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

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Simon Brooke 2026-08-12 13:02:43 +01:00
parent ce32cd285e
commit 970a320a84
8 changed files with 118 additions and 16 deletions
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4
README.md
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@ -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*
1. `:dew-point` *not yet implemented*
2. `:actual-vapour-pressure` *Pascals*
1. `:dew-point-celsius`
2. `:pressure-pascals`
3. `:pressure-millibars`
4. `:saturation-vapour-pressure` *Pascals*

25
src/humidity/actual_vp.clj
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@ -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)
:authority "https://www.weather.gov/media/epz/wxcalc/vaporPressure.pdf"
: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 &deg; Celsius."
[& {:keys [dew-point-celsius]}]
(when dew-point-celsius
(* (C 1) (expt (C 2)
(/ (* (C 3) dew-point-celsius)
(+ (C 4) dew-point-celsius))))))

5
src/humidity/constants.clj
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@ -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.

13
src/humidity/core.clj
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@ -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}]
@ -43,13 +46,17 @@
(?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))
:pressure-millibars (when-not pressure-millibars
(if pressure-pascals
(float (/ pressure-pascals 100))
(float (/ Pa 100))))
(float (/ pressure-pascals 100))
(float (/ Pa 100))))
:relative-humidity (when-not relative-humidity
(rel-humidity packet))
:saturation-vapour-pressure (when-not saturation-vapour-pressure

34
src/humidity/dew_point.clj Normal file
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@ -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 &deg; Celsius, given temperature
in &deg; 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 &deg; Celsius, given temperature
in &deg; 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')))))

4
src/humidity/volume.clj
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@ -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]}]

12
test/humidity/core_test.clj
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@ -26,14 +26,16 @@
(deftest resolution-test
(testing "resolution")
(let [expected {:relative-humidity 100,
:temperature-celsius 35,
(let [expected {:absolute-humidity 3953.618101887827,
: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,
:volume 1.1281347245103424}
:relative-humidity 100,
:saturation-vapour-pressure 5622.488734516426,
:actual-vapour-pressure 4765.10094206357}
packet {:relative-humidity 100 :temperature-celsius 35}
actual (resolution packet)]
(is (= actual expected))))

37
test/humidity/dew_point_test.clj
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@ -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)))))