Buzz
With record-breaking heatwaves across the U.S. and Europe, coupled with the planet's increasing temperatures, it's the perfect moment to tackle some key questions about the heat index—and why high humidity leaves us feeling drained and lifeless.
What is it about humidity that makes us feel so much hotter?
Let's dive into the role of sweat. As you might recall from your high school biology lessons, one of your body's methods for cooling down is sweating. As the sweat evaporates from your skin, it carries heat away, cooling your body in the process.
Humidity disrupts the body's natural cooling process. As the relative humidity rises, sweat evaporation slows down. Instead of evaporating, sweat just drips off, leaving you with a soaked T-shirt and no cooling effect. When the humidity spikes, we lose a crucial tool that typically helps cool us down.
What makes relative humidity 'relative'?
Humidity refers to the amount of water vapor present in the air. However, as the air temperature increases, so does the air's capacity to hold moisture. (Warmer air can contain more water vapor.) Relative humidity compares the current amount of moisture to the maximum water vapor the air can hold at any given temperature.
What's the heat index, and why does it matter?
While anyone who's spent time outside on a sticky, muggy day can feel the effects of humidity, the current heat index system owes much to Robert G. Steadman, a textile researcher. In his 1979 paper, “An Assessment of Sultriness, Parts I and II,” Steadman identified the factors influencing how hot a person feels in specific conditions, and meteorologists soon adopted his work to create a simplified formula for calculating the heat index.
The formula may seem complicated at first, but fortunately, it can be simplified into easy-to-read charts available online. All you need is the air temperature and the relative humidity, and the chart will provide the rest of the information.
Is the heat index calculation universal for everyone?
Not exactly, but it's very close. Steadman’s research was based on the concept of a “typical” person subjected to a specific set of conditions. This person was 5 feet, 7 inches tall, weighed 147 pounds, wore long pants and a short-sleeved shirt, and was walking at slightly over three miles per hour into a gentle breeze while staying in the shade. Any changes in these conditions will influence how the heat and humidity are perceived by a particular person.
What difference does being in the shade make to the heat index?
Quite a significant one. All of the National Weather Service’s charts for calculating the heat index assume that people will seek shade when the heat and humidity become unbearable. Direct sunlight can raise the calculated heat index by as much as 15 degrees.
How does wind influence the severity of heat?
Typically, when we imagine wind on a hot day, we picture a refreshing, cooling breeze. This is the usual scenario, but when the temperature is unbearably high—think 90s or above—a dry wind can actually make things worse. In extreme heat, wind pulls sweat away from the body before it can evaporate and cool us down. Instead of offering relief, it turns a once-comfortable breeze into something like a convection oven.
When should I start paying attention to high heat index warnings?
The National Weather Service uses a helpful four-tiered system to indicate how serious the heat conditions are. At the highest level, when the heat index exceeds 130°F, it’s classified as “extreme danger,” with a high risk of heat stroke from continued exposure. The severity decreases as you go down the scale, but even on “Danger” days, when the heat index is between 105°F and 130°F, staying outdoors is risky. At this level, prolonged exposure and/or physical activity could lead to sunstroke, heat cramps, and heat exhaustion, with heat stroke also being a possibility.
This article has been updated for 2022.
