“Feels like” temperature, also known as apparent temperature, offers a more comprehensive understanding of how outdoor conditions affect human comfort and safety. It provides a realistic sense of what the air truly feels like to the human body, considering various atmospheric elements beyond just air temperature. This measurement serves as an important tool in daily weather forecasts and public health advisories.
Why Air Temperature Isn’t Enough
Air temperature alone does not fully capture how hot or cold it feels because the human body constantly exchanges heat with its surroundings. This heat exchange is influenced by several environmental factors beyond just the air temperature, including humidity, wind, and solar radiation.
Humidity impacts how effectively the body cools itself through sweat evaporation. When the air is humid, sweat evaporates more slowly, hindering the body’s natural cooling and making it feel warmer. Wind influences the rate of heat loss, stripping away the insulating layer of warm air near the skin and accelerating heat transfer. Direct sunlight also adds considerable heat, making conditions feel hotter than indicated in the shade. These combined effects necessitate a more nuanced calculation to represent perceived temperature accurately.
Calculating Heat Stress
In warm conditions, the “feels like” temperature is represented by the Heat Index. This index quantifies how hot it feels when humidity is high, combining actual air temperature with relative humidity. Its purpose is to alert people to conditions that can increase the risk of heat-related illnesses.
The Heat Index relates to the body’s natural cooling mechanism: sweating. High relative humidity reduces the rate at which sweat can evaporate, meaning the body struggles to release heat efficiently. This leads to a sensation of feeling much hotter than the measured air temperature. For instance, 32°C (90°F) with 70% relative humidity can feel like 41°C (106°F). The Heat Index is for shaded areas; direct sunlight can increase the perceived temperature by up to 8°C (15°F).
Calculating Cold Stress
In cold environments, the “feels like” temperature is expressed through the Wind Chill. This calculation uses both the air temperature and the wind speed as its primary inputs. The Wind Chill indicates the rate of heat loss from exposed skin, providing an approximation of how cold the air feels on the body.
Wind Chill works because moving air rapidly removes heat from the body’s surface. Wind disrupts the insulating layer of warm air around the skin, constantly replacing it with colder air. This accelerates heat transfer, making the temperature feel significantly colder than what a thermometer would read in still air. For example, -18°C (0°F) with a 24 km/h (15 mph) wind can feel like -28°C (-19°F), increasing the risk of frostbite. Wind Chill affects living organisms but does not cool inanimate objects below the actual air temperature.
The Science Behind Apparent Temperature
“Feels like” temperatures, such as the Heat Index and Wind Chill, are derived values based on scientific models and human physiological responses. These models were developed through research, often involving experimental studies on human subjects to understand how the body gains or loses heat. Scientists observed how people perceive temperature under controlled settings to create mathematical formulas.
Meteorological organizations, such as the U.S. National Weather Service and Environment Canada, standardize these calculations using specific formulas. These indices are used for official weather forecasts and public safety advisories. While these models provide guidance, individual perception can vary due to personal factors like clothing, activity levels, and acclimatization.