The “feels like” temperature, technically known as the apparent temperature, is a calculation that adjusts the air temperature to reflect how the environment truly feels to a human being. This value is not measured directly by a thermometer but is computed using a model that incorporates atmospheric variables affecting the body’s ability to regulate its own temperature. The apparent temperature translates objective meteorological measurements into a subjective human experience of thermal comfort. It accounts for conditions that make a given air temperature feel significantly warmer or colder than the reading on a standard thermometer.
The Science of Thermal Regulation
The human body constantly works to maintain a stable internal temperature, a process called thermoregulation, by balancing heat production and heat loss. Heat is exchanged with the environment through four primary avenues: radiation, conduction, convection, and evaporation.
Evaporative cooling works because the conversion of liquid water on the skin into water vapor requires heat energy drawn directly from the body’s surface. The effectiveness of this process is influenced by the moisture present in the air, known as humidity. High humidity means the air is saturated with water vapor, decreasing the pressure gradient between the skin and the air, which severely limits the rate at which sweat can evaporate.
Convection involves the transfer of heat through the movement of air or water molecules across the skin. When the air is cooler than the skin, a thin, insulating layer of warmer air forms next to the body. Wind disrupts this boundary layer, stripping away the warm air and replacing it with cooler air, which accelerates heat loss.
The rate of convective heat loss is directly proportional to the speed of the air movement. In cold conditions, even a slight breeze can rapidly increase the rate at which the body loses heat. Understanding how humidity and wind affect these two primary cooling mechanisms—evaporation and convection—is the foundation for calculating the apparent temperature.
The Heat Index (Warm Weather Calculation)
The Heat Index is the specific calculation used to determine the apparent temperature in warm conditions, where high air temperature and high relative humidity create dangerous heat stress. This index measures how much humidity impedes the body’s ability to cool itself through sweat evaporation. When sweat cannot evaporate quickly, the body retains heat, causing the internal temperature to rise.
The calculation relies solely on two input variables: the air temperature and the relative humidity. The current Heat Index used by the U.S. National Weather Service is a complex multiple regression equation, developed in 1990 to approximate a biometeorological model. This model, originally developed by Robert G. Steadman in 1979, links environmental conditions to the physiological state of a hypothetical adult human.
The regression equation simplifies the original model, treating factors like wind speed and solar radiation as constants. The formula assumes a light wind speed (typically around 5 knots) and assumes a person is in the shade, with no direct solar radiation. This simplification allows for a quick and standardized calculation, but it means the Heat Index can underestimate the apparent temperature in direct sunlight.
The Heat Index is applied when the air temperature is 80°F or higher, as humidity’s effect on evaporative cooling becomes significant at these temperatures. High Heat Index values are correlated with an increased risk of heat-related illnesses, such as heat exhaustion and heatstroke. For example, a temperature of 90°F with a relative humidity of 70% results in a Heat Index of approximately 105°F, which is considered a danger level.
The Wind Chill Index (Cold Weather Calculation)
The Wind Chill Index is the counterpart calculation designed for cold, windy conditions, which quantifies the accelerated rate of heat loss from exposed skin. This index measures the cooling effect of wind on the human body, focusing on the convective heat transfer mechanism. The Wind Chill Index is defined only for temperatures at or below 50°F and relies on the air temperature and the wind speed as its two variables.
The index does not measure how cold an object (such as a car or a water pipe) will get; wind chill can never lower the temperature of an inanimate object below the actual air temperature. Instead, it measures the temperature equivalent that would result in the same heat loss on exposed skin if the wind were calm. The primary health risk associated with low wind chill is the potential for frostbite and hypothermia, which are results of rapid heat loss.
The current standard Wind Chill Temperature formula was implemented in 2001 by a joint action group of meteorologists and medical experts. This revision replaced an older index based on the freezing time of water in plastic bottles and was found to significantly overestimate the cooling effect. The updated formula is based on scientific modeling of heat transfer from a human face.
Key improvements in the 2001 model included adjusting the wind speed measurement from the standard height of 33 feet down to the average human face level of 5 feet. The model also incorporated modern heat transfer theory and assumed a worst-case scenario for solar radiation, such as a clear night sky. The resulting index is a more accurate representation of the thermal stress caused by wind on exposed skin, providing a better metric for issuing public safety warnings.