Clarify rh_from_tdew variable naming, comments and references#2782
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Clarify rh_from_tdew variable naming, comments and references#2782gaoflow wants to merge 2 commits into
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In rh_from_tdew the intermediate named `e` was computed from the air temperature (so it was actually the saturation vapor pressure) and `es` from the dew point (the actual vapor pressure) -- swapped relative to the usual convention, which led pvlib#2734 to read it as an inverted formula. The returned values are correct and unchanged (100 * es/e with the swapped names equals 100 * e/es with conventional ones); this computes `e` from the dew point and `es` from the air temperature so the source matches the convention, and fixes the matching derivation comment in tdew_from_rh. Also adds a Notes section and the Alduchov & Eskridge (1996) reference for the Magnus form, plus a physical-bounds regression test (saturation when dew point == air temperature, RH below 100% and monotonic in the dew point otherwise) so the direction cannot silently invert. Addresses pvlib#2734.
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@gaoflow in the future please include the PR template checklist when making PRs. I've added it back to this one. |
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Thanks, and apologies for omitting the checklist - I will include the PR template on future PRs. |
echedey-ls
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Jun 14, 2026
State the -45 to +60 C validity range of the Magnus form directly in the Notes, citing WMO-No. 8 [1], instead of deferring to the reference.
cwhanse
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Jun 15, 2026
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| :py:func:`pvlib.atmosphere.tdew_from_rh`) so the actual and saturation vapor | ||
| pressures are no longer swapped in the source. The returned values are |
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| :py:func:`pvlib.atmosphere.tdew_from_rh`) so the actual and saturation vapor | |
| pressures are no longer swapped in the source. The returned values are | |
| :py:func:`pvlib.atmosphere.tdew_from_rh`) related to the actual | |
| and saturation vapor pressures. Function output is |
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| Relative humidity is computed as ``100 * e / es``, where the actual vapor | ||
| pressure ``e`` is the saturation vapor pressure at the dew point and the |
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| Relative humidity is computed as ``100 * e / es``, where the actual vapor | |
| pressure ``e`` is the saturation vapor pressure at the dew point and the | |
| Relative humidity is computed as ``100 * e / es``, where ``e`` is the | |
| saturation vapor pressure at the dew point temperature, and ``es`` |
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| saturation vapor pressure ``es`` is evaluated at the air temperature, both | ||
| from the Magnus equation ``A * exp(B * T / (C + T))``. The default |
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| saturation vapor pressure ``es`` is evaluated at the air temperature, both | |
| from the Magnus equation ``A * exp(B * T / (C + T))``. The default | |
| is the saturation vapor pressureat the air temperature, both | |
| from the Magnus equation ``A * exp(B * T / (C + T))``. The default |
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docs/sphinx/source/referencefor API changes.docs/sphinx/source/whatsnewfor all changes. Includes link to the GitHub Issue with:issue:`num`or this Pull Request with:pull:`num`. Includes contributor name and/or GitHub username (link with:ghuser:`user`).remote-data) and Milestone are assigned to the Pull Request and linked Issue.This is a clarity change with no change to the returned values. As noted in #2734 and independently confirmed by @echedey-ls (cross-checked against an online calculator and NREL's pvdeg),
rh_from_tdewalready returns correct relative humidity — e.g.rh_from_tdew(20, 10) == 52.56 %,rh_from_tdew(25, 25) == 100 %, and it round-trips withtdew_from_rh. What confused the report is that the two intermediate variables were named backwards:ewas computed fromtemp_air(so it was really the saturation pressure) andesfromtemp_dew(the actual pressure), and100 * es/ewith those swapped names happens to equal the conventional100 * e/es.Changes:
efrom the dew point (actual vapor pressure) andesfrom the air temperature (saturation) so the source matches the usual convention, and write100 * e/es. Numerically identical — the existing value-pinning tests (test_rh_from_tdew,test_tdew_from_rh,test_tdew_to_rh_to_tdew) pass unchanged.tdew_from_rh(it referencedes/e).Notessection and the Alduchov & Eskridge (1996) reference for the Magnus form, since the WMO guide alone doesn't name it (per @echedey-ls).I left the
exp-division micro-optimisation @echedey-ls mentioned out of this first pass since he flagged maintainers may differ on it — happy to add it (or expand the applicability notes with a max-error / valid-range figure) if wanted.