Heat Index & Humidity Comfort Calculator

Understanding Heat Index and Thermal Comfort

Heat index, also called apparent temperature or "feels like" temperature, accounts for how humidity combines with air temperature to affect how hot the environment truly feels. On a 90°F day with 70% humidity, people feel significantly hotter than on a dry 90°F day with 20% humidity. This difference occurs because humidity impairs the body'"'"'s ability to cool itself through perspiration, the primary mechanism for thermoregulation in hot environments. Understanding heat index is essential for assessing heat stress risk, planning outdoor activities, and protecting vulnerable populations from heat illness.

The human body maintains a relatively constant core temperature (approximately 98.6°F) through metabolic heat production and heat dissipation. When environmental temperature rises, the body loses less heat through radiation and convection. In response, the body increases perspiration to promote evaporative cooling. However, evaporation only occurs when sweat can evaporate from the skin surface. When humidity is high, the air is already saturated with water vapor, slowing or preventing evaporation. This traps heat in the body, raising core temperature and increasing cardiovascular strain. Extended exposure to high heat index conditions can lead to heat cramps, heat exhaustion, heat stroke, and potentially fatal hyperthermia.

Heat illness occurs along a spectrum of severity. Heat cramps are involuntary muscular spasms caused by electrolyte depletion through heavy sweating. Heat exhaustion is characterized by excessive sweating, weakness, dizziness, nausea, and a core body temperature of 104–105°F. The skin may be cool and clammy. Heat exhaustion is reversible with cooling and hydration but requires immediate intervention. Heat stroke is life-threatening; it occurs when core body temperature exceeds 104°F and the body'"'"'s thermoregulatory system fails. The person may stop sweating despite high temperature, become confused, lose consciousness, or experience seizures. Heat stroke is a medical emergency requiring immediate hospital care.

Several factors influence individual susceptibility to heat illness. Age is significant: very young children and elderly individuals have reduced thermoregulatory capacity. Children'"'"'s bodies heat up faster and cool down slower than adults'"'"', and elderly individuals have diminished sweating responses and may be taking medications that interfere with thermoregulation. Fitness level matters: sedentary individuals dissipate heat less efficiently than those who exercise regularly. Dehydration increases risk substantially; even mild dehydration impairs thermoregulation. Certain medical conditions and medications elevate risk. Prior heat illness significantly increases susceptibility to future episodes. Heat acclimatization—physiological adaptation over 1–2 weeks in hot environments—substantially reduces risk by improving cardiovascular stability and sweat efficiency.

MathML Formula:

where T is temperature in Fahrenheit and RH is relative humidity as a percentage. The nonlinear formula accounts for the interaction between temperature and humidity and provides accurate heat index values for realistic environmental conditions. Research shows this formula provides accurate estimates for temperatures above 80°F.

Worked Example: Consider a scenario with measured air temperature of 95°F and 60% relative humidity. The heat index calculation yields approximately 107°F. To a person outdoors, the environment feels like 107°F, not 95°F. This difference is critical for assessing heat stress. An athlete exercising in these conditions experiences significantly greater cardiovascular strain and heat illness risk. If the humidity rises to 80% while temperature remains 95°F, the heat index climbs to approximately 121°F. In such conditions, heat stroke risk becomes severe for most populations.

Comparison table showing heat index categories and risk levels:

Limitations and Assumptions: This calculator uses the standard Rothfusz heat index formula, most accurate for outdoor conditions with shade and light wind. It does not account for solar radiation, which significantly increases thermal stress. Conditions in direct sunlight can feel 10–15°F hotter than calculated heat index. Always monitor for heat illness symptoms regardless of calculated heat index: excessive fatigue, weakness, dizziness, nausea, or disorientation warrant immediate cooling and medical attention. Heat illness risk varies based on prior heat exposure, fitness level, medical conditions, medications, hydration status, and genetics. Always consult medical professionals in extreme conditions.