This scale is based on how the body cools itself through sweat evaporating off the skin, the NWS said. Higher humidity slows down the rate that the sweat evaporates, making the body feel warmer.
The NWS created a heat index chart for easy reference, though it specifically said that the values are for shady locations, and being in direct sun can push the values up by as much as 15 degrees.
So, for example, if it is 94 degrees outside with 55% relative humidity, to the body it will feel like it is 106 degrees in the shade.
The heat index is often discussed when values rise to dangerous levels, but low relative humidity can actually make it feel colder, the NWS said. For example, it said if the air temperature is 100 degrees but humidity is 15%, the index will be 96 degrees.
The NWS uses a four-tier classification for heat indexes.
A heat index between 80 and 90 degrees is labeled “Caution,” with fatigue possible with prolonged exposure or physical activity.
At 90 to 103 degrees, the classification is “Extreme Caution,” as heat stroke, heat cramps and heat exhaustion are possible with prolonged exposure or physical activity.
When heat indexes reach 103 to 124 degrees, it rises to the “Danger” level, as heat cramps or heat exhaustion become likely, and heat stroke is possible with prolonged exposure or activity.
At 125 degrees or higher, the NWS classifies the index as “Extreme Danger,” and says heat stroke is highly likely.
In addition to the chart, the NWS offers a heat index calculator, as well as an equation that closely approximates the heat index.
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