How to Calculate Wind Chill and Heat Index

The thermometer on your porch might read 25 degrees Fahrenheit, but a 20 mph wind can make it feel like 13 degrees. In summer, a 90-degree afternoon at 70 percent humidity can feel like 106 degrees on your skin. Wind chill and heat index are the two measurements that bridge the gap between what the thermometer says and what your body actually experiences.

This guide explains both concepts using the official National Weather Service formulas, walks you through real calculations, and provides safety thresholds that can protect you and your family during extreme weather.

What Is Wind Chill?

Wind chill describes how cold the air feels on exposed skin when wind is factored in. Your body constantly radiates heat, and in calm conditions, a thin layer of warmed air sits against your skin, providing mild insulation. When wind blows, it strips away this insulating boundary layer, forcing your body to lose heat faster. The stronger the wind, the faster the heat loss, and the colder it feels.

The National Weather Service updated its wind chill formula in 2001, basing it on modern heat transfer theory and clinical trials where volunteers walked on a treadmill in a wind tunnel. The current formula is calibrated for a face height of approximately 5 feet, walking speed of 3 mph, and assumes no direct sunlight. Wind chill values only apply when the actual air temperature is at or below 50 degrees Fahrenheit and wind speeds are above 3 mph.

The Wind Chill Formula

The official NWS wind chill temperature index formula uses air temperature in Fahrenheit (T) and wind speed in miles per hour (V):

Sample calculation: If the air temperature is 10 degrees Fahrenheit and the wind speed is 25 mph, the calculation proceeds as follows. First, compute V raised to the 0.16 power: 25^0.16 = 1.741. Then: 35.74 + (0.6215 x 10) - (35.75 x 1.741) + (0.4275 x 10 x 1.741) = 35.74 + 6.215 - 62.24 + 7.44 = -12.8 degrees Fahrenheit. So 10 F with 25 mph winds feels like approximately -13 F on exposed skin.

What Is Heat Index?

Heat index, sometimes called the apparent temperature, measures how hot it feels when relative humidity is factored in with the actual air temperature. Your body cools itself primarily through perspiration — sweat evaporates from the skin, carrying heat away. When humidity is high, the air is already saturated with moisture, slowing evaporation and reducing your body's ability to cool down. This makes high-humidity heat far more dangerous than dry heat at the same temperature.

The NWS heat index is calculated for shaded conditions. If you are in direct sunlight, the heat index can increase by up to 15 degrees Fahrenheit. The heat index applies when air temperature is at or above 80 degrees Fahrenheit and relative humidity is at or above 40 percent.

The Heat Index Formula

The NWS uses a regression equation developed by Lans P. Rothfusz. The full equation uses air temperature (T) in Fahrenheit and relative humidity (R) as a percentage:

Simple check first: The NWS recommends starting with a simple average: HI = 0.5 x (T + 61.0 + (T - 68.0) x 1.2 + R x 0.094). If this result averages above 80 F when averaged with the temperature, use the full regression equation above. This two-step approach avoids unnecessary complexity for milder conditions.

Sample calculation: With an air temperature of 96 F and relative humidity of 65 percent, the full Rothfusz regression yields a heat index of approximately 121 degrees Fahrenheit — firmly in the "danger" zone where heat stroke is likely with prolonged exposure.

Practical Examples

Here is how wind chill and heat index affect real decisions in fictional but realistic scenarios.

Wind Chill and Heat Index Reference Tables

These tables provide quick lookups for common conditions. Wind chill values show the feels-like temperature based on actual temperature and wind speed.

Safety Tips and Complete Guide

Understanding the numbers is only useful if you take appropriate action. These guidelines help you stay safe in both extreme cold and extreme heat.

• Dress in layers for cold weather. A base layer wicks moisture, a middle layer insulates, and an outer layer blocks wind. This system traps warm air while allowing moisture from sweat to escape, preventing the dangerous combination of wet skin in cold wind.
• Cover all exposed skin when wind chill drops below 0 F. Frostbite can begin on exposed ears, nose, cheeks, and fingers within 30 minutes at -20 F wind chill and within 10 minutes at -40 F. A balaclava or neck gaiter protecting the face is essential, not optional.
• Hydrate aggressively in high heat index conditions. Drink water before you feel thirsty. By the time thirst signals reach your brain, you may already be mildly dehydrated. In heat index values above 105 F, aim for at least one cup of water every 15 to 20 minutes during physical activity.
• Recognize early warning signs. For hypothermia: uncontrollable shivering, slurred speech, confusion, and drowsiness. For heat exhaustion: heavy sweating, weakness, nausea, dizziness, and headache. Both conditions can progress rapidly to life-threatening emergencies if untreated.
• Check conditions before outdoor activities. Always check the wind chill or heat index before exercising, working, or recreating outdoors. Adjust intensity, duration, and protective measures based on the feels-like temperature, not just the thermometer reading.
• Account for sun exposure. The official heat index is calculated for shade. Direct sunlight can add up to 15 F to the heat index value. If you are in the sun, mentally add 15 degrees to the reported heat index to assess your true risk level.

Common Mistakes to Avoid

• Ignoring wind chill because the thermometer seems mild. A 25 F reading might not sound dangerous, but with 35 mph gusts, the wind chill drops to 8 F — cold enough for frostbite on prolonged exposure.
• Assuming dry heat is not dangerous. While heat index is lower in dry climates, temperatures above 105 F in low humidity can still cause heat stroke, especially during physical exertion when the body cannot cool fast enough through sweating alone.
• Overdressing children in cold weather. Over-bundling can cause sweating, and wet clothing in cold wind accelerates heat loss. Dress children in breathable layers and check periodically for dampness.
• Relying on indoor temperature readings. A thermometer inside your house or car does not reflect outdoor wind chill or humidity conditions. Always check an outdoor-calibrated source or weather service for accurate feels-like temperatures.
• Exercising during peak heat index hours. The highest heat index values typically occur between 2 PM and 5 PM. Scheduling outdoor exercise before 9 AM or after 7 PM dramatically reduces heat-related risk.

Last updated: February 23, 2026