Heat index tells truth of temperature

In 1979, R.I. Steadman of the National Weather Service developed today’s widely-used “heat index,” which shows the temperature that the human body actually feels.

The heat index is calculated by combining air temperature, dew-point and relative humidity levels. For example, an air temperature of 85 degrees Fahrenheit combined with a relative humidity reading of 80 percent results in a heat index of 97 degrees Fahrenheit. Although the reading is only 85 degrees, the high humidity levels would make it feels like 97 degrees to the human body. A 90-degree temperature combined with a 90 percent relative humidity level would push the heat index up to a very dangerous 122 degrees Fahrenheit.

Combined high temperatures and humidity levels, sunstroke, heat exhaustion, muscle cramps and numerous other health problems, including heart attacks, are likely to increase both in number and severity. The age and the health of the individual become extremely important factors in coping with such hazardous weather conditions.

Prolonged exposure to high heat indices can prove fatal. Since the ‘Dust Bowl Days’ of the 1930s, some 25,000 Americans have died during various heatwaves, mainly in the Desert Southwest and regions east of the Rockies, particularly the Deep South and the Southeast.

The heat index readings are normally far lower in the northern and western parts of the country, including the Inland Northwest. That’s because humidity and dew-point levels are often much lower than most areas east of the Continental Divide during the hot summer months. In our region, the warm, humid air from the Gulf of Mexico is blocked by the Rocky Mountains. Although we do see some triple-digit readings like those which occurred last July, the relative humidity levels are generally less than 20 percent, hence only a slight rise in the overall heat index.

One should use ‘caution’ in the outdoors if heat index readings are between 85 and 91 degrees. Use ‘extreme caution’ with heat index temperatures from 92 to 103 degrees. Conditions become ‘dangerous’ with indices from 104 to 125 degrees. It becomes ‘extremely dangerous’ or deadly with heat index levels above 125 degrees Fahrenheit.

One of the most deadly heatwaves in our nation’s history occurred in Chicago between July 13-15, 1995.

In the southwestern part of that city on July 14, 1995, the heat index soared to near 130 degrees Fahrenheit killing more than 500 people. The day’s high was a record 109 degrees with a relative humidity level of 80 percent.

This upcoming summer season should once again be hot, but not as dry as last year. In 2006, the airport recorded 30 days with highs at or above 90 degrees. Coeur d’Alene gauged a remarkable 38 days, both near-double the normal. During an average summer season, we typically see 19 days at the airport and approximately 22 days in the Spokane Valley area at or above 90 degrees. July was certainly a torrid period as almost half of that month (14 days) recording those temperatures at or above 90 degrees. In fact, July 23rd was the hottest day with a high of 102 degrees. We actually saw three days in-a-row with highs at or above the century mark, mighty hot indeed!

This summer season, I’m still looking for about 25 to 30 afternoons with highs of 90 degrees or above. There should be a couple of egg-frying days near the century mark. The hottest periods should be in early July and again in late July and early August.

Fire season across the Inland Northwest may be tough again this year as we’ll see long stretches of generally dry weather, but there will be occasional thunderstorm activity.