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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
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