code: def usaheatindex(temp, humidity, dew=None):
      
      if temp is None or humidity is None:
          return None
      if dew is None:
          dew = dew_point(temp, humidity)
      if temp < 26.7 or humidity < 40 or dew < 12.0:
          return temp
      T = (temp * 1.8) + 32.0
      R = humidity
      c_1 = -42.379
      c_2 = 2.04901523
      c_3 = 10.14333127
      c_4 = -0.22475541
      c_5 = -0.00683783
      c_6 = -0.05481717
      c_7 = 0.00122874
      c_8 = 0.00085282
      c_9 = -0.00000199
      return ((c_1 + (c_2 * T) + (c_3 * R) + (c_4 * T * R) + (c_5 * (T**2)) +
               (c_6 * (R**2)) + (c_7 * (T**2) * R) + (c_8 * T * (R**2)) +
               (c_9 * (T**2) * (R**2))) - 32.0) / 1.8
docs: Calculate Heat Index as per USA National Weather Service Standards
  
      See http://en.wikipedia.org/wiki/Heat_index, formula 1. The
      formula is not valid for T < 26.7C, Dew Point < 12C, or RH < 40%