The common phrase “feels like temperature” appears frequently in weather forecasts, often alongside the actual air temperature. This apparent temperature represents how current atmospheric conditions affect the human body, which can vary significantly from what a standard thermometer measures. Understanding the distinction between the overarching concept of “feels like” and the specific metric known as the Heat Index is important for personal comfort and safety. Public health officials use these metrics to warn the public about conditions that can quickly lead to heat-related illnesses or dangerous cold exposure.
The Specific Metric: Heat Index
The Heat Index is a specific, standardized calculation used exclusively during warm conditions. It quantifies the combined effect of air temperature and relative humidity on the human body. This metric is a public health tool utilized by organizations like the National Weather Service (NWS) to communicate risk, based on studies modeling how the body regulates internal temperature.
The primary mechanism for human cooling is the evaporation of sweat from the skin’s surface, which draws heat away from the body. High relative humidity inhibits this process because the air is already saturated with water vapor. Since the body’s cooling system becomes less efficient, it experiences a sensation of much greater heat than the thermometer indicates. For example, 90°F air temperature with 70% humidity results in a Heat Index of 106°F, a level considered immediately dangerous.
The NWS uses the Heat Index to determine when to issue advisories and warnings. This typically occurs when values are expected to exceed 100°F for a sustained period. These thresholds indicate conditions where the risk of heat exhaustion and heat stroke rises sharply. The Heat Index value is generally calculated assuming a person is standing in the shade with light wind.
The Broad Concept: “Feels Like” Temperature
The term “Feels Like” temperature is the broader, non-technical umbrella term used to describe how the environment affects a person’s thermal comfort. This single value encapsulates the effects of temperature, humidity, and wind speed. When temperatures are high, “Feels Like” is often used interchangeably with the Heat Index because humidity is the dominant factor influencing thermal perception.
The “Feels Like” temperature can incorporate environmental factors not included in the official Heat Index calculation. Exposure to direct sunlight, for example, can increase the perceived temperature by as much as 15°F above the calculated Heat Index. Since the Heat Index assumes conditions in the shade, the “Feels Like” temperature may be higher if the sun is intense. The “Feels Like” concept also accounts for wind, which offers a slight cooling effect in warm weather through increased evaporative cooling.
“Feels Like” is a dynamic indicator that changes based on the season and the most impactful atmospheric element. It is the single value displayed on weather applications that switches its underlying calculation depending on whether conditions are hot or cold. During warm months, the calculation defaults to the Heat Index to reflect the danger of high humidity.
The Cold Weather Counterpart: Wind Chill
To provide a comprehensive “Feels Like” temperature year-round, the calculation switches to the Wind Chill Index when conditions are cold. Wind Chill is the specific metric describing how cold people and animals feel outside due to the combination of air temperature and wind speed. This calculation is entirely separate from the Heat Index and is typically only defined for temperatures at or below 50°F.
The Wind Chill Index quantifies the rate of heat loss from exposed skin, which accelerates as wind speed increases. Moving air continuously draws away the thin layer of warmth the body naturally creates, driving down skin temperature more rapidly. The faster the wind blows, the quicker the body loses heat, resulting in a much colder sensation than the actual air temperature.
The Wind Chill calculation is based on a human face model and standardized to accurately reflect the risk of frostbite and hypothermia. This value is the primary driver of the “Feels Like” temperature during the colder half of the year, warning the public about dangerously cold conditions. For example, a 20°F air temperature with a 15 mph wind creates a Wind Chill of 4°F, indicating a high risk of frostbite within 30 minutes.