What Is Heat Index?
Heat index, also called "apparent temperature," is what the temperature feels like when humidity is factored in. High humidity reduces your body's ability to cool through sweating, making the effective temperature much higher than the actual air temperature.
Quick Reference Heat Index Chart
Heat Index Table (Temperature °F vs. Relative Humidity %)
| Temp | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% |
|---|---|---|---|---|---|---|---|---|---|
| 80°F | 80 | 80 | 81 | 81 | 82 | 82 | 83 | 84 | 84 |
| 82°F | 81 | 82 | 83 | 84 | 84 | 85 | 86 | 88 | 89 |
| 84°F | 83 | 84 | 85 | 86 | 88 | 89 | 90 | 92 | 94 |
| 86°F | 85 | 87 | 88 | 89 | 91 | 93 | 95 | 97 | 100 |
| 88°F | 88 | 89 | 91 | 93 | 95 | 98 | 100 | 103 | 106 |
| 90°F | 91 | 93 | 95 | 97 | 100 | 103 | 106 | 109 | 113 |
| 92°F | 94 | 96 | 99 | 101 | 105 | 108 | 112 | 116 | 121 |
| 94°F | 97 | 100 | 103 | 106 | 110 | 114 | 119 | 124 | 129 |
| 96°F | 101 | 104 | 108 | 112 | 116 | 121 | 126 | 132 | — |
| 98°F | 105 | 109 | 113 | 117 | 123 | 128 | 134 | — | — |
| 100°F | 109 | 114 | 118 | 124 | 129 | 136 | — | — | — |
| 102°F | 114 | 119 | 124 | 130 | 137 | — | — | — | — |
| 104°F | 119 | 124 | 131 | 137 | — | — | — | — | — |
| 106°F | 124 | 130 | 137 | — | — | — | — | — | — |
| 108°F | 130 | 137 | — | — | — | — | — | — | — |
| 110°F | 136 | — | — | — | — | — | — | — | — |
Heat Index Danger Levels
Risk Categories
| Heat Index | Category | Risk | Action Required |
|---|---|---|---|
| Under 80°F | Low | Low heat illness risk | Normal precautions |
| 80-90°F | Caution | Fatigue possible with prolonged exposure | Basic precautions |
| 91-103°F | Extreme Caution | Heat exhaustion possible | Enhanced precautions |
| 103-124°F | Danger | Heat exhaustion likely, heat stroke possible | Protective measures required |
| 125°F+ | Extreme Danger | Heat stroke highly likely | Limit or stop outdoor work |
Work/Rest Guidelines by Heat Index
Recommended Schedules
Below 91°F: Moderate work at 50 minutes work per 10 minutes rest per hour. Heavy work at 40 minutes work per 20 minutes rest.
91-103°F: Moderate work at 40 minutes work per 20 minutes rest. Heavy work at 30 minutes work per 30 minutes rest.
103-115°F: Moderate work at 30 minutes work per 30 minutes rest. Heavy work at 20 minutes work per 40 minutes rest.
Above 115°F: If work is absolutely required, very frequent breaks and enhanced monitoring are mandatory. Prepare for emergency response.
Work Intensity Classifications
Light Work
- Standing, sitting
- Light tool use
- Operating equipment (air-conditioned cab)
- Walking on flat ground
Moderate Work
- Walking on uneven ground
- Light lifting (under 25 lbs)
- Moderate tool use
- General ranch maintenance
Heavy Work
- Digging, chopping
- Heavy lifting (over 25 lbs)
- Working livestock
- Fence building
- Hay baling/stacking
Hydration Requirements by Heat Index
Fluid Intake Guidelines
| Heat Index | Recommended Intake |
|---|---|
| Below 80°F | 8 oz every 20-30 minutes |
| 80-90°F | 8 oz every 15-20 minutes |
| 91-103°F | 8-12 oz every 15 minutes |
| 103-115°F | 12+ oz every 15 minutes |
| Above 115°F | Continuous hydration, 12-16 oz every 15 minutes |
How to Find Current Heat Index
Sources
- Weather.gov - Enter your location for current heat index
- Weather apps - Most display "feels like" temperature
- OSHA-NIOSH Heat Safety Tool - Free smartphone app
- NOAA Weather Radio - Includes heat advisories
Monitoring Throughout Day
Check the morning forecast for the daily high heat index, then monitor actual conditions during work. Heat index typically peaks between 2-5 PM, so adjust your work as conditions change.
Additional Factors That Increase Risk
Beyond the heat index itself, direct sun exposure (which adds up to 15°F to the effective heat index), lack of airflow or breeze, radiant heat from hot surfaces, wearing heavy or non-breathable clothing, physical exertion level, lack of acclimatization, and certain medications or health conditions all increase your risk. When any of these factors are present, treat the situation as though you're in the next-higher risk category.
Action Checklist by Heat Index Level
CAUTION (80-90°F Heat Index)
- Ensure water is available
- Take regular breaks in shade
- Monitor for early symptoms
- Wear appropriate clothing
EXTREME CAUTION (91-103°F Heat Index)
- Implement work/rest schedule
- Require hydration breaks
- Check on workers frequently
- Have first aid supplies ready
- Consider rescheduling heavy work
DANGER (103-115°F Heat Index)
- Shortened work periods mandatory
- Mandatory hydration every 15 minutes
- Designated rest areas with shade/cooling
- Buddy system for monitoring
- Emergency response plan reviewed
EXTREME DANGER (Above 115°F Heat Index)
- Postpone non-essential work
- Emergency-only operations
- Maximum protection measures
- Immediate response capability
- Consider complete work stoppage
Quick Mobile Reference
``` HEAT INDEX QUICK GUIDE
Under 91°F = Caution - Normal precautions 91-103°F = Extreme Caution - Enhanced precautions 103-115°F = Danger - Protective measures required Over 115°F = Extreme Danger - Limit/stop work
WATER INTAKE Drink 8-12 oz every 15-20 minutes More in higher heat index
WARNING SIGNS
- Heavy sweating then suddenly stopping
- Confusion or disorientation
- Nausea or vomiting
- Rapid pulse
- Hot, red skin
