Humidity makes the difference
I always tell people that summer doesn’t really start around here until after July 4 and that the actual summer solstice is just a day on the calendar. That couldn’t have been more true this year. Last month, we not only saw rainfall more than an inch above normal in both Spokane and Coeur d’Alene; our temperature average for the month was about 2.5 degrees below normal. Only 10 days out of 30 had afternoon highs above 75 degrees.
While the past week has brought summer heat to both the Northwest and New England, spawning heat advisories in places like Portland and Boston, the circumstances are quite different.
We have all heard the saying, “It’s not the heat, it’s the humidity.” That is true for a couple of reasons. In New England, as well as most locations east of the Rockies, humid air adds a whole other level to hot weather. When the air is moist, our bodies have a hard time utilizing their natural coping ability, sweating, to cool us off. Sweating can lower our body temperature by evaporational cooling. When we sweat, the evaporation of the moisture from our skin uses heat energy, resulting in a net cooling effect. When the air is moist, there is less evaporation, and this process does not work as efficiently.
Another effect of moist air is that it does not cool as rapidly at night as dry air. For people without adequate air conditioning, nighttime heat can be just as dangerous as daytime, with outside temperatures remaining in the mid-70s or higher evenovernight . In the Northwest, while temperatures may reach the 90s or even 100 degrees, the moisture content is relatively low. This is most evident when you take a dip in the lake or pool when it’s hot outside, only to feel a distinct chill as soon as you get out of the water as the moisture rapidly evaporates. Overnight, it is not uncommon in our neck of the woods to see temperatures drop 40 or more degrees, allowing folks without air conditioning to open windows and let the nighttime air cool things off.
When trying to determine how much moisture is in the air, we need to look at a term called “dewpoint” rather than relative humidity. Relative humidity is exactly as it says: relative. It tells us the level of saturation of the air relative to its temperature. During the cold season, we often see relative humidity above 90 percent in the early morning hours, and I assure you it is not “muggy” outside. The dewpoint temperature gives us a better idea of the absolute moisture content of the air. As a general rule, the air starts to feel “sticky” with dewpoint temperatures above 55 degrees. Dewpoint temperatures in places like Philadelphia and Boston have ranged from the mid-60s to the lower 70s this past week. In contrast, the dewpoint in Portland, as well as in Spokane, has only been in the upper 40s.
Michelle Boss can be reached at email@example.com.