The reason? An inversion, or an increase in temperature with height. That “caps” the atmosphere, preventing cooler surface air from rising. It also helps turn the lower atmosphere into an echo chamber, allowing sound waves to propagate across long distances.
A firework-induced rumbling was heard as far away as Silver Spring, Huntington, Bethesda and Hyattsville.
A weather balloon launched at 7 p.m. Wednesday from Washington Dulles International Airport didn’t show too much of an inversion near the ground, but it did reveal an increase in temperature with height about a mile up. The sounding did show very dry air near the surface, however, which would allow temperatures to fall quickly after dark, thanks to radiational cooling.
The next weather balloon sounding we have didn’t come until 7 a.m. Thursday, when an extremely strong inversion was noted. The ground temperature was measured at 24.6 degrees, but at 1,150 feet it had spiked to 30.7 degrees.
This adds to the evidence that a pronounced inversion probably developed overnight, between 7 p.m. and 7 a.m., and was likely to have been in place when the fireworks occurred around 10 p.m.
The dew point, a measure of how much moisture is contained in the air, was quite low — only 14 degrees at Reagan National Airport. With little humidity or cloud cover to trap heat, that means temperatures can fall quickly.
Since National is on the mild Potomac, it’s not surprising the 10 p.m. observation reported temperatures of 34 degrees. However, with such dry air in place and a lack of cloud cover, it’s probable that areas like the National Mall may have been in the upper 20s to around 30 degrees.
In Gaithersburg, for example, the temperature was 26 degrees at 10 p.m., indicating the significant drop-off in surface temperatures away from the Potomac. Surface winds were calm, further enhancing radiational cooling.
Radiational cooling is most effective near the surface, and it’s likely ground temperatures chilled much faster than temperatures a couple thousand feet above the ground. That induced a near-surface inversion that intensified over the course of the night.
The inversion acted as a ceiling, putting a cap on the lower atmosphere that allowed sound waves from the fireworks to ricochet up and down. At the same time, sound translated horizontally, essentially echoing between the lower atmosphere and the ground across long distances. Similarly propagating sound waves have been observed during thunderstorms that occur amid inversions and near airports, where the sound of jets taking off can extend well beyond the typical noise footprint.
The existence of an inversion is also indicated by the presence of thick smoke long after the fireworks cleared, with a somewhat flat top to the smoke clouds. That indicates there was a layer in the atmosphere the smoke couldn’t really make its way into and above, pointing to an inversion. (An air parcel will keep rising only if it is warmer than the surrounding air.)
However, the fact that the smoke didn’t gather above the ground, nor cause widespread pollution issues, points to the inversion being at least a few thousand feet up.
“None of the regulatory monitors in the D.C. region had a notable uptick in particle pollution last night,” Ryan Stauffer, an atmospheric scientist with NASA, wrote in an email. “It didn’t seem like a strong inversion was fully established by the time the fireworks went off (but apparently it was in place enough so that people could hear it for miles!).”
Shallower and stronger inversions have contributed to significant pollution buildup during summertime Independence Day firework displays.
Andrew Freedman contributed to this report.