Heat Index Calculator

The heat index, also known as the "apparent temperature" or "feels like" temperature, is a measure of how hot it actually feels when relative humidity is factored in with the air temperature.

It's important because high humidity reduces the body's ability to cool itself through sweating, making it feel hotter than the thermometer reading. This can have serious health implications, as the human body's ability to regulate temperature is compromised in high heat index conditions.

The heat index calculation used in this calculator is based on the Rothfusz regression, which is the standard formula used by the National Weather Service and meteorologists worldwide. It provides a good approximation of how hot it feels to the human body.

However, it's important to note that the heat index calculation assumes:

• You are in the shade
• There is a light wind
• You are not doing strenuous activity

Direct sunlight can increase the heat index by up to 15°F (8°C), and physical activity generates body heat that can make it feel even hotter.

Heat index values are grouped into risk categories:

• 80-90°F (27-32°C): Caution - Fatigue possible
• 90-105°F (32-41°C): Extreme Caution - Heat exhaustion possible
• 105-130°F (41-54°C): Danger - Heat exhaustion likely, heat stroke possible
• Above 130°F (54°C): Extreme Danger - Heat stroke highly likely

You should be particularly vigilant once the heat index exceeds 90°F (32°C), especially if you are elderly, very young, have health conditions, or plan to be physically active.

Heat Exhaustion symptoms include:

• Heavy sweating
• Cold, pale, clammy skin
• Fast, weak pulse
• Nausea or vomiting
• Muscle cramps
• Fatigue or weakness
• Dizziness
• Headache

Heat Stroke is a medical emergency with symptoms including:

• High body temperature (103°F/39.4°C or higher)
• Hot, red, dry or damp skin
• Fast, strong pulse
• Confusion or loss of consciousness
• Possible seizures

Heat stroke requires immediate medical attention. Call emergency services if you suspect someone is experiencing heat stroke.

When the heat index is high, take these precautions:

• Stay in air-conditioned spaces as much as possible
• Drink plenty of fluids, even if you don't feel thirsty
• Schedule outdoor activities for cooler parts of the day
• Wear lightweight, light-colored, loose-fitting clothing
• Take cool showers or baths
• Use a buddy system when working in extreme heat
• Never leave children or pets in vehicles
• Check on elderly relatives and neighbors
• Avoid hot and heavy meals

Humidity makes it feel hotter because it affects the body's primary cooling mechanism - sweating. When we sweat, the evaporation of moisture from our skin removes heat and cools us down.

In high humidity, the air is already saturated with water vapor, which slows down the evaporation of sweat from our skin. This reduces our body's ability to cool itself, making us feel hotter than the actual air temperature.

This is why a 90°F (32°C) day with high humidity can feel much hotter than a 100°F (38°C) day with very low humidity. In dry conditions, sweat evaporates quickly, efficiently cooling the body.