vapour_pressure

vapour_pressure(q[, p])

Computes the vapour pressure for a given specific humidity (q) and pressure (p).

Parameters

  • q (number, ndarray or Fieldset) – specific humidity (kg/kg)

  • p (number, ndarray or Fieldset) – pressure (Pa)

Return type

same type as q or None

The result is the vapour pressure in Pa units. On error None is returned. The following rules are applied when q is a Fieldset:

  • if q is a pressure level 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 Fieldset defining the pressure on the same levels as q

  • for other level types p must be a Fieldset defining the pressure on the same levels as q.

The computation is based on the following formula:

\[\frac{p\;q}{\epsilon\; (1 + q(\frac{1}{\epsilon} -1 ))}\]

with

\[\epsilon = \frac{R_{dry}}{R_{vapour}} = 0.621981\]