(time_shift, "h")
+    if isinstance(time_shift, str):
+        time_shift = t2m.coords[time_shift]
+    if isinstance(time_shift, xr.DataArray):
+        unique_shifts = np.unique(time_shift.values)
+        # Can only proceed if all timeseries are in the same time zone. If
+        # there are multiple time zones: split, process separately, merge.
+        if unique_shifts.size == 1:
+            time_shift = unique_shifts[0]
+        else:
+            assert __DMT_TIME_SHIFT_COORD not in t2m.coords
+            # Merging of groups where different partial days were removed
+            # fails after map when the time coordinate is of pandas period
+            # dtype. The period dtype works as long as the same partial days
+            # are in all groups removed.
+            return (
+                t2m
+                # Groups don't know their time shift unless we attach it here.
+                # Grouping by the time shift means the shift value per group
+                # is unique and the recursion ends immediately.
+                .assign_coords({__DMT_TIME_SHIFT_COORD: time_shift})
+                .groupby(__DMT_TIME_SHIFT_COORD)
+                .map(daily_mean_temperature, day_start=day_start, time_shift=__DMT_TIME_SHIFT_COORD, **kwargs)
+                # Don't expose the internal shift coordinate to the user
+                .drop_vars(__DMT_TIME_SHIFT_COORD)
+            )
+    time_shift = np.asarray(time_shift)
+    assert time_shift.size == 1
+    assert np.issubdtype(time_shift.dtype, np.timedelta64)
+
+    agg_kwargs = {"time_shift": time_shift - day_start, "remove_partial_periods": True, **kwargs}
+    tmin = earthkit.transforms.temporal.daily_min(t2m, **agg_kwargs)
+    tmax = earthkit.transforms.temporal.daily_max(t2m, **agg_kwargs)
+    return 0.5 * (tmin + tmax)
+
+
+@_with_metadata("ehi_sig", long_name="Significance index")
+def significance_index(dmt, threshold=("quantile", 0.95), ndays=3, time_dim=None):
+    """Significance index
+
+    Parameters
+    ----------
+    dmt : xr.DataArray
+        Daily mean temperature.
+    threshold : number
+        TODO
+    ndays : number
+        Length of evaluation time window.
+    time_dim : None | str
+        Name of time dimension in dmt DataArray.
+
+    Returns
+    -------
+    xr.DataArray
+        Significance index.
+    """
+    # Time coordinate detection compatible with earthkit.transforms
+    if time_dim is None:
+        time_dim = _ekt_tools.get_dim_key(dmt, "t", raise_error=True)
+    # Compute threshold as quantile
+    if isinstance(threshold, tuple):
+        assert len(threshold) == 2
+        if threshold[0] == "quantile":
+            threshold = dmt.quantile(threshold[1], dim=time_dim)
+        else:
+            raise NotImplementedError
+    # TODO: also support day-of-year climatology to detect warm spells
+    current = _rolling_mean(dmt, ndays, shift_days=(1 - ndays))
+    return current - threshold
+
+
+@_with_metadata("ehi_accl", long_name="acclimatisation_index")
+def acclimatisation_index(dmt, ndays=3, ndays_ref=30):
+    """Acclimatisation index
+
+    Parameters
+    ----------
+    dmt : xr.DataArray
+        Daily mean temperature.
+    ndays : number
+        Length of evaluation time window.
+    ndays_ref : number
+        Length of reference time window (recent past).
+    """
+    current = _rolling_mean(
+        dmt, ndays, shift_days=(1 - ndays)
+    )  # TODO: shared with significance index, would be nice to not compute it twice
+    reference = _rolling_mean(dmt, ndays_ref, shift_days=1)
+    return current - reference
+
+
+# https://codes.ecmwf.int/grib/param-db/261024
+@_with_metadata("exhf", long_name="Excess heat factor")
+def excess_heat_factor(ehi_sig, ehi_accl, nonnegative=True):
+    """Excess heat factor
+
+    Parameters
+    ----------
+    ehi_sig : xr.DataArray | array_like
+        Significance index.
+    ehi_accl : xr.DataArray | array_like
+        Acclimatisation index.
+    nonnegative : bool
+        Whether to clip the lower value range by zero.
+    """
+    if nonnegative:
+        ehi_sig = np.maximum(0, ehi_sig)
+    return ehi_sig * np.maximum(1.0, ehi_accl)
+
+
+# https://codes.ecmwf.int/grib/param-db/261025
+@_with_metadata("excf", long_name="Excess cold factor")
+def excess_cold_factor(ehi_sig, ehi_accl, nonnegative=True):
+    return NotImplementedError  # TODO
diff --git a/tests/extreme/test_excess_heat.py b/tests/extreme/test_excess_heat.py
new file mode 100644
index 0000000..681dd63
--- /dev/null
+++ b/tests/extreme/test_excess_heat.py
@@ -0,0 +1,167 @@
+# (C) Copyright 2025 - ECMWF and individual contributors.
+
+# This software is licensed under the terms of the Apache Licence Version 2.0
+# which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
+# In applying this licence, ECMWF does not waive the privileges and immunities
+# granted to it by virtue of its status as an intergovernmental organisation nor
+# does it submit to any jurisdiction.
+
+import numpy as np
+import pandas as pd
+import pytest
+import xarray as xr
+
+from earthkit.meteo.extreme import excess_heat
+
+
+class TestDailyMeanTemeratureArgDayStart:
+
+    @pytest.fixture
+    def t2m(self):
+        nt = 4 * 24
+        return xr.DataArray(
+            data=np.arange(nt),
+            dims=["valid_time"],
+            coords={"valid_time": pd.date_range("2026-01-01", periods=nt, freq="1h")},
+            name="t2m",
+        )
+
+    @pytest.mark.parametrize("start", [-5, 0, 3])
+    def test_number_value_interpreted_as_hours(self, t2m, start):
+        dmt_tdelta = excess_heat.daily_mean_temperature(t2m, day_start=np.timedelta64(start, "h"))
+        dmt_number = excess_heat.daily_mean_temperature(t2m, day_start=start)
+        np.testing.assert_allclose(dmt_tdelta, dmt_number)
+
+    def test_zero_means_day_starts_at_midnight(self, t2m):
+        dmt = excess_heat.daily_mean_temperature(t2m, day_start=0, time_shift=0)
+        np.testing.assert_allclose(dmt, [0.5 * (0 + 23), 0.5 * (24 + 47), 0.5 * (48 + 71), 0.5 * (72 + 95)])
+
+    @pytest.mark.parametrize("start", [-9, -5, -2])
+    def test_negative_value_means_day_starts_early(self, t2m, start):
+        dmt = excess_heat.daily_mean_temperature(t2m, day_start=start, time_shift=0)
+        # Partial periods removed: days 2, 3, 4 returned; 1 and 5 partial
+        np.testing.assert_allclose(
+            dmt, [0.5 * (24 + 47) + start, 0.5 * (48 + 71) + start, 0.5 * (72 + 95) + start]
+        )
+
+    @pytest.mark.parametrize("start", [2, 5, 9])
+    def test_positive_means_day_starts_late(self, t2m, start):
+        dmt = excess_heat.daily_mean_temperature(t2m, day_start=start, time_shift=0)
+        # Partial periods removed: days 1, 2, 3 returned; 0 and 4 partial
+        np.testing.assert_allclose(
+            dmt, [0.5 * (0 + 23) + start, 0.5 * (24 + 47) + start, 0.5 * (48 + 71) + start]
+        )
+
+
+class TestDailyMeanTemperatureArgTimeShift:
+
+    @pytest.fixture
+    def t2m(self):
+        nx, nt = 3, 4 * 24
+        return xr.DataArray(
+            # Values differ by order of magnitude in space, increase over time
+            data=np.logspace(0, nx - 1, nx)[None, :] * np.arange(nt)[:, None],
+            dims=["valid_time", "x"],
+            coords={
+                "valid_time": pd.date_range("2026-01-01", periods=nt, freq="1h"),
+                "x": np.arange(nx),
+            },
+            name="t2m",
+        )
+
+    @pytest.mark.parametrize("shift", [-5, 0, 3])
+    def test_number_value_interpreted_as_hours(self, t2m, shift):
+        dmt_tdelta = excess_heat.daily_mean_temperature(
+            t2m, day_start=0, time_shift=np.timedelta64(shift, "h")
+        )
+        dmt_number = excess_heat.daily_mean_temperature(t2m, day_start=0, time_shift=shift)
+        np.testing.assert_allclose(dmt_tdelta, dmt_number)
+
+    def test_field_access_with_str_value(self, t2m):
+        tz = xr.DataArray(
+            data=[np.timedelta64(1, "h"), np.timedelta64(5, "h"), np.timedelta64(7, "h")],
+            dims=["x"],
+            coords={"x": t2m.coords["x"]},
+            name="timezone",
+        )
+        dmt_array = excess_heat.daily_mean_temperature(t2m, day_start=0, time_shift=tz)
+        dmt_coord = excess_heat.daily_mean_temperature(
+            t2m.assign_coords({"timezone": tz}), day_start=0, time_shift="timezone"
+        )
+        np.testing.assert_allclose(dmt_array, dmt_coord)
+
+    @pytest.mark.parametrize("shift", [-5, 0, 3])
+    def test_scalar_value_is_applied_everywhere(self, t2m, shift):
+        dmt_scalar = excess_heat.daily_mean_temperature(t2m, day_start=0, time_shift=shift)
+        dmt_array = excess_heat.daily_mean_temperature(
+            t2m.assign_coords({"timezone": ("x", 3 * [np.timedelta64(shift, "h")])}),
+            day_start=0,
+            time_shift="timezone",
+        )
+        np.testing.assert_allclose(dmt_scalar, dmt_array)
+
+    @pytest.mark.parametrize("shift", [-2, -5, -9])
+    def test_negative_value_means_day_starts_late(self, t2m, shift):
+        dmt = excess_heat.daily_mean_temperature(t2m, day_start=0, time_shift=shift)
+        ref = (np.asarray([11.5, 35.5, 59.5]) - shift)[:, None] * np.asarray([1e0, 1e1, 1e2])[None, :]
+        np.testing.assert_allclose(dmt, ref)
+
+    @pytest.mark.parametrize("shift", [2, 5, 9])
+    def test_positive_value_means_day_starts_early(self, t2m, shift):
+        dmt = excess_heat.daily_mean_temperature(t2m, day_start=0, time_shift=shift)
+        ref = (np.asarray([35.5, 59.5, 83.5]) - shift)[:, None] * np.asarray([1e0, 1e1, 1e2])[None, :]
+        np.testing.assert_allclose(dmt, ref)
+
+    def test_multiple_timezones_grouping_of_duplicate_values(self, t2m):
+        t2m = t2m.assign_coords(
+            {"timezone": ("x", [np.timedelta64(1, "h"), np.timedelta64(4, "h"), np.timedelta64(4, "h")])}
+        )
+        dmt = excess_heat.daily_mean_temperature(t2m, day_start=0, time_shift="timezone")
+        np.testing.assert_allclose(dmt, [[34.5, 315.0, 3150.0], [58.5, 555.0, 5550.0], [82.5, 795.0, 7950.0]])
+
+    def test_multiple_timezones_fills_with_nan_to_preserve_outputs(self, t2m):
+        t2m = t2m.assign_coords(
+            {"timezone": ("x", [np.timedelta64(-2, "h"), np.timedelta64(0, "h"), np.timedelta64(10, "h")])}
+        )
+        dmt = excess_heat.daily_mean_temperature(t2m, day_start=0, time_shift="timezone")
+        np.testing.assert_allclose(
+            dmt,
+            [
+                [13.5, 115.0, np.nan],
+                [37.5, 355.0, 2550.0],
+                [61.5, 595.0, 4950.0],
+                [np.nan, 835.0, 7350.0],
+            ],
+        )
+
+
+def test_daily_mean_temperature_combined_day_start_and_time_shift_args():
+    nx, nt = 4, 4 * 24
+    t2m = xr.DataArray(
+        data=np.arange(nt).repeat(nx).reshape((nt, nx)),
+        dims=["valid_time", "x"],
+        coords={
+            "valid_time": pd.date_range("2026-01-01", periods=nt, freq="1h"),
+            "x": np.arange(nx),
+            "timezone": (
+                "x",
+                [
+                    np.timedelta64(-5, "h"),
+                    np.timedelta64(1, "h"),
+                    np.timedelta64(3, "h"),
+                    np.timedelta64(12, "h"),
+                ],
+            ),
+        },
+        name="t2m",
+    )
+    dmt = excess_heat.daily_mean_temperature(t2m, day_start=3, time_shift="timezone")
+    np.testing.assert_allclose(
+        dmt,
+        [
+            [19.5, 13.5, 11.5, np.nan],
+            [43.5, 37.5, 35.5, 26.5],
+            [67.5, 61.5, 59.5, 50.5],
+            [np.nan, np.nan, 83.5, 74.5],
+        ],
+    )