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Posted at 11:00 AM ET, 07/23/2009

It's Not the Heat...

By Don Lipman

Debunking misconceptions about humidity

* Finally, Some Rain? Full Forecast | NatCast | D.C.'s Lightning Rod *
* Outside Now? Radar, Temps & More: Weather Wall | Lightning Map *

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.

By Don Lipman  | July 23, 2009; 11:00 AM ET
Categories:  Education, Extreme Heat, Lipman, Local Climate  
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Interestingly enough, there's plenty of humidity during the summer in many locations near the Persian Gulf, but little or no rain. The reason seems to involve the lack of triggering mechanisms such as frontal boundaries, airmass troughs and vorticity lobes. Generally, the major triggering mechanism in the tropics, the ITCZ, known also as the doldrum trough, monsoon trough, or meteorological equator, tends to settle well to the south of the Persian Gulf area, and the semipermanent subtropical ridge tends to sit over the Arabian peninsula during the summer. The easterly tropical waves which comprise such an important feature of the tropical Atlantic and Caribbean summer season are rare or nonexistent between Africa and Southeast Asia.

The ITCZ DOES move over the Indian subcontinent in June or July, and the southwesterly flow of deep humid "Equatorial" air thus introduced over the subcontinent is responsible for the heavy rainfall of the summer monsoon. The monsoon is heralded by intense thunderstorms, followed by extremely heavy rains, and an actual cooling of the subcontinent from the triple-digit temperatures and drought generally prevailing from April to early June in advance of the monsoon.

Posted by: Bombo47jea | July 23, 2009 12:21 PM | Report abuse

OK, I think I might be coming around on dew points. The only problem is that most cheap home weather stations (like mine) only give you temp and humidity. Please comment if someone recommends an inexpensive device to monitor dew points. I did find this calculator online if interested.

Posted by: spgass1 | July 23, 2009 12:42 PM | Report abuse

Monsoon Note #2. The term "monsoon" derives from the Arabic "mausim" or "season", and refers to the wet and dry seasons, not to the influx of humidity and heavy rain characteristic of the summer monsoon. In September the ITCZ recedes from the Indian subcontinent, and the resultant dry northerly or northwesterly winds herald the arrival of the dry or winter monsoon.

The monsoon of the American Southwest has a similar pattern, but is not related to the ITCZ. However both the Indian and Southwestern American monsoons have a similar cause. In India, and neighboring countries, the heating of the subcontinent draws the humid tropical/equatorial air northward with the ITCZ, and the monsoon rains begin. In the American Southwest, the heating of the Great Basin and Intermountain deserts draws moist tropical air northwestward from the Gulf of Mexico and to a lesser extent northward from the Gulf of California, the water of which is nearly as warm as the Persian Gulf.

Both the Indian and American monsoons are influenced by the tropical circulation patterns which involve the El Nino/Southern Oscillation and the Madden-Julian Oscillation as driving factors--a reason why El Nino years have different climate patterns than neutral and La Nina years. It is now suspected that the Kelvin waves of the MJO may influence the Rossby westerlies of both the Northern and Southern Hemispheres by way of feedback, and that both the northern and southern Rossby [jet stream] patterns in turn may exert a feedback influence on the tropical Hadley cell which separates them and drives the ITCZ. It is due to this feedback interaction that El Ninos and La Ninas arise, and monsoon failures may lead to famines on the Indian subcontinent. Feedback interactions probably affect both the oceanic wind and water circulation patterns; the resultant variations in sea surface temperatures then drive the wind circulation patterns in a continuous feedback cycle driven by the Sun's "movement" due to the tilt of the Earth's axis of rotation.

Posted by: Bombo47jea | July 23, 2009 12:58 PM | Report abuse

So you "debunk" the notion that it gets that hot and humid in DC during the summers by comping us to a standard not even surpassed by the Deep South? FAIL.

Posted by: rick1977 | July 23, 2009 1:11 PM | Report abuse

BTW, the warmest ocean water in the world is not the Persian Gulf, but the Red Sea on the other side of the Arabian Peninsula.
Water in the Red Sea may rise to a summer temperature as high as 96 to 100 Fahrenheit, and some of the world's most oppressive heat and humidity occurs in the coastal deserts of Eritrea and Djibouti, as well as the ports of Yemen and the Persian Gulf. Again, the lack of triggering mechanisms means little or no precipitation though thunderstorms do occur over the mountains of northern interior Yemen. This area is the original home of the coffee tree, Coffea arabica and the Yemeni port of Mocha is renowned for its coffee, which was originally drunk by Arabs as a beverage, then spread by the Muslims to Indonesia ["Java"] before eventual dispersion over the trade routes to the great New World coffee-growing regions in Brazil, Colombia and Central America. Starbucks is a relative latecomer to the scene, and the taste of a coffee blend depends largely on the soil where the coffee crop is grown, with rich, volcanic soils being the best. What Starbucks and others are doing today is improving the taste by mixing coffee blends from various regions of the world, something to ponder when settling down with that cup of latte.

Posted by: Bombo47jea | July 23, 2009 1:21 PM | Report abuse


Sounds like you misinterpreted the post. It states quite clearly at the very beginning that D.C. summers are typically hot and humid. The point was that relative humidity does not have to come in at a very high number for it to feel quite humid outside, thus why dew point is more effective indicator of comfort level.

Posted by: CapitalWeatherGang | July 23, 2009 1:58 PM | Report abuse

Why, when I click on your animated radar, is it a full two hours behind?

Posted by: waterfrontproperty | July 23, 2009 2:01 PM | Report abuse


We just noticed this ourselves about 30 minutes ago. It's only the zoomed-in local radar. The wider view (see the Weather Wall, and the forecast post below this post on the main page) radar is up to date. Our vendor is already working on it. (Great timing, huh? First rainy day in weeks)

Posted by: CapitalWeatherGang | July 23, 2009 2:16 PM | Report abuse

I knew what you were saying Don! BTW, here at 18th and E NW we just measured a tad under 2 inches of rain in the last hour. That was a solid cell.

Posted by: DullesARC | July 23, 2009 2:36 PM | Report abuse

I lived in Yemen in the 1960s, before air conditioning was widely utilized, and I can assure you that the worst day of humidity in Washington, DC is only 1/10 as bad as a hot&humid day on the coast of the Red Sea or the Gulf of Aden. It's like being constantly slapped with a steaming hot towel.

Posted by: FloralPattern | July 23, 2009 2:55 PM | Report abuse


Thank you.

Don, of the CWG

Posted by: Weatherguy | July 23, 2009 3:32 PM | Report abuse

Why is Accuweather reporting no precipitation for D.C. from 2pm to 3pm today? I work about 4 blocks from Reagan National and it poured for a good 45 minutes here.

I demand accuracy when measuring rainfall amounts!

Posted by: shoveit | July 23, 2009 4:36 PM | Report abuse

Ah, you've happened upon the true definition of "hit-or-miss" showers and storms. While you got poured on about 4 blocks away, the reporting station at National has a big goose egg for precipitation so far today, and radar confirms the heavy rain cell just narrowly missed the airport itself.

Posted by: CapitalWeatherGang | July 23, 2009 5:07 PM | Report abuse

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