Clime
← Back to Blog
Guides

Understanding the Heat Index: A Step-by-Step Guide

June 18, 2026 · The Clime Team
Understanding the Heat Index: A Step-by-Step Guide

The heat index, often referred to as the "apparent temperature," combines air temperature and relative humidity to determine how hot it feels to the human body. This metric is crucial for assessing heat-related health risks and making informed decisions during hot and humid conditions.

What Is the Heat Index?

The heat index represents the human-perceived equivalent temperature, considering both air temperature and humidity. For instance, a temperature of 90°F with 70% humidity may feel like 105°F to the human body. Understanding the heat index helps in recognizing potential heat-related health risks, such as heat exhaustion or heat stroke.

How Is the Heat Index Calculated?

Calculating the heat index involves a complex formula developed by the National Weather Service (NWS). The general equation is:

HI = c₁ + c₂T + c₃R + c₄TR + c₅T² + c₆R² + c₇T²R + c₈TR² + c₉T²R²

Where:

  • HI = Heat Index (°F)
  • T = Air Temperature (°F)
  • R = Relative Humidity (%)

The constants (c₁ to c₉) are specific values used in the formula. For temperatures above 80°F and relative humidity above 40%, this formula provides a good approximation of the heat index. (en.wikipedia.org)

Step-by-Step Calculation

  1. Gather Data: Obtain the current air temperature and relative humidity for your location.

  2. Apply the Formula: Substitute the values of T and R into the heat index formula.

  3. Compute the Heat Index: Perform the calculations to determine the heat index value.

Example Calculation

Suppose the air temperature is 90°F, and the relative humidity is 70%. Using the formula:

HI = -42.379 + 2.04901523(90) + 10.14333127(70) - 0.22475541(90)(70) - 6.83783×10⁻³(90)² - 5.481717×10⁻²(70)² + 1.22874×10⁻³(90)²(70) + 8.5282×10⁻⁴(90)(70)² - 1.99×10⁻⁶(90)²(70)²

After performing the calculations, the heat index is approximately 105°F. (en.wikipedia.org)

Limitations of the Heat Index

While the heat index is a valuable tool, it has limitations:

  • Sun Exposure: The heat index is calculated for shaded areas; direct sunlight can increase the apparent temperature by up to 15°F.

  • Wind: Wind can affect the body's ability to cool itself, influencing the perceived temperature.

  • Altitude: At higher elevations, the air is less dense, which can affect the heat index.

Using the Heat Index for Safety

Understanding the heat index is essential for preventing heat-related illnesses. The Occupational Safety and Health Administration (OSHA) provides guidelines based on the heat index to help assess heat-related risks in the workplace and recommend protective measures. (heat-calculator.com)

Conclusion

The heat index is a crucial metric for understanding how temperature and humidity combine to affect human comfort and safety. By accurately calculating and interpreting the heat index, individuals can take appropriate precautions during hot and humid conditions to prevent heat-related health issues.

Highlights:

Frequently Asked Questions