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.. py:function:: saturation_vapour_pressure(t, [phase])
Computes the saturation vapour pressure for a given temperature (``t``) and ``phase``.
:param t: temperature (K)
:type t: number, ndarray or :class:`Fieldset`
:param phase: is either "water", "ice" or "mixed". When it is not specified the "water" phase is used.
:type phase: str
:rtype: same type as ``t`` or None
The result is the saturation vapour pressure in Pa units. On error None is returned. The computations for saturation over "water" and "ice" are based on the Tetens formula:
.. math::
e_{sat} = a_{1}\;exp \left(a_{3}\frac{T-273.16}{T-a_{4}}\right)
where the parameters are set as follows:
* "water": a\ :sub:`1` =611.21 Pa, a\ :sub:`3` =17.502 and a\ :sub:`4` =32.19 K
* "ice": a\ :sub:`1` =611.21 Pa, a\ :sub:`3` =22.587 and a\ :sub:`4` =-0.7 K
For the "mixed" phase the linear combination of the "water" and "ice" phases is used as described in the IFS documentation (see `here <https://www.ecmwf.int/en/elibrary/18714-part-iv-physical-processes>`_ on p116 for details for model cycle CY45R1).