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.. py:function:: dewpoint_from_specific_humidity(q, [p])
Computes the dewpoint temperature from the given specific humidity (``q``) and pressure (``p``).
:param q: specific humidity (kg/kg)
:type q: number, ndarray or :class:`Fieldset`
:param p: pressure (Pa)
:type p: number, ndarray or :class:`Fieldset`
:rtype: same type as ``q`` or None
The result is the dewpoint temperature in K units. On error None is returned. The following rules are applied when ``q`` is a :class:`Fieldset`:
* if ``q`` is a pressure level :class:`Fieldset` no ``p`` is needed
* if ``q`` is defined on ECMWF model levels (hybrid/eta) ``p`` must be either a single LNSP (logarithm of surface pressure, identified by paramId=152) field or a :class:`Fieldset` defining the pressure on the same levels as ``q``
* for other level types ``p`` must be a :class:`Fieldset` defining the pressure on the same levels as ``q``.
The computation is based on the following equation:
.. math::
e(q, p) = e_{wsat}(td)
where:
* e is the vapour pressure (see :func:`vapour_pressure`)
* e\ :sub:`wsat` is the saturation vapour pressure over water (see :func:`saturation_vapour_pressure`)
* td is the dewpoint temperature