# File lib/wx/metar.rb, line 257
    def relative_humidity
      if (!dewpoint.nil? && !temp.nil?)
        es0 = 6.11 # hPa
        t0 = 273.15 # kelvin
        td = self.dewpoint.to('tempK').abs
        t = self.temp.to('tempK').abs
        lv = 2500000 # joules/kg
        rv = 461.5 # joules*kelvin/kg
        e  = es0 * Math::exp(lv/rv * (1.0/t0 - 1.0/td))
        es = es0 * Math::exp(lv/rv * (1.0/t0 - 1.0/t))
        rh = 100 * e/es
        (rh.to_s+'%').unit
      else
        nil
      end
    end