Heat Index Calculator - Feels Like Temp
Calculate the heat index (apparent temperature) from air temperature and relative humidity. Shows NWS heat danger categories and heat illness risk levels.
Formula
HI = -42.379 + 2.049T + 10.143R - 0.225TR + adjustments
The NWS Heat Index formula (Rothfusz regression) combines air temperature and relative humidity to calculate how hot it feels. High humidity prevents sweat evaporation, making it feel hotter.
Worked Examples
Example 1: Summer Afternoon Assessment
Problem: It's 95°F with 65% humidity. Is it safe for outdoor sports?
Solution: Using NWS Heat Index equation:\nTemperature: 95°F\nRelative Humidity: 65%\n\nHI = -42.379 + 2.04901523(95) + 10.14333127(65)\n - 0.22475541(95)(65) - 0.00683783(95²)\n - 0.05481717(65²) + adjustments...\n\nHeat Index ≈ 117°F\n\nThis falls in the DANGER category:\n• Heat exhaustion likely\n• Heat stroke possible\n• Outdoor sports should be CANCELLED
Result: Heat Index: 117°F - DANGER level, cancel outdoor activities
Example 2: Early Morning Exercise
Problem: At 6 AM it's 78°F with 85% humidity. Safe for a 5K run?
Solution: Temperature: 78°F\nRelative Humidity: 85%\n\nSince temp < 80°F, use simpler formula:\nHI = 0.5(78 + 61 + (78-68)×1.2 + 85×0.094)\nHI = 0.5(78 + 61 + 12 + 7.99)\nHI ≈ 79.5°F\n\nBut this is borderline, check full equation:\nFull calculation yields ~82°F\n\n82°F = CAUTION level\n• Fatigue possible\n• 5K run is reasonable with precautions\n• Stay hydrated, watch for symptoms\n• Consider slower pace
Result: Heat Index: 82°F - Exercise OK with caution
Example 3: Workplace Safety Decision
Problem: Construction site at 92°F, 75% humidity. What work/rest schedule?
Solution: Temperature: 92°F, Humidity: 75%\n\nHeat Index calculation:\nHI ≈ 112°F (DANGER level)\n\nOSHA/NIOSH recommendations for this level:\n• Light work: 25 min work / 35 min rest per hour\n• Moderate work: 15 min work / 45 min rest per hour\n• Heavy work: Should be postponed\n\nAdditional requirements:\n• Shade/cooling stations required\n• Mandatory water breaks\n• Buddy system for symptom monitoring\n• Consider early start (5-6 AM)
Result: HI: 112°F - Limited work, extensive rest breaks required
Frequently Asked Questions
What is the heat index and how is it calculated?
Heat index (also called 'apparent temperature' or 'feels like') combines air temperature and relative humidity to show how hot it feels. High humidity prevents sweat from evaporating, reducing your body's ability to cool itself. The NWS uses the Rothfusz regression equation, which considers non-linear interactions between temperature and humidity. At 90°F with 70% humidity, the heat index is about 106°F.
Why does humidity make heat feel worse?
Your body cools itself primarily through sweating. When sweat evaporates, it removes heat from your skin. High humidity means the air already contains a lot of water vapor, so sweat can't evaporate as quickly. At 100% humidity, sweat can't evaporate at all. This is why 90°F at 90% humidity (heat index 132°F) is dangerous, while 90°F at 30% humidity (heat index 87°F) feels almost pleasant.
At what heat index should I avoid outdoor activities?
80-89°F: Use caution, stay hydrated. 90-104°F: Limit strenuous activity, take frequent breaks. 105-129°F: Cancel outdoor activities if possible, essential work only with precautions. 130°F+: No outdoor exposure except emergencies. For athletes and outdoor workers, most organizations recommend work/rest schedules starting at 90°F heat index.
What are the signs of heat-related illness?
Heat cramps: Muscle cramps and spasms, usually during/after exercise. Heat exhaustion: Heavy sweating, weakness, cold/clammy skin, nausea, dizziness, fast pulse. Move to cool area, apply cool water, hydrate. Heat stroke (EMERGENCY): High body temperature (103°F+), hot/red/dry skin, rapid pulse, confusion, unconsciousness. Call 911 immediately, cool the person rapidly.
Who is most at risk from high heat index?
Elderly (65+): Reduced ability to regulate temperature, may not feel thirsty. Infants and young children: Higher metabolic rate, less able to sweat. Outdoor workers: Prolonged exposure, physical exertion. Athletes: Intense activity generates significant body heat. People with chronic conditions: Heart disease, obesity, diabetes increase risk. Those on certain medications: Diuretics, beta-blockers, antihistamines can impair heat response.
How does the heat index affect exercise safety?
At 80°F heat index: Normal activity, stay hydrated. 85-90°F: Reduce intensity, extend rest breaks. 90-95°F: Limit to light activity, 10-15 min work/rest cycle. 95-100°F: Reduce activity to essential only, 1:1 work/rest. Above 100°F: Cancel strenuous activity. Acclimatization takes 10-14 days - gradually increase exposure. Never ignore symptoms; heat illness can progress rapidly.