Physics

Heat Index Calculator | Feels-Like Temperature & Heat Safety Levels

Calculate the heat index (feels-like temperature) from air temperature and relative humidity using the official NWS Rothfusz regression equation. Shows heat safety category — Caution through Extreme Danger — with practical cooling recommendations and heat illness warning signs.

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Air Temperature

Relative Humidity (%)

60%

What Is the Heat Index?

The heat index — also called "feels like" temperature — combines air temperature and relative humidity to determine how hot the air feels to the human body. High humidity reduces the efficiency of sweating, the body's primary cooling mechanism, making the same air temperature feel significantly hotter.

Heat-Related Illness Progression

Condition	Symptoms	Action
Heat Cramps	Painful muscle spasms, heavy sweating	Rest, cool fluids, move to shade
Heat Exhaustion	Heavy sweating, weakness, cold/pale skin, fast pulse	Cool environment, lay down, hydrate
Heat Stroke	High body temp 103°F+, no sweating, confusion	Call 911 immediately — life-threatening

Staying Safe in Extreme Heat

Drink water before you feel thirsty — by the time you feel thirst, you may already be mildly dehydrated. Avoid strenuous outdoor activity between 10 AM and 4 PM. Check on elderly neighbors and never leave children or pets in parked vehicles. Heat stroke is a medical emergency requiring immediate cooling and 911 activation.

Note: Heat index values are calculated for shady conditions. Direct sunlight can increase the effective temperature by an additional 10–15°F. This is especially important for outdoor workers, athletes, and those without access to air conditioning.

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