Heat Index Calculator
Calculate Perceived Temperature & Heat Illness Risk - Heat Index Assessment Tool
🌡️ Pro Tips for Heat Index Calculations:
Heat index accounts for humidity's effect on body cooling. Formula: complex polynomial using temperature + humidity. Valid for 80°F+ at 40%+ humidity. Below 80°F: minimal effect. High humidity dramatically increases perceived temperature. Heat index > 103°F: heat exhaustion risk. Heat index > 125°F: heat stroke imminent.
Heat Index Calculator - Using Relative Humidity
✅ Heat Index Results:
Heat Index Calculator - Using Dew Point
✅ Heat Index Results:
Heat Index Chart with Risk Levels
Temperature (°F) × Relative Humidity (%) = Heat Index (°F)
| RH % | 80°F | 82°F | 84°F | 86°F | 88°F | 90°F | 92°F | 94°F | 96°F | 98°F | 100°F | 105°F | 110°F |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 40% | 80 | 81 | 83 | 85 | 88 | 91 | 94 | 97 | 101 | 105 | 109 | 114 | 124 |
| 45% | 80 | 82 | 84 | 87 | 89 | 93 | 96 | 100 | 104 | 109 | 114 | 119 | 137 |
| 50% | 81 | 83 | 85 | 88 | 91 | 95 | 99 | 103 | 108 | 113 | 118 | 124 | |
| 55% | 81 | 84 | 86 | 89 | 93 | 97 | 101 | 106 | 112 | 117 | 124 | ||
| 60% | 82 | 84 | 88 | 91 | 95 | 100 | 105 | 110 | 115 | 123 | |||
| 65% | 82 | 85 | 89 | 93 | 98 | 103 | 108 | 114 | 121 | ||||
| 70% | 83 | 86 | 90 | 95 | 100 | 105 | 112 | 119 | |||||
| 75% | 84 | 88 | 92 | 97 | 103 | 109 | 116 | 124 | |||||
| 80% | 84 | 89 | 94 | 100 | 106 | 113 | 121 | 129 | |||||
| 85% | 85 | 90 | 96 | 102 | 110 | 117 | 126 | 135 | |||||
| 90% | 86 | 91 | 98 | 105 | 113 | 122 | 137 | ||||||
| 95% | 86 | 93 | 100 | 108 | 117 | 127 | |||||||
| 100% | 87 | 95 | 103 | 112 | 121 | 132 |
Caution (80-90°F)
Extreme Caution (90-105°F)
Danger (105-130°F)
Extreme Danger (130°F+)
Heat-Related Illness & Prevention
Heat Index Risk Levels & Effects:
| Heat Index | Risk Level | Potential Effects & Precautions |
|---|---|---|
| 80-90°F | Caution | Fatigue possible with prolonged exposure. Continuing activity could cause heat cramps. Stay hydrated, take breaks. |
| 90-105°F | Extreme Caution | Heat cramps, heat exhaustion possible. Continuing activity increases heat stroke risk. Limit outdoor time, frequent breaks, hydrate. |
| 105-130°F | Danger | Heat cramps and exhaustion likely. Heat stroke probable with continued activity. Reduce strenuous activity. Hydrate constantly. |
| 130°F+ | Extreme Danger | Heat stroke imminent. Avoid outdoor activity. Seek air conditioning. Medical attention may be required. |
🚨 Heat-Related Illness Symptoms:
Heat Cramps: Muscle pain/spasms. Heavy sweating. Stop activity, seek cool area, hydrate.
Heat Exhaustion: Excessive sweating, weakness, dizziness, nausea, headache, cool/clammy skin. Seek cool area, hydrate, rest. Apply cool, wet cloths.
Heat Stroke (MEDICAL EMERGENCY): Confusion, loss of consciousness, seizures, high body temperature (103°F+), hot/dry skin, no sweating. Call 911 immediately. Move to cool area. Apply ice packs to neck, armpits, groin.
Heat Exhaustion: Excessive sweating, weakness, dizziness, nausea, headache, cool/clammy skin. Seek cool area, hydrate, rest. Apply cool, wet cloths.
Heat Stroke (MEDICAL EMERGENCY): Confusion, loss of consciousness, seizures, high body temperature (103°F+), hot/dry skin, no sweating. Call 911 immediately. Move to cool area. Apply ice packs to neck, armpits, groin.
Heat Safety Prevention Strategies:
- Hydration: Drink water continuously. Sports drinks for extended activity. Thirst is late sign of dehydration.
- Activity Timing: Avoid outdoor activity 10am-4pm (peak heat). Exercise early morning or evening.
- Clothing: Wear light, loose, breathable fabrics. Light colors reflect heat. Hat provides head protection.
- Breaks: Take frequent rest breaks in shade or air conditioning. Reduce intensity during heat index extremes.
- Vulnerable Groups: Extra monitoring for elderly, children, chronic conditions. Ensure adequate hydration/rest.
- Never Leave Unattended: Children/pets in cars reach dangerous temperatures in minutes (120°F+ interior in hot weather).
Heat Index Reference & Information
Heat Index Formula (National Weather Service):
HI = c₁ + c₂T + c₃RH + c₄TRH + c₅T² + c₆RH²
+ c₇T²RH + c₈TRH² + c₉T²RH²
Where: T = temperature (°F), RH = relative humidity (%)
Formula coefficients determined empirically
Valid for: T ≥ 80°F and RH ≥ 40%
Developed by George Winterling 1978
+ c₇T²RH + c₈TRH² + c₉T²RH²
Where: T = temperature (°F), RH = relative humidity (%)
Formula coefficients determined empirically
Valid for: T ≥ 80°F and RH ≥ 40%
Developed by George Winterling 1978
Below 80°F or humidity below 40%: use actual temperature (minimal humidity effect)
Relative Humidity vs. Dew Point:
| Measurement | Definition | Advantages | Usage |
|---|---|---|---|
| Relative Humidity | Percentage of maximum moisture air holds at current temperature | Commonly reported, familiar to public | Heat index formula primary input |
| Dew Point | Temperature at which moisture condenses | Direct humidity indicator, less variable, directly related to comfort | Alternative input, weather services report |
Key Heat Index Facts:
Heat index only affects living organisms with exposed skin. Does NOT affect air temperature or thermometer readings. High humidity most problematic at high temperatures (exponential effect). Sunlight can increase heat index 14°F+ above calculation. Vulnerable groups at greater risk. Water immersion/activities increase dehydration risk.
🌡️ Understanding Heat Index & Summer Safety
What is Heat Index?:
Heat index: perceived temperature accounting for air temperature and humidity. Body cools through perspiration evaporation. High humidity reduces evaporative efficiency. Result: body feels hotter than thermometer reads. Example: 85°F with 70% humidity feels like 93°F. Formula developed empirically by George Winterling 1978 for temperatures 80°F+ with 40%+ humidity.
Physiological Effects of Heat:- Thermoregulation: Body attempts maintaining 98.6°F core temperature. Sweating primary cooling mechanism. Extreme heat overwhelms thermoregulation.
- Dehydration: Excessive sweating depletes body fluids. Thirst is late dehydration sign. Leads to reduced blood volume, compromised cooling.
- Heat Exhaustion: Mild-to-moderate heat illness. Body still sweating (functional thermoregulation). Treatable through cooling and hydration.
- Heat Stroke: Medical emergency. Thermoregulation fails. Core temperature exceeds 104°F. Sweating may stop. Requires immediate emergency treatment.
- Elderly: Less efficient heat regulation, reduced thirst, chronic conditions increase risk.
- Children: Immature thermoregulation, less aware of overheating, tendency to overexert.
- Chronic Conditions: Obesity, diabetes, heart disease, respiratory conditions impair heat tolerance.
- Medications: Some drugs impair heat tolerance. Stimulants increase heat production.
- Continuous Hydration: Drink water regularly, not just when thirsty. Sports drinks for extended activity.
- Limit Outdoor Exposure: Peak heat 10am-4pm. Schedule outdoor activities early morning/evening.
- Appropriate Clothing: Light colors, loose fit, breathable fabrics. Hats provide head protection.
- Frequent Breaks: Rest in shade/AC. Reduce activity intensity at extreme heat index.
- Buddy System: Monitor companions for heat illness symptoms. Check on vulnerable individuals regularly.
- Never Leave Unattended: Cars reach 130°F+ interior temperature in minutes. Children/pets cannot be left alone.
❓ Frequently Asked Questions
Why does humidity make temperature feel hotter?
+
Body cools through perspiration evaporation. High humidity saturates air with moisture, reducing evaporation rate. Sweat accumulates on skin without cooling effect. Heat traps in body. Result: body feels hotter than actual temperature. Example: 85°F with 90% humidity feels 10°F hotter than 85°F with 40% humidity.
Can dew point predict discomfort level?
+
Yes, dew point directly indicates humidity/discomfort. Below 60°F: comfortable. 60-70°F: normal. 70°F+: humid/uncomfortable. 80°F+: oppressive. Dew point less variable than relative humidity. At high temperatures with high dew point: extremely uncomfortable/dangerous.
What should I do if someone has heat stroke?
+
CALL 911 IMMEDIATELY. Heat stroke is medical emergency. Move person to cool area. Apply ice packs to neck, armpits, groin (major blood vessels). Cool with water spray if available. Monitor breathing. Do NOT give oral fluids if unconscious. Continue cooling until emergency arrives.
How much hotter does sun exposure increase heat index?
+
Full sunshine can increase heat index 14°F above calculation. Shade reduces apparent temperature. Reflective surfaces (pavement, water) increase heating. Dark clothing absorbs more heat. Light-colored, breathable clothing recommended for hot conditions.
Why is heat index minimal below 80°F?
+
Below 80°F: body cooling mostly effective regardless of humidity. Perspiration evaporates efficiently. Formula valid for 80°F+ where humidity significantly impairs cooling. At lower temperatures: actual temperature is adequate descriptor.
How quickly does dehydration cause problems?
+
Even 1-2% dehydration impairs physical/cognitive performance. Heat exhaustion risk increases significantly. Thirst is late dehydration sign. Continuous hydration critical in heat. Monitor urine color (dark = dehydrated). Drink water before feeling thirsty.
Can children handle heat as well as adults?
+
No, children less able to handle heat. Immature thermoregulation, lower sweat capacity, higher heat production during activity. Children more vulnerable to heat-related illness. Larger skin surface relative to body mass. Often unaware of overheating. Require aggressive hydration/cooling strategies.
Why is alcohol consumption risky in heat?
+
Alcohol impairs thermoregulation and judgment. Promotes dehydration (diuretic effect). Reduces sweating efficiency. Vasodilation increases heat loss/core temperature drop. Prevents recognition of heat-related illness symptoms. Avoid alcohol during heat exposure/outdoor activities in heat.