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+{-# LANGUAGE NamedFieldPuns #-}
+
+module Web.OpenWeatherMap.Formulas
+  ( absoluteHumidity
+  ) where
+
+import Web.OpenWeatherMap.Types.Main
+
+{-- | Calculate absolute humidity (g/m³)
+
+Returns 'Nothing' if the temperature is out of range (−30°C,  +35°C).
+
+-}
+absoluteHumidity :: Main -> Maybe Double
+absoluteHumidity Main {temp, humidity}
+  | tC > -30 && tC < 35 = Just $ ahBolton1980 temp humidity
+  | otherwise = Nothing
+  where
+    tC = k2c temp
+
+{-- | Calculate absolute humidity (g/m³)
+
+Ref.: Bolton D. "The Computation of Equivalent Potential Temperature",
+Monthly Weather Review, 1980, 108(7):1046–1053.
+
+-}
+ahBolton1980 ::
+     Double -- ^ Temperature in Kelvins
+  -> Double -- ^ Relative humidity in %
+  -> Double
+ahBolton1980 temp humidity =
+  13.25 * humidity * exp (17.67 * tC / (tC + 243.5)) / temp
+  where
+    tC = k2c temp
+
+k2c :: Double -> Double
+k2c t = t - 273.15