diff --git a/.gitignore b/.gitignore
index 6e0bb90..cdea907 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,5 @@
 .DS_Store
 *.nc
 data/
+
+*.egg-info/
diff --git a/eurec4a_environment/interface.py b/eurec4a_environment/interface.py
new file mode 100644
index 0000000..c5c1c50
--- /dev/null
+++ b/eurec4a_environment/interface.py
@@ -0,0 +1,67 @@
+# Example function for testing calculate
+def _thingness(u, v, T):
+    return u*v*T
+
+
+def calculate(name, ds):
+    """Calculate a variable from the given dataset by CF standard name
+
+    Args:
+        name (str): The CF standard name of the variable to be calculated
+        ds (xarray.DataSet): The dataset to calculate the variable from
+
+    Returns:
+        xarray.DataArray:
+
+    Raises:
+        ValueError: If the requested variable (or a variable required to calculate it)
+            is not available in the dataset
+    """
+    # If the variable is in the dataset, just return it
+    if name in ds:
+        return ds[name]
+
+    # Also check if the name matches a CF standard name
+    # Create a mapping of CF standard names in the dataset to their variable name
+    translation_table = dict()
+    for varname in ds:
+        try:
+            translation_table[ds[varname].attrs["standard_name"]] = varname
+        except KeyError:
+            # Skip variables without a standard_name attribute
+            pass
+
+    # Check whether the requested variable matches a standard name
+    if name in translation_table:
+        result = ds[translation_table[name]].copy()
+        _rename_xarray(result, name)
+        return result
+
+    # Otherwise calculate the requested variable using variables in the dataset
+    if name in available:
+        arguments = []
+        for argument_name in available[name]["arguments"]:
+            arguments.append(calculate(argument_name, ds))
+
+        result = available[name]["function"](*arguments)
+        _rename_xarray(result, name)
+        return result
+
+    # If you get this far without returning a variable then something has gone wrong
+    raise ValueError("Can not calculate {} from dataset".format(name))
+
+
+def _rename_xarray(array, name):
+    array.rename(name)
+    array.attrs["standard_name"] = name
+    array.attrs["long_name"] = name
+
+
+# A dictionary mapping variables that can be calculated to the functions to calculate
+# them and the required input arguments (by CF standard names)
+available = dict(
+    thingness=dict(
+        function=_thingness,
+        arguments=["eastward_wind", "northward_wind", "air_temperature"],
+    )
+)
diff --git a/setup.py b/setup.py
index fa898a3..3b38cae 100644
--- a/setup.py
+++ b/setup.py
@@ -2,7 +2,14 @@
 from setuptools import setup, find_packages
 import versioneer
 
-INSTALL_REQUIRES = ["xarray", "netcdf4", "typhon", "tqdm"]
+INSTALL_REQUIRES = [
+    "xarray",
+    "netcdf4",
+    "typhon",
+    "tqdm",
+    "requests",
+    "pyyaml",
+]
 
 setup(
     name="eurec4a-environment",
diff --git a/tests/test_interface.py b/tests/test_interface.py
new file mode 100644
index 0000000..43bcf52
--- /dev/null
+++ b/tests/test_interface.py
@@ -0,0 +1,21 @@
+import numpy as np
+
+from eurec4a_environment.source_data import open_joanne_dataset
+from eurec4a_environment.interface import calculate
+
+
+def test_calc():
+    ds_joanne = open_joanne_dataset()
+
+    result = calculate("thingness", ds_joanne)
+    expected = ds_joanne.u*ds_joanne.v*ds_joanne.T
+
+    expected = _mask_nans(expected.values)
+    result = _mask_nans(result.values)
+
+    assert (result == expected).all()
+
+
+def _mask_nans(array):
+    # NaNs mess up array-wise equality checks
+    return np.ma.masked_where(np.isnan(array), array)