The “feels like” or apparent temperature measurement is an attempt to quantify human perceptions of
temperature by taking into account additional factors such as relative humidity and wind speed. This is
represented in two calculations, the heat index and wind chill.
The Heat Index correlates measured air temperature with relative humidity. When air temperature is
combined with high relative humidity, the body’s evaporative cooling system (perspiration) becomes
less efficient, causing the temperature to feel warmer than the measured value. Conversely, on a warm
day with low relative humidity, the perceived temperature may be lower than the measured value due
to the accelerated evaporation of perspiration.
The heat index calculation applies only when the relative humidity is 40% or higher, and the air
temperature is 26.6 C or higher. At lower temperatures and relative humidity values, it is assumed that
the measured temperature is similar to the perceived temperature.
On a cold day, even small increases in wind speed will more efficiently transfer heat away from exposed
skin and cause one to feel much colder. This is represented as wind-chill.
The original calculations of wind-chill, used widely until 2001, were based on the rate at which water
froze as wind speed increased. This calculation tended to produce values much colder than were
accurate. Since 2001, the National Weather Service has adopted new wind-chill calculations based on
the heat transfer of bare skin exposed to the wind. The USU Environmental Observatory uses this newer
Wind-chill is calculated when temperatures are at or below 10 C and wind speeds are above 1.33 meters
"Feels Like" Temperature