It's Not the Heat...
Debunking misconceptions about humidity
Unless you've just arrived from Dallas or New Orleans, you would probably agree that usually (but fortunately not this summer, at least until now) (a) we have quite warm summers around here; (b) we have fairly humid summers around here; and (c) there's little you can do about either unless you constantly stay indoors or move away.
But this summer, even on the warmest of days, the relative humidity (RH) -- a function of how much moisture the air can hold at any given temperature -- has generally been under 50%. Is this an aberration? After all, to many people, 50% RH probably doesn't sound so bad.
Keep reading to see misconceptions about humidity debunked...
As it turns out, the National Weather Service says that when Washington's temperature exceeds 90, RH has probably NEVER exceeded 70% and rarely, if ever, 60%.
From a casual look at some of our hottest, most oppressive days in the past, I've found no instances (where humidity information was available) of 90 degrees with 60% RH or higher. The reality is that with a temperature of 90 or higher, it begins to feel very sultry when RH exceeds even 40%. At 100 degrees, just 35% RH can cause major wilting.
As indicated in earlier postings (here and here), the dew point is a much better indicator of comfort level, as a dew point over 70 degrees is uncomfortable for almost everyone, regardless of the temperature. The heat index is also a useful measure.
Not always noted for its progressive policies in such matters, even the federal government, for the well-being of its own workforce, long ago decided to recognize the danger of working in high heat and humidity. Therefore, in the past (and possibly even now in non-air conditioned environments), early dismissal was allowed when any of the following combinations of temperature and RH were recorded: 95°/55%, 96°/52%, 97°/49%, 98°/45%, 99°/42% and 100°/38%. Was someone standing by to take these readings?
The government certainly had the right idea. Bear in mind, however, that I believe these were outdoor readings, which means that conditions inside buildings without air conditioning would have been even worse. Anyone out there know for sure -- were they indoor or outdoor readings?
As I mentioned earlier, the combination of 90°-plus temperatures and 70%-plus RH is unknown around here. But it is also rare even in the Deep South. This is because as temperatures rise through the 80s, 90s and beyond, the atmosphere can hold far more moisture. (At 90º, air can hold four times more water vapor than at 50º.) Therefore, if no additional moisture creeps in, then RH, which is based on the atmosphere's moisture-carrying ability, declines as the temperature rises.
For RH to be sustained at the 80-90% level on an increasingly hot summer day, it would take an enormous influx of superheated air previously conditioned over a cauldron-like body of water, such as the Persian Gulf. There, water temperatures can reach 95. Fortunately for us, even the Gulf of Mexico hasn't gotten that hot -- yet.
If you doubt that the humidity normally falls as the mercury rises on a summer day, check the humidity at 7 a.m. on what is predicted to be one of the hottest, most humid days of the summer. The temperature will probably be around 78-80 and the RH may be over 80% or even 90%, especially if it is foggy. Then check conditions once more at 3-4 p.m. -- around the hottest time of the day -- as the temperature peaks at 95 or higher. Although it may feel extremely oppressive, you will find that the RH has gone down, probably to somewhere between 35 and 45 percent.
Does the combination of 90°/90% occur anywhere in the world? Official records are unclear on this, but if it ever has, it could have been in Saudi Arabia, where on July 8, 2003, the city of Dhahran on the Persian Gulf reported a temperature of 108 with an RH of 68% and a dew point of 95! By comparison, Washington summers are almost fall-like.
FOOTNOTE: We use the term relative humidity rather than absolute humidity because the latter -- a measurement of how many units of moisture are in a parcel of air -- would be meaningless to most people.
| July 23, 2009; 11:00 AM ET
Categories: Education, Extreme Heat, Lipman, Local Climate
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