From f681b8e861052766f9ea5882cbf48dda9d4f72ac Mon Sep 17 00:00:00 2001
From: Frankie <frankie.lc@gmail.com>
Date: Thu, 2 Apr 2020 11:28:58 +0100
Subject: [PATCH] heat index with refinements from NWS for lower values

---
 DHT.cpp | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/DHT.cpp b/DHT.cpp
index 14e1d85..96bc17e 100644
--- a/DHT.cpp
+++ b/DHT.cpp
@@ -175,9 +175,12 @@ float DHT::computeHeatIndex(bool isFahrenheit) {
 }
 
 /*!
- *  @brief  Compute Heat Index
- *  				Using both Rothfusz and Steadman's equations
+ *  @brief  Compute Heat Index (based on Robert G. Steadman tables)
+ *  				Using Rothfusz equations with refinements by the NWS
  *					(http://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml)
+ *                  and NWS algorithm that respects lower values of temperature
+ *                  and humidity
+ *                  (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3801457/)
  *  @param  temperature
  *          temperature in selected scale
  *  @param  percentHumidity
@@ -193,6 +196,15 @@ float DHT::computeHeatIndex(float temperature, float percentHumidity,
   if (!isFahrenheit)
     temperature = convertCtoF(temperature);
 
+  if(temperature <= 40) {
+    return isFahrenheit ? temperature : convertFtoC(temperature);
+  }
+
+  float varA = -10.3 + 1.1 * temperature + 0.047 * percentHumidity;
+  if (varA < 79) {
+    return isFahrenheit ? varA : convertFtoC(varA);
+  }
+
   hi = 0.5 * (temperature + 61.0 + ((temperature - 68.0) * 1.2) +
               (percentHumidity * 0.094));