A hot July day in downtown Washington in 2010. (Linda Davidson/The Washington Post)

It’s no secret the world is warming, but thanks in part to climate change, humidity is also beginning to surge. Here in Washington, that means the punishing combination of heat and humidity is becoming more oppressive.

We reviewed data from four locations in the District and broader region. Through this, we calculated specific humidity, which is the mass of water in a volume of air. (It’s a much better indicator of how oppressive it feels than the relative humidity, to which we are accustomed.)

To calculate these trends in humidity, it took a lot of number-crunching — involving close to 2 million data points — with observations stretching back as far as the 1930s. In the end, we found that the District is, on average, a little more than 5 percent more humid than it was in 1970, and slightly more than 10 percent juicier than in 1950.

This is important because humidity affects how we feel. Take Wednesday, for instance. The average high for this time of year at Dulles Airport is 86 degrees. With 1970s humidity, a typical summertime afternoon would produce a heat index (a measure of how hot it feels) of 89 degrees.

Now, when you factor in a higher humidity, it feels closer to 91 or 92 — and that’s at the same 86 degrees.

Consider average summer temperatures have also warmed three degrees since then, and the new heat/humidity combination would produce a heat index of 94 degrees: a five-degree bump in how hot it feels. That puts significantly more strain on the body, and suddenly, you’re feeling the heat.

The results

The increases in humidity we computed are for the entire year, but they also apply to each of the four seasons, including summer, at most of the stations we reviewed.

The spots we examined in detail are Reagan National Airport; Dulles International Airport; Quantico, Va.; and Hot Springs in west-central Virginia.

At National, humidity has increased slightly more than 5 percent since 1970. At Dulles Airport, it’s a little less than 10 percent.

Why the difference? National is on the Potomac. Its average specific humidity is already 10 percent higher than Dulles. It’s likely that because National is already so moist, it’s difficult to add more to the air.

Farther inland, it’s drier. That makes any incremental increases in moisture more significant. Dulles has seen roughly the same moisture increase since just the 1990s that National has since 1970.


Long-term trends in dew point temperatures, an indicator of humidity, at Quantico, Reagan National Airport, Dulles International Airport and Hot Springs.

Like National, Quantico is near the Potomac. Similarly, it saw a humidity increase of just above 5 percent since 1970.

Hot Springs, more than 100 miles inland, has historically been much drier — partly due to its elevation — but its rate of increase in humidity, compared with its baseline, is three times higher than that of Quantico. Just since 2000, it has seen about a 10 percent increase in its humidity.

It would seem we’re all experiencing an increase. Those of us closer to the Potomac are seeing a slower increase, but we started higher; in other words, we’re used to extreme mugginess. It’s still getting worse. Those farther inland started with a lower baseline, but are moistening much faster.


The Washington Monument is silhouetted against the morning sky as the sun rises at the start of a hot day in the District on July 20. (J. David Ake/AP)

The implications of this increasing humidity are significant.

“When it comes to dangerous heat, climate change packs a one-two punch,” wrote Michael Mann, a climate scientist at Pennsylvania State University, in an email interview. “Not only do we see higher temperatures, but a warmer atmosphere holds more moisture, so we tend to see higher levels of humidity.”

Heat is a silent killer, particularly dangerous to vulnerable low-income populations and older adults who may not be able to afford air conditioning.

“The more humid the air, the more difficult it is for human beings to cool off through evaporation,” Mann wrote. “Episodes of extreme heat and humidity, such as we saw in the eastern and central U.S. this July, pose a particular danger to us.”

Methodology

We pulled archived weather data to find hourly dew point observations at a variety of locations. At Dulles Airport, our data stretched back nearly continuously to 1963, and at National, to 1938. (The latter data from 1938 to 1945 was recorded at 24th and M streets before observations were moved to the airport.) At Quantico, observations weren’t taken at night until late 1953. To combat irregularities in observation, we only used periods that featured complete data, and we focused on the period from 1970 onward.

We also wanted to examine what was happening away from urban areas, lest infrastructure development play a role. Our data dive included Quantico, as well as Hot Springs in the George Washington and Jefferson National Forest. Quantico is a little over 30 miles south of the District, just inland from the Potomac, while Hot Springs is seated in a rural forested region of the Appalachian Mountains.

The reason we chose these two latter locations? Variety, so we could compare urban, rural, river and mountain locations.

We know that a fraction of observed warming in big cities stems from the “urban heat island” effect, but while cities can make it warmer, it’s unlikely they make the air any wetter. In fact, they may slow down moistening of the atmosphere. That’s because cityscapes feature less evapotranspiration, a process through which vegetation evaporate water into the air, raising the dew point. Fewer trees in urban areas means less evapotranspiration, so any observed dew point trends should be strictly climatic.

Richard “Heat wave” Berler, a Texas-based broadcast meteorologist specializing in temperature and humidity, told us there should be no “dew point island.” Dew point is proportional to specific humidity.

“In 1971, a friend and I got a . . . grant to pursue a study on this very topic,” Berler wrote in an email interview. “Amazingly . . . the dew point varied by less than one degree throughout! No discernible dew point island!” That means the moisture increase we’ve seen is likely not tied to urbanization.


A man walks in the heat at the Jefferson Memorial in the District. (Alastair Pike/AFP/Getty Images)

This analysis is certainly not exhaustive, nor has it been peer reviewed. The data are subject to uncertainties due to the quality of instrumentation and any changes in observing practices. However, the results presented here are broadly consistent with published studies and independent analyses that reveal similar trends.

In addition to the four locations at which we closely analyzed humidity trends, we also checked them at six additional locations in the region with long-term records, and all showed increases. Dew points rose at the rate of 2.5 to 5 degrees per century at Andrews Air Force Base, Baltimore-Washington International Marshall Airport, Annapolis, Salisbury, all in Maryland; and Charlottesville and Richmond in Virginia.