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How to Calculate the THW Index

High humidity can make the outdoors feel warmer and exacerbate allergic reactions. At the same time, high winds can make a warm day feel cooler than the reported temperature. By calculating the temperature-humidity-wind (THW) index, you will gain offer a more accurate understanding of outdoor weather conditions and your expected comfort level.

Things You'll Need

  • Anenometer
  • Psychrometer
  • Calculator
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Instructions

  1. Instructions

    • 1

      Obtain the outdoor temperature in degrees Fahrenheit using the psychrometer. The psychrometer measures a dry bulb and a wet bulb temperature. The outdoor temperature is the dry bulb temperature.

    • 2

      Calculate the outdoor relative humidity using the psychrometer. The relative humidity is the difference between the dry bulb and wet bulb values, expressed as a rounded percentage.

    • 3

      Calculate the heat index using the formula HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 - 5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-4TR2 - 1.99x10-6T2R2. The variable T is the outdoor temperature and the variable R is the relative humidity.

    • 4

      Obtain the outdoor wind speed in miles per hour using the anenometer. To improve accuracy, hold the anenometer a few feet above the ground.

    • 5

      Calculate the THW Index using the formula THW = HI - (1.072 * W). The variable HI is the heat index, and the variable W is the wind speed.

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