The temperature humidity wind index, or THW, determines what the temperature actually feels like based on the heat index and wind chill combined. For example, the temperature could be 50 degrees Fahrenheit, but due to high wind chill and heat index factors, the THW determines it feels like 40 F.
The heat index alone calculates perceived temperature based on humidity factors. A higher humidity generally results in a higher perceived temperature. In THW, the temperature-reducing effect of wind is also considered in determining what a temperature actually feels like. The wind chill is based on the flow of air molecules from high pressure areas to low pressure ones. The greater the difference in air pressure, the faster the wind blows.