I have not yet got the overall architecture right, but the functions are

beginning to work.

project.clj

@@ -0,0 +1,7 @@
+(defproject humidity "0.1.0-SNAPSHOT"
+  :description "FIXME: write description"
+  :url "http://example.com/FIXME"
+  :license {:name "EPL-2.0 OR GPL-2.0-or-later WITH Classpath-exception-2.0"
+            :url "https://www.eclipse.org/legal/epl-2.0/"}
+  :dependencies [[org.clojure/clojure "1.11.1"]]
+  :repl-options {:init-ns humidity.core})

resources/katzenberger.txt

@@ -0,0 +1,49 @@
+# I'm guessing this is Python
+
+# Saturated water vapor pressure in hPa, Magnus Formula
+# Parameters from Sonntag1990, for −45 °C ≤ T ≤ 60 °C (error ±0.35 °C).
+def saturatedVaporPressureMagnusSonntag1990(temperatureCelsius):
+    a = 6.112
+    b = 17.62
+    c = 243.12
+
+    saturatedVaporPressure = a * math.exp((b * temperatureCelsius) / (c + temperatureCelsius))
+    return saturatedVaporPressure
+
+
+# Saturated water vapor pressure in hPa, Tetens Formula
+def saturatedVaporPressureTetens(temperatureCelsius):
+    a = 6.1078
+    b = 17.27
+    c = 237.3
+
+    saturatedVaporPressure = a * math.exp((b * temperatureCelsius) / (c + temperatureCelsius))
+    return saturatedVaporPressure
+
+
+# Saturated water vapor pressure in hPa, Buck 1996 Formula
+def saturatedVaporPressureBuck1996(temperatureCelsius):
+    if temperatureCelsius < 0:
+        return 6.1115 * math.exp(
+            (23.06 - temperatureCelsius / 333.7) * (temperatureCelsius / (279.82 + temperatureCelsius)))
+    else:
+        return 6.1121 * math.exp(
+            (18.678 - temperatureCelsius / 234.5) * (temperatureCelsius / (257.14 + temperatureCelsius)))
+
+
+# Actual water vapor pressure in hPa
+def vaporPressure(relativeHumidity, temperatureCelsius):
+    vaporPressure = relativeHumidity / 100.0 * saturatedVaporPressureBuck1996(temperatureCelsius)
+    return vaporPressure
+
+
+# Absolute humidity in g/m³
+def absoluteHumidity(relativeHumidity, temperatureCelsius):
+    molarMassOfWaterVapor = 18.01528
+    universalGasConstant = 8314.46261815324
+    zeroCelsiusInKelvin = 273.15
+
+    absoluteHumidity = 10 ** 5 * molarMassOfWaterVapor / universalGasConstant * vaporPressure(relativeHumidity,
+                                                                                              temperatureCelsius) / (
+                               temperatureCelsius + zeroCelsiusInKelvin)
+    return absoluteHumidity

src/humidity/absolute.clj

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+(ns ^{:doc "Function to calculate wet bulb temperature (in ° C) given temperature in 
+       ° C and relative-humidity percentage (0...100).
+            
+            See [authority](https://carnotcycle.wordpress.com/2012/08/04/how-to-convert-relative-humidity-to-absolute-humidity/)"}
+ humidity.absolute 
+  (:require
+    [humidity.constants :refer [Rw]]))
+
+;; Absolute Humidity (grams/m3) = 6.112 × e^[(17.67 × T)/(T+243.5)] × rh × 2.1674
+;; -----
+;;  (273.15+T)
+
+(def ^:const magic-numbers
+  "The magic number constants from the derived expression. It would give me 
+   more comfort if there were some agreement between these magic numbers and 
+   those used in the wet bulb function, but there is not."
+  [0 6.112 17.67 243.5 2.1674 273.15])
+
+(def C magic-numbers)
+
+;; (defn abs-humidity
+;;   "Returns absolute humidity in grammes per cubic metre given this 
+;;    `temperature` in  ° C and `relative-hunidity` percentage (0...100).
+;;    ![function for absolute humidity given temperature and relative humidity](https://carnotcycle.wordpress.com/wp-content/uploads/2014/09/ah3.gif)"
+;;   [temperature relative-humidity]
+;;   (when (empty? (filter false? (map number? [temperature relative-humidity])))
+;;     (let [T temperature
+;;           RHP relative-humidity]
+;;       (/ (* (C 1) (pow (* (/ (C 2) T) (+ T (C 3))) e) RHP (C 4))
+;;          (+ celsius-offset temperature)))))
+
+;; AH=(RH × Ps)​​/(Rw ​× T × 100)
+
+(defn abs-humidity [& {:keys [relative-humidity saturation-pressure temperature-kelvin]}]
+  (when (and relative-humidity saturation-pressure temperature-kelvin)
+    (/ (* relative-humidity saturation-pressure) (* Rw temperature-kelvin 100)))
+  )
+

src/humidity/constants.clj

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+(ns humidity.constants)
+
+
+(def ^:const celsius-offset
+  "Value of 0° Celsius in ° Kelvin"
+  273.15)
+
+(def ^:const e
+  "Base of natural logarithms"
+  2.71828)
+
+(def ^:const Pc
+  "Critical pressure for water, in MPa (Megapascal)"
+  22.064)
+
+(def ^:const Rw
+  "Specific gas constant for water vapour (J/(kg⋅K))"
+  461.5)
+
+(def ^:const Tc
+  "Critical temperature for water, ° Kelvin"
+  647.096)

src/humidity/core.clj

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+(ns humidity.core
+  (:require [humidity.absolute :refer [abs-humidity]]
+            [humidity.constants :refer [celsius-offset]]
+            [humidity.saturation-vp :as saturation-vp]
+            [humidity.utils :refer [?assoc]]
+            [humidity.wet-bulb :refer [wet-bulb-temperature]]))
+
+(defn resolve-humidity
+  "Resolve humidity equations given the data in this packet."
+  [& {:keys [temperature temperature-celsius temperature-kelvin relative-humidity 
+             pressure-kilopascals pressure-millibars] :as packet}]
+  (println packet)
+  (if temperature (resolve-humidity (assoc (dissoc packet :temperature) :temperature-celsius temperature))
+      (?assoc packet
+              :temperature-celsius (when
+                                    (and temperature-kelvin (not temperature-celsius))
+                                     (- temperature-kelvin celsius-offset))
+              :temperature-kelvin (when
+                                   (and temperature-celsius (not temperature-kelvin))
+                                    (+ temperature-celsius celsius-offset))
+              :wet-bulb-temperature (when
+                                     (and temperature-celsius relative-humidity)
+                                      (wet-bulb-temperature temperature-celsius relative-humidity))
+              :absolute-humidity (when (and temperature-celsius relative-humidity)
+                                   (abs-humidity temperature-celsius relative-humidity))
+              :pressure-kilopascals (when-not pressure-kilopascals
+                                      (if pressure-millibars (/ pressure-millibars 10) 100))
+              :pressure-millibars (when-not pressure-millibars
+                                    (if pressure-kilopascals (* pressure-kilopascals 10) 1000))
+              :saturation-pressure (saturation-vp/saturation-vp packet))))
+
+
+;; the example in the paper gives this as 13.7
+(wet-bulb-temperature 20 50)
+;; by definition, this must equal 35
+(wet-bulb-temperature 35 100)
+;; from published graphs, I interpolatethis in the range 33...35
+(wet-bulb-temperature 40 70)
+
+;; (resolve-humidity {:relative-humidity 100 :temperature 35})
+
+(defn resolution
+  "Repeatedly run `resolve-humidity`, q.v., until we can infer no further
+   information."
+  [packet]
+  (loop [p packet]
+    (let [p' (resolve-humidity p)]
+      (if (= (keys p) (keys p')) p
+          (recur p')))))

src/humidity/relative.clj

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+(ns humidity.relative)

src/humidity/saturation_vp.clj

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+(ns ^{:doc "Saturation vapour pressure function"
+      :authority "https://en.wikipedia.org/wiki/Tetens_equation"
+      :author "simon"}
+ humidity.saturation-vp
+  (:require [clojure.math :refer [pow]]
+            [humidity.constants :refer [celsius-offset e]]))
+
+
+(defn saturation-vp-tetens
+  "Calculate and return saturation vapour pressure in hPa using Tetens' 
+   formula, given temerature in ° Celsius."
+  [temperature-celsius]
+  (let [a 6.1078 ;; yet more magic numbers...
+        b 17.27
+        c 237.3]
+    (* a (pow e (/ (* b temperature-celsius) (+ c temperature-celsius))))))
+
+(defn saturation-vp
+  "Calculate and return saturation vapour pressure, is possible,
+   given the data in this packet."
+  [packet]
+  (let [Tc (packet :temperature-celsius)
+        Tk (packet :temperature-kelvin)]
+    (cond Tc (saturation-vp-tetens Tc)
+          Tk (saturation-vp-tetens (- Tk celsius-offset)))))

src/humidity/utils.clj

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+(ns ^{:doc "Utility functions for humidity calculation"} humidity.utils
+  (:require [clojure.math :refer [pow]]))
+
+(def ^:const celsius-offset
+  "Value of 0° Celsius in ° Kelvin"
+  273.15)
+
+(def ^:const e
+  "Base of natural logarithms"
+  2.71828)
+
+
+(defmacro expt
+  "Clojure.math `pow` function does not allow ratios as exponents;
+   this is equivalent but coerces the exponent argument to float."
+  [base exponent]
+  `(pow ~base (float ~exponent)))
+
+(defn ?assoc
+  "Same as assoc, but skip the assoc if v is nil.
+   Borrowed from https://stackoverflow.com/questions/16356888/assoc-if-in-clojure"
+  [m & kvs]
+  (->> kvs
+       (partition 2)
+       (filter second)
+       (map vec)
+       (into m)))

src/humidity/wet_bulb.clj

@@ -0,0 +1,26 @@
+(ns ^{:doc "Function to calculate wet bulb temperature (in ° C) given temperature in 
+   ° C and relative-humidity percentage (0...100)"} 
+  humidity.wet-bulb
+  (:require [clojure.math :refer [atan]]
+            [humidity.utils :refer [expt]]))
+
+(def ^:const magic-numbers
+  "The magic numbers (empirical constants) from the derived expression. It
+   bothers me that all these formulae depend on 'empirical constants', *and*
+   that there is no commonality between the empirical constants of different
+   formulae."
+  [0 0.151977 8.313659 1.676331 0.00391838 0.023101 4.686035])
+
+(def C magic-numbers)
+
+(defn wet-bulb-temperature
+  "Returns wet bulb temperature (in ° C) given this `temperature` in 
+   ° C and `relative-humidity` percentage (0...100)"
+  [temperature relative-humidity]
+  (let [T temperature
+        RHP relative-humidity]
+    (+ (* T (atan (* (C 1) (expt (+ RHP (C 2)) 1/2))))
+       (atan (+ T RHP))
+       (- 0 (atan (- RHP (C 3))))
+       (* (C 4) (expt RHP 3/2) (atan (* (C 5) RHP)))
+       (- 0 (C 6)))))

test/humidity/wet_bulb_test.clj

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+(ns humidity.wet-bulb-test
+  (:require [clojure.test :refer [deftest is testing]]
+            [humidity.wet-bulb :refer [wet-bulb-temperature]]))
+
+
+(deftest wet-bulb-test
+  (testing "Example from paper"
+    (let [expected 13.7
+          temperature 20
+          rel-humidity 50
+          actual (wet-bulb-temperature temperature rel-humidity)]
+      (println (format "Expected: %s; actual: %s" expected actual))
+      (is (< (abs (- expected actual)) 0.001))))
+
+  (testing "Interpolations from graph"
+    (let [min-expected 31
+          max-expected 33
+          temperature 37
+          rel-humidity 70
+          actual (wet-bulb-temperature temperature rel-humidity)]
+      (println (format "Expected: %s...%s; actual: %s" min-expected max-expected actual))
+      (is (< min-expected actual max-expected)))
+
+    (let [min-expected 34
+          max-expected 36
+          temperature 39.7
+          rel-humidity 70
+          actual (wet-bulb-temperature temperature rel-humidity)]
+      (println (format "Expected: %s...%s; actual: %s" min-expected max-expected actual))
+      (is (< min-expected actual max-expected)))
+
+
+    (let [min-expected 31
+          max-expected 34
+          temperature 40
+          rel-humidity 60
+          actual (wet-bulb-temperature temperature rel-humidity)]
+      (println (format "Expected: %s...%s; actual: %s" min-expected max-expected actual))
+      (is (< min-expected actual max-expected)))
+
+    (let [min-expected 32
+          max-expected 35
+          temperature 51
+          rel-humidity 30
+          actual (wet-bulb-temperature temperature rel-humidity)]
+      (println (format "Expected: %s...%s; actual: %s" min-expected max-expected actual))
+      (is (< min-expected actual max-expected)))))