Yes, it’s hot, but it’s a ‘dry heat’
We have all heard the saying “…but it’s a dry heat,” when referring to the hot, desertlike conditions of the southwestern U.S. Our arid summers can be very similar, though fortunately for us we don’t have to endure weeks of consecutive 100-plus-degree weather.
Last weekend, when temperatures across the Inland Northwest reached into the upper 90s and even low 100s, it might surprise you that the term “heat index” was never mentioned. The heat index formula incorporates the amount of moisture in the air (the dew point) and the air temperature to produce what we often hear in other parts of the country as the “feels like” temperature.
The heat index is not a real temperature in the sense that you can’t measure it with any thermometer. What happens under high dew point/high temperature conditions, is that your body’s natural cooling mechanism –sweating – does not work as efficiently. Normally, when we get hot and sweat, the sweat evaporates from our body providing a cooling effect. When the air is thick with moisture, which happens with much more frequency east of the Rockies, sweat does not evaporate as readily, leaving us feeling hotter than we would otherwise.
In the Midwest’s corn belt, transpiration from area crops can often push the dew point into the lower 70s. A temperature of 95 degrees with a dew point of 70 degrees leads to a heat index of 103 degrees. This type of heat-humidity combination is in stark contrast to what we saw last weekend. Locally, Spokane and Coeur d’Alene saw highs last Sunday of 96 degrees (broke a record in CdA, but missed it by one degree in Spokane). The dew point during the hottest part of the day was only 42 degrees. That temperature-dew point combination results in a relative humidity of only 15 percent – pretty dry air.
Therefore, when we are outside sweating in the sun, the evaporation rate is pretty high. The heat index with that situation comes out to only 91 degrees – cooler than that actual air temperature. Growing up in a severely “muggy” Oklahoma, I always have a hard time at the pools here because I chill so easily even with temperatures in the upper 80s. Evaporative cooling is pretty intense in arid summer climates such as this. Of course, a heat index which is lower than the actual air temperature is probably a moot point when highs surge above 100 degrees. In Lewiston, last Monday’s high was 105 degrees with a dew point of 44 degrees. The heat index would have come out to only 101 degrees. That is hot no matter how you spin the numbers.
When we do get the heat and moisture combination – and add to that a little push from Mother Nature – severe weather is the result. We got a taste of that this past week. The more common dry heat, however, is part of the reason that we don’t experience much in the way of tornado producing storms.
Despite the late June heat, the hottest days of the season have yet to be realized. Our warmest average temperatures occur in late July into early August. When it comes to reaching the century mark, though, the odds are stacked against. In Spokane, only seven of the past 18 summers saw highs which reached 100 degrees or more.