That “dry heat” chestnut tells you only that sweltering at 95 in Back Bay is probably even more uncomfortable than mummifying at 95 in the desert. Like we said, big fucking deal.
Nevertheless, there are real issues involved — vital issues involving health and environmental decay. In recent years, climatologists and health experts have derived a new and meaningful statistic from all this called the “apparent temperature,” a concept best summed up in the phrase, “but it feels like . . . ”
Just how drastic is the difference between infernal dry heat and miserable wet heat? Conspicuously drastic, depending on circumstances.
On a typical mild early-June day, the 91-degree air temperature in Arizona feels like . . . wait for it . . . 91 degrees, because the relative humidity out there is only 10 percent. On the same day in Massachusetts — temperature 85, relative humidity 25 percent — the air feels like — hey! — 85 degrees. It’s called a mild day for a reason.
But at 95 degrees (that’s hot!) and a relative humidity of 75 percent (that’s humid!), the air feels like it’s 130 degrees; though at only 25 percent relative humidity, 95 feels like 94. Really. One less. This is where the not-so-bad dry-desert-heat notion comes from.
Observing the obvious
You don’t have to study one of those color-coded humidity maps too closely to see that on any given day, the air in the American Southwest (painted reds and yellows) is usually dry, compared with the wetter air in the rest of the country (greens and blues).
It may be of some comfort — though not much on one of those tacky 95-degree summer days — to know that we’re not so bad off here in Boston. Our muggy old town doesn’t even make the list of top 100 most humid cities (by annual average) in America. That honor goes, somewhat unexpectedly, to Bellingham, Washington, at 79.4 percent, followed closely by a bunch of places in Texas and Louisiana.
Statistics vary among sources, but Boston enjoys somewhere around an annual average humidity of 65.5 percent. The least humid city on the America’s 100 Most Humid list, Clearwater, Florida, by comparison, averages 73.1 percent. New Orleans, which has a well-earned reputation for being intolerably hot and humid, averages an annual 76 percent. And Phoenix, Arizona? Roughly 36.5 percent. Big difference.
Merely two ways to die
All of this is important for two reasons. First, there’s that now-famous global-warming thing, which, by its very name suggests that heat is on the rise. More heat means more hot air means more air that’s capable of being more humid. That adds up to more horribly uncomfortable feels-like days, which, on top of the burgeoning biting-insect populations, the daily increasing threat from ultraviolet rays, and the proliferation of reality-TV shows can make summers pretty miserable.
Second, heat — especially when abetted by humidity — can kill you. For one thing, high humidity discourages evaporation because air can hold only so much water before becoming saturated. (We could explain the term dew point here, but you really wouldn’t enjoy it because it involves a maze of counter-intuitive reasoning. Suffice to say that the higher the dew point, the worse off you are.)
