Heat index

Before you rely on it

What to check

Note that standard values assume shade and light wind.

The common error

Where people go wrong with heat index

Planning outdoor work off the air temperature. Direct sun adds substantially, and the published index has not accounted for it.

Worked through

Heat index in plain numbers

A day at 90°F with 40% relative humidity has a heat index near 91°F, so it feels about as hot as it reads. Hold the temperature at 90°F and raise humidity to 70% and the heat index climbs to about 105°F; the air did not get warmer. Sweat evaporates more slowly when the air is already moist, so the body sheds heat less well and the strain rises. National weather services publish charts that group these values into caution and danger bands. Across those bands, the risk of heat cramps, exhaustion and heat stroke increases with time outdoors and exertion.

Easily confused

Heat index and its neighbours

The heat index is not the actual air temperature, and it is not the same as the dew point or wind chill; the dew point states how much moisture is in the air. The heat index combines temperature and humidity into a feels-like value for shade conditions. Wind chill does the equivalent for cold and wind. Direct sun can add up to 15 degrees to the heat index, and some agencies use other measures, such as wet-bulb globe temperature, which also includes radiant heat and wind, and is used for work and sport guidance.

In real decisions

Where heat index turns up

People check it before outdoor work, sports, events and travel. Employers use it to schedule breaks and hydration. Coaches use it to decide whether to modify practice. Weather alerts often trigger at set heat index values. Someone planning a run or a hike can compare the heat index with their own tolerance. Vulnerable groups, including older adults, young children and people with certain conditions or medications, need extra caution.

Know the edges

Where heat index stops being reliable

The formula was built for average conditions and can be off in extremes, and it assumes shade and light wind; it does not capture acclimatization, clothing or exertion. People differ in how they respond to heat. This page is educational, not medical or safety advice. In dangerous heat, follow guidance from local health and weather authorities, drink water, rest in cool places and seek medical help for warning signs.

Quick answers

What people ask about heat index

Does the heat index apply in direct sun? The published values assume shade. Direct sunshine can raise the effective value by as much as fifteen degrees Fahrenheit, so take extra care when working or exercising outdoors.

When is heat dangerous? When the heat index is high and exertion is long. Warning signs include dizziness, nausea, confusion and cessation of sweating. Seek help promptly and follow local health authority advice.