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: Temperature: 95°F Relative Humidity: 65% HI = -42.379 + 2.04901523(95) + 10.14333127(65) - 0.22475541(95)(65) - 0.00683783(95²) - 0.05481717(65²) + adjustments... Heat Index ≈ 117°F This falls in the DANGER category: • Heat exhaustion likely • Heat stroke possible • 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 Relative Humidity: 85% Since temp < 80°F, use simpler formula: HI = 0.5(78 + 61 + (78-68)×1.2 + 85×0.094) HI = 0.5(78 + 61 + 12 + 7.99) HI ≈ 79.5°F But this is borderline, check full equation: Full calculation yields ~82°F 82°F = CAUTION level • Fatigue possible • 5K run is reasonable with precautions • Stay hydrated, watch for symptoms • 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% Heat Index calculation: HI ≈ 112°F (DANGER level) OSHA/NIOSH recommendations for this level: • Light work: 25 min work / 35 min rest per hour • Moderate work: 15 min work / 45 min rest per hour • Heavy work: Should be postponed Additional requirements: • Shade/cooling stations required • Mandatory water breaks • Buddy system for symptom monitoring • 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.
What's the difference between heat index and wet bulb temperature?
Heat index shows perceived temperature based on humidity's effect on evaporative cooling. Wet bulb temperature (WBT) is the lowest temperature achievable through evaporation alone - measured by a thermometer wrapped in wet cloth. At 100% humidity, wet bulb = air temperature. WBT above 95°F is potentially fatal for healthy humans regardless of shade or water, as the body cannot cool itself. Heat index better represents discomfort; wet bulb represents physiological limits.
How do I calculate heat index without a calculator?
For a quick estimate: At 80°F with 50% humidity, heat index ≈ actual temp. Each 10% increase in humidity above 50% adds about 3-5°F to heat index. Each 10°F increase in temp (above 80°F) with high humidity adds 15-20°F to heat index. But these are rough - use a calculator or chart for accurate values, especially in dangerous conditions.
Does wind reduce the heat index?
Standard heat index assumes calm conditions in shade. Wind can help if temp is below body temperature (98.6°F) by improving evaporation. However, wind when heat index exceeds body temp actually makes things worse - like a convection oven. Sun exposure adds 10-15°F to perceived heat. So actual conditions may be significantly more dangerous than calculated heat index.
What precautions should I take during high heat index?
Hydration: Drink before you're thirsty, avoid alcohol/caffeine. Clothing: Light colors, loose-fitting, moisture-wicking. Timing: Do outdoor activities in early morning or evening. Cooling: Use fans, AC, cool showers, wet towels. Rest: Take frequent breaks in cool areas. Buddy system: Watch others for heat illness signs. Avoid: Leaving anyone in parked cars (temps can reach 150°F+).
Background & Theory
The heat index measures how hot it feels when humidity is factored in with actual air temperature. Understanding heat index is critical for preventing heat-related illness and death.
How Your Body Cools Itself:
Your body maintains ~98.6°F core temperature through:
- Radiation: Heat transfers to cooler surroundings
- Convection: Air movement carries heat away
- Evaporation: Sweating (most important in heat)
When air temp approaches body temp, evaporation becomes the primary cooling method. High humidity severely impairs evaporation.
The Heat Index Formula:
The Rothfusz regression (NWS):
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²
Where T = temperature (°F), R = relative humidity (%)
Danger Levels:
| Heat Index | Category | Risk |
|---|---|---|
| < 80°F | Safe | Normal activity |
| 80-89°F | Caution | Fatigue possible |
| 90-104°F | Extreme Caution | Cramps/exhaustion possible |
| 105-129°F | Danger | Cramps/exhaustion likely |
| ≥ 130°F | Extreme Danger | Heat stroke imminent |
Heat-Related Illnesses:
Heat Cramps:
- Muscle cramps during/after exertion
- Caused by electrolyte loss
- Treatment: Rest, fluids, light stretching
Heat Exhaustion:
- Heavy sweating, weakness, nausea
- Cool, clammy skin
- Fast, weak pulse
- Treatment: Cool environment, fluids, remove excess clothing
Heat Stroke (EMERGENCY):
- Body temp > 103°F
- Hot, red, dry OR clammy skin
- Rapid, strong pulse
- Confusion, loss of consciousness
- Treatment: Call 911, rapid cooling
Risk Factors:
Individual:
- Age (elderly and very young)
- Obesity
- Dehydration
- Fever or illness
- Poor cardiovascular fitness
- Previous heat illness
Medication Effects:
- Diuretics (increase dehydration)
- Beta-blockers (limit heart rate)
- Antihistamines (reduce sweating)
- Antipsychotics (impair thermoregulation)
Environmental:
- Full sun adds 10-15°F to heat index
- No wind reduces evaporative cooling
- Physical activity generates body heat
Prevention Strategies:
Hydration:
- 8 oz water every 15-20 min during activity
- Don't wait until thirsty
- Avoid alcohol and caffeine
- Sports drinks for prolonged activity
Clothing:
- Light colors reflect heat
- Loose fit allows air circulation
- Moisture-wicking fabrics
Activity Modification:
- Schedule outdoor work for coolest hours
- Take frequent breaks in shade
- Use work/rest schedules based on heat index
- Acclimatize gradually (10-14 days)
Workplace Requirements:
OSHA recommends (and may require):
- Access to water, shade, and rest
- Heat illness prevention training
- Acclimatization procedures
- Emergency response plan
- Buddy system to watch for symptoms
Special Considerations:
Sports:
ACSM guidelines recommend:
- Cancel when heat index > 104°F
- Modify practice at 90°F+
- Mandatory water breaks
- Extended rest periods
- Monitor athletes closely
Children:
- Higher metabolic rate per body mass
- Less efficient sweating
- May not recognize dehydration
- Never leave in parked cars
Elderly:
- Reduced thirst sensation
- Less efficient thermoregulation
- May not perceive heat as dangerous
- Check on regularly during heat waves