How Hot is It and What Does the Heat Index Mean?

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By Bill Koch, editor

Forecasting the weather may not be an exact science. The meteorologist announces it’s going to be sunny and cool, and storm clouds dampen your day.

With recent temperatures lingering and loitering in the upper 90s, managing your time in the Florida sun can get complicated. Recently recorded heat indexes (what it feels like) have climbed well into the triple digits – in some areas reaching nearly 120.

The Old Farmer’s Almanac predicts Florida’s summer to be slightly cooler and rainier than normal. The hottest temperatures are expected through July and into late August.

Don’t expect noticeable relief in September and October. Predictions for those months are for hotter and drier than normal temperatures.

The U.S. Climate Data expects average daily high temperatures for July and August of 93. September high temperatures are expected to drop by 2 degrees. October may provide some relief with daytime highs of 85.

Florida hit a record high in June 29, 1931 of 109 in Monticello, and that was before most people had air conditioning.

Arizona’s Lake Havasu City recorded a high of 128 on June 29, 1994. Even Alaska isn’t exempt from summertime heat. Temperatures in North Pole, Alaska, are expected to reach into the upper 80s today.

What makes the temperature worse is not the actual reading, but the heat index. According to the National Weather Service, the heat index is the apparent temperature, what it feels like when factoring humidity levels. Arizona’s dry heat, for example, may be higher, but the heat index may read lower compared to Florida’s typically high humidity levels.

Heat indexes may be more accurate assessments of heat levels since they take into account the body’s ability to perspire. Perspiration helps the body cool. As sweat evaporates, it cools the body temperature. Higher humidity levels thwart that process. In other words, you sweat less in climates with high humidity levels because your body can’t effectively cool itself.

The combination of high temperature and high humidity foster greater discomfort. Here are some examples: a 100% humidity level at 90 puts the heat index at 132, the extreme danger zone. However, a temperature of 106 with a 40% humidity levels puts the index at 124, only the danger zone.

The National Weather Service advices caution for indexes above 90. It considers indexes above 103 to be dangerous. Those warnings apply to the likelihood of suffering heat disorders with prolonged exposure to the higher indexes or strenuous activities in those conditions.

Under the service’s classification systems, the heat index “caution” level (80-90) may produce fatigue for people outdoors. Heat stroke and cramps are possible in heat indexes of 90-103. Heat cramps and exhaustion are likely and strokes are possible with physical activity in the “danger” classification of heat indexes of 102-124. Heat stroke is highly likely when the heat index is above 124.

The U.S. Centers for Disease Control and Prevention urge residents in areas prone to high heat indexes to take precautions:

  • Drink adequate levels of water.
  • Stay indoors when indexes are high.
  • Limit outdoor activity.
  • Wear sunscreen.
  • Wear loose, lightweight, light-colored clothing.
  • Schedule workouts or other activities earlier or later in the day.