Added dew point; implemented actual vapour pressure, but don't have test data.

README.md

@@ -25,8 +25,8 @@ The 'packet', in the terminology of this library, is a map which has bindings
 for some of the following keywords:
 
 1. `:absolute-humidity` *grammes per metre<sup>3</sup>.*
-2. `:actual-vapour-pressure` *not yet implemented*
+2. `:actual-vapour-pressure` *Pascals*
-1. `:dew-point` *not yet implemented*
+1. `:dew-point-celsius` 
 2. `:pressure-pascals` 
 3. `:pressure-millibars` 
 4. `:saturation-vapour-pressure` *Pascals*

src/humidity/actual_vp.clj

@@ -1,7 +1,8 @@
 (ns ^{:author "simon"
       :authority "https://www.weather.gov/media/epz/wxcalc/vaporPressure.pdf"
-  :doc "Compute actual vapour pressure."} humidity.actual-vp)
+      :doc "Compute actual vapour pressure."}
-
+ humidity.actual-vp
+  (:require [humidity.utils :refer [expt]]))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;
@@ -23,3 +24,21 @@
 ;;; USA.
 ;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(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."
+   ;; 1000 because are units are Pascals
+  [0 6.11 1000 7.5 237.3])
+
+(def C magic-numbers)
+
+(defn actual-vapour-pressure-fn
+  "Compute actual vapour pressure in Pascals given dew point in ° Celsius."
+  [& {:keys [dew-point-celsius]}]
+  (when dew-point-celsius
+    (* (C 1) (expt (C 2)
+                   (/ (* (C 3) dew-point-celsius)
+                      (+ (C 4) dew-point-celsius))))))

src/humidity/constants.clj

@@ -1,4 +1,5 @@
 (ns ^{:doc "Constants used in humidity calculations."
+  :authority ["https://journals.ametsoc.org/view/journals/apme/35/4/1520-0450_1996_035_0601_imfaos_2_0_co_2.xml"]
   :author "simon"} humidity.constants)
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -30,6 +31,10 @@
   "Base of natural logarithms"
   2.71828)
 
+(def ^:const magnus-coefficients
+  "See https://www.omnicalculator.com/physics/dew-point#how-to-calculate-dew-point-how-to-calculate-relative-humidity"
+  {:a 17.625 :b 243.04})
+
 (def ^:const Pa
   "Standard atmospheric pressure, in Pascals. I don't know why this isn't
   100,000, but it isn't.

src/humidity/core.clj

@@ -2,7 +2,9 @@
       :author "simon"}
  humidity.core
   (:require [humidity.absolute :refer [abs-humidity]]
+            [humidity.actual-vp :refer [actual-vapour-pressure-fn]]
             [humidity.constants :refer [celsius-offset Pa]]
+            [humidity.dew-point :refer [dew-point-celsius-fn]]
             [humidity.relative :refer [rel-humidity]]
             [humidity.saturation-vp :refer [saturation-vp]]
             [humidity.utils :refer [?assoc]]
@@ -32,7 +34,8 @@
 
 (defn resolve-humidity
   "Resolve humidity equations given the data in this packet."
-  [& {:keys [absolute-humidity pressure-pascals pressure-millibars
+  [& {:keys [absolute-humidity actual-vapour-pressure dew-point-celsius
+             pressure-pascals pressure-millibars
              relative-humidity saturation-vapour-pressure temperature
              temperature-celsius temperature-kelvin volume
              wet-bulb-temperature] :as packet}]
@@ -43,6 +46,10 @@
       (?assoc packet
               :absolute-humidity (when-not absolute-humidity
                                    (abs-humidity packet))
+              :actual-vapour-pressure (when-not actual-vapour-pressure
+                                        (actual-vapour-pressure-fn packet))
+              :dew-point-celsius (when-not dew-point-celsius
+                                   (dew-point-celsius-fn packet))
               :pressure-pascals (when-not pressure-pascals
                                   (if pressure-millibars
                                     (* pressure-millibars 100) Pa))

src/humidity/dew_point.clj

@@ -0,0 +1,34 @@
+(ns ^{:author "simon"
+  :authority ["https://www.omnicalculator.com/physics/dew-point#what-is-dew-point-dew-point-definition"
+  "https://en.wikipedia.org/wiki/Dew_point#Calculating_the_dew_point"]
+  :doc "Function to compute the dew point, in ° Celsius, given temperature
+  in ° Celsius and relative humidity percentage."} humidity.dew-point
+(:require [clojure.math :refer [log]]
+  [humidity.constants :refer [magnus-coefficients]]))
+
+(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 6.11 10 7.5 237.3])
+
+(def C magic-numbers)
+
+(defn- alpha
+  "See https://www.omnicalculator.com/physics/dew-point#how-to-calculate-dew-point-how-to-calculate-relative-humidity"
+  [T RH]
+  (let [a (:a magnus-coefficients)
+    b (:b magnus-coefficients)]
+  (+ (log (/ RH 100)) (/ (* a T) (+ b T)))))
+
+;; Ts = (b × α(T,RH)) / (a - α(T,RH))
+(defn dew-point-celsius-fn
+  "Compute and return the dew point, in ° Celsius, given temperature
+  in ° Celsius and relative humidity percentage; or `nil` if this data is
+  not available."
+  [&{:keys [temperature-celsius relative-humidity]}]
+  (when (and temperature-celsius relative-humidity)
+    (let [a' (alpha temperature-celsius relative-humidity)]
+      (/ (* (:b magnus-coefficients) a')
+        (- (:a magnus-coefficients) a')))))

src/humidity/volume.clj

@@ -2,7 +2,7 @@
       :authority "https://en.wikipedia.org/wiki/Gas_laws"
       :doc "In computing the relative humidity of air after it has warmed, I need
   to account for the change in volume. By Charles' Law, given stable
-  pressure, volume is proportinal to absolute temperature (i.e. V=Tk, where K
+  pressure, volume is proportional to absolute temperature (i.e. V=Tk, where K
   is a constant). However, the sources I have read do not give that constant,
   for air. But seeing what I'm interested in is that amount of air which is
   contained in a cubic metre at standard temperature and pressure, then *I
@@ -14,7 +14,7 @@
   (:require [humidity.constants :refer [celsius-offset Pa]]))
 
 (defn volume-fn
-  "Compute the volume, in m<sup>3</sup> of a quantity of air which would be
+  "Compute the volume, in m<sup>3</sup>, of a quantity of air which would be
   1m<sup>3</sup> at standard temperature and pressure, given temperature in
   &deg; Kelvin and pressure in Pascals."
   [& {:keys [temperature-kelvin pressure-pascals]}]

test/humidity/core_test.clj

@@ -26,14 +26,16 @@
 
 (deftest resolution-test
   (testing "resolution")
-  (let [expected {:relative-humidity 100,
+  (let [expected {:absolute-humidity 3953.618101887827,
-                  :temperature-celsius 35,
                   :pressure-pascals 101325,
+                  :temperature-celsius 35,
+                  :dew-point-celsius 35.00000000000001,
                   :pressure-millibars 1013.25,
-                  :saturation-vapour-pressure 5622.488734516426,
+                  :volume 1.1281347245103424,
                   :temperature-kelvin 308.15,
-                  :absolute-humidity 3953.618101887827,
+                  :relative-humidity 100,
-                  :volume 1.1281347245103424}
+                  :saturation-vapour-pressure 5622.488734516426,
+                  :actual-vapour-pressure 4765.10094206357}
         packet {:relative-humidity 100 :temperature-celsius 35}
         actual (resolution packet)]
     (is (= actual expected))))

test/humidity/dew_point_test.clj

@@ -1 +1,36 @@
-(ns humidity.dew-point-test)
+(ns humidity.dew-point-test
+  (:require [clojure.test :refer [deftest is testing]]
+            [humidity.constants :refer [celsius-offset]]
+            [humidity.dew-point :refer [dew-point-celsius-fn]]))
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;
+;;; Copyright (C) 2026 Simon Brooke
+;;;
+;;; This program is free software; you can redistribute it and/or
+;;; modify it under the terms of the GNU General Public License
+;;; as published by the Free Software Foundation; either version 2
+;;; of the License, or (at your option) any later version.
+;;;
+;;; This program is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with this program; if not, write to the Free Software
+;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
+;;; USA.
+;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(deftest dew-point-test
+  (testing "definitional"
+    (let [min-expected 34.9
+          max-expected 35.1
+          actual (dew-point-celsius-fn :relative-humidity 100
+                                       :temperature-celsius 35)]
+      (is (< min-expected actual max-expected))))
+  (testing "missing data"
+    (let [expected nil
+          actual (dew-point-celsius-fn :relative-humidity 100)]
+      (is (= expected actual)))))