Clarify rh_from_tdew variable naming, comments and references

[gaoflow]

• Closes Inverted vapor pressure and saturation vapor pressure #2734
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• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
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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_tdew already returns correct relative humidity — e.g. rh_from_tdew(20, 10) == 52.56 %, rh_from_tdew(25, 25) == 100 %, and it round-trips with tdew_from_rh. What confused the report is that the two intermediate variables were named backwards: e was computed from temp_air (so it was really the saturation pressure) and es from temp_dew (the actual pressure), and 100 * es/e with those swapped names happens to equal the conventional 100 * e/es.

Changes:

• Compute e from the dew point (actual vapor pressure) and es from the air temperature (saturation) so the source matches the usual convention, and write 100 * 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.
• Fix the now-inaccurate derivation comment in tdew_from_rh (it referenced es/e).
• Add a short Notes section and the Alduchov & Eskridge (1996) reference for the Magnus form, since the WMO guide alone doesn't name it (per @echedey-ls).
• Add a physical-bounds regression test: saturation when dew point == air temperature, RH below 100 % otherwise and monotonic in the dew point, so the direction can't silently invert.

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.

[kandersolar]

@gaoflow in the future please include the PR template checklist when making PRs. I've added it back to this one.

[gaoflow]

Thanks, and apologies for omitting the checklist - I will include the PR template on future PRs.

[cwhanse]

Suggested change

	: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

[cwhanse]

Suggested change

	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``

[cwhanse]

Suggested change

	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 pressure at the air temperature, both
	from the Magnus equation ``A * exp(B * T / (C + T))``. The default

[cwhanse]

@gaoflow can you tick off the checklist items that are complete?

[gaoflow]

Done, checklist ticked. I left only the docs/sphinx/source/reference API-changes box unchecked since this is a clarity change with no API change (the whatsnew entry is in v0.15.2.rst). Tests and the new physical-bounds regression are in tests/test_atmosphere.py.

[cwhanse]

@gaflow there are three editorial suggestions, if you can accept or indicate otherwise. Then I think this is ready to go.

[gaoflow]

Thanks, I accepted the three editorial suggestions in e9e4723.

Local checks:

• python -m pytest tests/test_atmosphere.py -k 'rh_from_tdew or tdew_from_rh' -q
• python -m flake8 pvlib/atmosphere.py
• git diff --check fork/fix-2734-rh-from-tdew-naming..HEAD

[adriesse]

I always found the terminology confusing here, and still do because both numerator and denominator calculations are shown as saturation pressures. It helps (me) to remember that the numerator is actually the partial pressure under current conditions (temperature and water vapor content). Then it reads: RH = (partial pressure under current conditions) / (saturation pressure under current conditions).