Calculate Relative Humidity via Magnus Tetens Equation #2286

kurt-rhee · 2024-10-30T18:45:38Z

Closes implement Tdew to RH conversions #1744
I am familiar with the contributing guidelines
Tests added
Updates entries in docs/sphinx/source/reference for API changes.
Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for 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`).
[ x ] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
Pull request is nearly complete and ready for detailed review.
Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Reasoning

There are some occasions where dew point temperature is available, but relative humidity is not.
Relative humidity can be converted to precipitable water in pvlib via guemard94.
Precipitable water is used by first solar to calculate spectral correction factors.
This function would allow a user with dew point temperature to convert it to relative humidity.

Caveats

Caveat 1

In the issue thread implement Tdew to RH conversions #1744 there is some controversy about the simpleness of this approach. Some users would prefer a more well known solution.
I would argue that because this approach was used in the creation of the First Solar spectral correction function, that it has historical significance and should therefore be included.
If any user should desire, they could add an alternative function and then create a controller function which allows the user to choose different relative humidity calculation methods, which is a pattern than exists already inside pvlib.

Caveat 2

I placed these functions inside of spectrum, but I could also see them living inside of pvlib.atmosphere.py where the gueymard94 function exists. I'd like the maintainers opinion before proceeding to update the docs and what's new file.

adriesse · 2024-10-30T20:03:37Z

@kurt-rhee Perhaps it would be better to continue the discussion in #1744 rather than start a new thread here.

kurt-rhee · 2024-10-30T20:25:58Z

@adriesse good idea, I've copied the comment above to the thread in question

pvlib/spectrum/magnus_tetens.py

pyproject.toml

uv.lock

pvlib/spectrum/magnus_tetens.py

…, changed type hints in docstring, changed to suggested coefficient format

pvlib/atmosphere.py

Co-authored-by: Anton Driesse <[email protected]>

kurt-rhee · 2024-11-18T20:54:24Z

@adriesse agreed, anecdotal and unnecessary, it has been removed.

I have left the formatting of functions outside of the tdew and rh conversion functions. I can format them to satisfy flake8 linter if that is desired by maintainers. Just let me know how I can help.

kandersolar

Some more comments. A what's new entry will be needed before too long!

pvlib/spectrum/magnus_tetens.py

pvlib/atmosphere.py

pvlib/tests/test_atmosphere.py

Co-authored-by: Kevin Anderson <[email protected]>

kandersolar

A few more comments, and the old magnus_tetens.py file still needs to be removed.

Otherwise I think we just need to tick off the remaining checklist items at the top of this thread: list the new functions in the reference docs, and create a what's new entry!

pvlib/tests/conftest.py

pvlib/tests/test_atmosphere.py

pyproject.toml

kandersolar

A few more nitpicks, otherwise LGTM!

docs/sphinx/source/whatsnew/v0.11.2.rst

pvlib/atmosphere.py

pvlib/tests/test_atmosphere.py

pvlib/atmosphere.py

kandersolar · 2024-12-06T03:16:37Z

pvlib/tests/test_atmosphere.py

+# Unit tests
+def test_rh_from_tdew():
+
+    # dewpoint temp calculated with who and aekr coefficients


several comments and variables in these two test functions say "who" instead of "wmo" :)

pyproject.toml

adriesse · 2024-12-06T19:42:30Z

pvlib/atmosphere.py

@@ -337,6 +337,82 @@ def gueymard94_pw(temp_air, relative_humidity):
    return pw


+def rh_from_tdew(temperature, dewpoint, coeff=(6.112, 17.62, 243.12)):
+    """
+    Calculate relative humidity from dewpoint temperature using the Magnus equation.


Suggested change

Calculate relative humidity from dewpoint temperature using the Magnus equation.

Calculate relative humidity from dew-point temperature using the Magnus equation.

(inner grammarian emerges) the term is either "dewpoint" or "dew point": NOAA glossary. The AMS glossary prefers "dewpoint".

I can't find any instances of "dew-point" as a hyphenated adjective, although I understand that's what grammar rules suggest it should be, rather than "dew point".

Admittedly I didn't search beyond the WMO document we referenced, so it could be an old-world / new-world thing.

Happy to use any term that you all prefer

Looks like EPW and TMY3 files use the hyphen. Most of the pvlib docs use "dew point", with the exception of the Nomenclature page which says "Dewpoint".

I suggest we leave this for a potential package-wide cleanup and not worry about it here.

pvlib/atmosphere.py

adriesse · 2024-12-06T19:45:41Z

pvlib/atmosphere.py

+    # First calculate ln(es/A)
+    ln_term = (
+        (coeff[1] * temperature) / (coeff[2] + temperature)
+        + np.log(relative_humidity/100)


looks like RH zero might throw an error or warning here

Should we add a check for zero values or leave the original message in the stack trace?

I don't think it's necessary, RH=0% is practically impossible (source).

adriesse · 2024-12-06T19:48:27Z

pvlib/atmosphere.py

+    Returns
+    -------
+    numeric
+        Dewpoint temperature in degrees Celsius


Suggested change

Dewpoint temperature in degrees Celsius

Dew-point temperature in degrees Celsius

Co-authored-by: Anton Driesse <[email protected]>

Co-authored-by: Kevin Anderson <[email protected]>

AdamRJensen

Does it make sense to add a test that demonstrates that you converting to rh and then back to tdew gives the same answer? If possible it would be a nice simple test.

pvlib/atmosphere.py

pvlib/tests/test_atmosphere.py

Co-authored-by: Kevin Anderson <[email protected]>

Co-authored-by: Adam R. Jensen <[email protected]>

Added a new test to show that you can calculate relative humidity from dewpoint and then calculate dewpoint from relative humidity and it will be the same as the original dewpoint values

kurt-rhee · 2024-12-11T20:06:23Z

Does it make sense to add a test that demonstrates that you converting to rh and then back to tdew gives the same answer? If possible it would be a nice simple test.

Just added this test in the latest commit

kandersolar · 2024-12-12T14:19:36Z

pvlib/atmosphere.py

@@ -337,6 +337,86 @@ def gueymard94_pw(temp_air, relative_humidity):
    return pw


+def rh_from_tdew(temperature, dewpoint, coeff=(6.112, 17.62, 243.12)):


I noticed earlier but forgot to comment: we should rename this parameter to temp_dew, the name used by other pvlib models and iotools functions. I'll push a commit for that to give @kurt-rhee a break from this PR :P

edit: and same for temp_air of course

kandersolar · 2024-12-12T14:49:36Z

I think it's time to merge this.

Thanks @kurt-rhee for the PR, @adriesse for putting us on the right course, and everyone else that contributed to the discussion!

Kurt Rhee added 5 commits October 29, 2024 12:52

changed default types to float in solarposition.py

70e0156

switched to floats in the docstrings and removed type hints

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added magnus_tetens equations

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line too long in solarposition.py

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revert solarposition.py to pre-PR state

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set default magnus coefficients per conversation in #1744

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Kurt Rhee added 6 commits November 11, 2024 08:14

moved equations to atmosphere.py, updated function names to suggested…

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…, changed type hints in docstring, changed to suggested coefficient format

moved tests to atmosphere.py tests

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changed reference to WMO

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dry-bult temperature in the docstring

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revert pyproject.toml

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remove uv.lock

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cwhanse added the enhancement label Nov 11, 2024

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Update pvlib/atmosphere.py

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Co-authored-by: Anton Driesse <[email protected]>

fixing flake8 errors for tdew/rh functions

6312f92

I have left the formatting of functions outside of the tdew and rh conversion functions. I can format them to satisfy flake8 linter if that is desired by maintainers. Just let me know how I can help.

kandersolar changed the title ~~Calculate Relative Humidity via Magnus Tetens Equation with AEKR coefficients~~ Calculate Relative Humidity via Magnus Tetens Equation Nov 25, 2024

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kurt-rhee and others added 6 commits November 25, 2024 12:53

Update pvlib/atmosphere.py

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Co-authored-by: Kevin Anderson <[email protected]>

Update pvlib/atmosphere.py

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Co-authored-by: Kevin Anderson <[email protected]>

Update pvlib/atmosphere.py

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Co-authored-by: Kevin Anderson <[email protected]>

Update pvlib/atmosphere.py

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Co-authored-by: Kevin Anderson <[email protected]>

Update pvlib/atmosphere.py

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Co-authored-by: Kevin Anderson <[email protected]>

Update pvlib/atmosphere.py

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added some unit tests for different coefficients and input types

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refactored tests, removed magnus_tetens, updated whatsnew

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reference and whatsnew

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revert line 36 (linter)

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kurt-rhee and others added 5 commits December 6, 2024 16:01

Update pvlib/atmosphere.py

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Co-authored-by: Anton Driesse <[email protected]>

Update docs/sphinx/source/whatsnew/v0.11.2.rst

2bdc1dc

Co-authored-by: Kevin Anderson <[email protected]>

Update pvlib/atmosphere.py

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Co-authored-by: Kevin Anderson <[email protected]>

Update pvlib/tests/test_atmosphere.py

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Update pyproject.toml

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AdamRJensen and others added 7 commits December 9, 2024 20:57

Merge branch 'main' into relative-humidity

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Update pvlib/atmosphere.py

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Update pvlib/atmosphere.py

71a165e

Co-authored-by: Adam R. Jensen <[email protected]>

Update pvlib/atmosphere.py

25527d6

Co-authored-by: Adam R. Jensen <[email protected]>

added a round-trip test for magnus tetens

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Added a new test to show that you can calculate relative humidity from dewpoint and then calculate dewpoint from relative humidity and it will be the same as the original dewpoint values

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kandersolar added 4 commits December 12, 2024 09:28

temperature -> temp_air; dewpoint -> temp_dew

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miscellaneous other cleanup

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tests: put assertions next to calculations

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linter

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kandersolar merged commit cb37129 into pvlib:main Dec 12, 2024
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kurt-rhee deleted the relative-humidity branch February 11, 2025 22:38

	Calculate relative humidity from dewpoint temperature using the Magnus equation.
	Calculate relative humidity from dew-point temperature using the Magnus equation.

	Dewpoint temperature in degrees Celsius
	Dew-point temperature in degrees Celsius

		@@ -337,6 +337,86 @@ def gueymard94_pw(temp_air, relative_humidity):
		return pw


		def rh_from_tdew(temperature, dewpoint, coeff=(6.112, 17.62, 243.12)):

Calculate Relative Humidity via Magnus Tetens Equation #2286

Calculate Relative Humidity via Magnus Tetens Equation #2286

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kurt-rhee commented Oct 30, 2024 • edited by cwhanse Loading Uh oh! There was an error while loading. Please reload this page.

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Reasoning

Caveats

Caveat 1

Caveat 2

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adriesse commented Oct 30, 2024

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kurt-rhee commented Oct 30, 2024

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kurt-rhee commented Nov 18, 2024

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cwhanse Dec 6, 2024 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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kurt-rhee commented Oct 30, 2024 •

edited by cwhanse

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