BACKGROUND: The Heat Stress Index is a new, comprehensive summer index that evaluates daily relative stress for local locations throughout the United States. The HSI could serve as a guidance system for issuing heat warnings, and as a useful research tool for environmental issues.
HOW IT WORKS: The index is based on daily maximum and minimum temperature, cloud cover, cooling degree hours, and consecutive days of extreme heat. Each of these measurements are taken every day, and scientists then figure out the likely range these measurements usually fall into. (For instance, the average high temperature in your area on July 31 could be between 83 and 105, and the low between 65 and 80.) Then they take all the different measurements taken today, rate them and average them, producing a number between 1 and 10. A 1 day is a very unstressfully hot day, with most other days giving you more stress due to heat at your location at that time of year. A 10 day would be awful, making it the worst day to be stressed by heat that year. A daily value of 9.7, for example, would indicate that only 3 percent of days would be more stressful than the present day at the given location during this time of the year. The same weather conditions in Boston and Atlanta will yield different heat stress index values, based on the city's weather history. The same conditions at the same place would also yield different values if they occurred in early June and mid July.
WHY IT'S NEEDED: It's not the heat, it's the humidity. But why does humidity make you hotter? Humidity is the measure of how much moisture is in the air. The hotter the air, the more moisture it can hold. For example, a high relative humidity of 100 percent means that the air is saturated with moisture and can't hold any more. Our bodies rely on the evaporation of moisture from our skin to keep cool. When relative humidity is low, sweat evaporates from the skin very quickly. But when it is high, body sweat does not evaporate as easily, and we feel sticky and hot.
HEALTH RISKS: Overheating can be very dangerous. Heat exhaustion occurs when the body loses fluid and salt through perspiration faster than they can be replaced, causing dizziness. Exercising in hot weather can cause muscle cramps, especially in the legs, because of brief imbalances in body salts. People not used to exercising in heat may experience a quick drop in blood pressure that can lead to fainting. And in some cases, extreme heat can cause body temperature to rise to 105 degrees or higher, causing a heat stroke, with confusion and unconsciousness. Low humidity makes us feel cooler than the actual temperature because our sweat evaporates quickly. But low humidity can cause dry, itchy skin and chapped lips.