By Brad Rhen
Some people refer to it as “The sound of freedom.”
Others may call it a nuisance.
Whatever you call it, noise from military training is fairly common at Fort Indiantown Gap. The 18,000-acre installation is one of the busiest National Guard training centers in the country, hosting over 100,000 service members annually.
“Fort Indiantown Gap is the only live-fire training base in Pennsylvania, and we need to ensure our young Soldiers and Airmen are well trained and ready to be called to defend our country,” said garrison commander Col. Lane Marshall. “To maintain combat proficiency, live-fire service practice is critically necessary for safety in wartime, and, of course, that means noise.”
Occasionally, the noise generated from certain types of training – such as field artillery training – can sound louder than usual. That’s because there are several factors that can affect noise from training.
Among the things that can affect noise from training are clouds, temperature, humidity, wind, barometric pressure, snow on the ground and vegetation on trees, said Tech Sgt. Robert Capella, a staff weather officer with the 193rd Special Operations Wing’s 203rd Weather Flight.
“Sound travels slower in, and bends toward, cold air since it is denser than warm air,” Capella said. “Humidity and pressure affect sound like temperature – high humidity and low pressure air is less dense than dry and high pressure air. Vegetation on trees or snow on the ground act to dampen sounds that are bouncing around, where each leaf might reflect away a small bit of a sound wave or a ‘sponge-like’ snow surface absorbs sound waves like soundproofing acoustic foam.”
When two or more of those factors work in sync, they have an even greater effect on noise, said Capella, who is finishing a master's degree in Atmospheric Science at the University of Wyoming and has worked as a forecaster for the National Weather Service.
“If you have a cold, dry, still fall morning with little vegetation on the trees and no snow on the ground, you will notice sounds louder and from farther away,” he said.
One of those days when several factors worked in concert with each other to create increased noise levels at Fort Indiantown Gap occurred on March 15. That day, field artillery live-fire training was being conducted and louder-than-usual “booms” could be heard as far away as Harrisburg.
Capella explained that cold air near the surface caused sound waves to bend – or refract – back toward the ground, bounce – or reflect – off the surface, and repeat to skip along the ground – ducting – outwards toward nearby homes and towns.
This phenomenon is prone to happen when air is coolest near the ground, like on a crisp, dry, spring or fall morning.
“Oftentimes during these seasons, you might have a beautiful, warm, sunny day,” Capella said. “On a still, clear, dry night, the air nearest the surface cools very quickly. This cooling eventually affects a layer a few hundred feet deep. The air above this cooling is the same warm air from the day before. In this case, the temperature is coldest near the surface with a much warmer air above, which is also known as an inversion. Formally, this process is known as stable boundary layer development via ideal radiational cooling.”
Additionally, Capella said, there is phenomenon common to Fort Indiantown Gap and the eastern seaboard in general known as a Cold Air Dam. In short, a cold air wedge from Canada bulges southward, along and east of the Appalachians, and locks against the mountains, he said.
This is a shallow – a few thousand feet deep – albeit deeper than the aforementioned inversion, cold surface air mass, he said.
Both conditions affected Fort Indiantown Gap on March 15, Capella said.
“The day prior warmed up to a lovely 58 degrees, and that night, the surface cooled to a chilly 24 degrees,” he said. “The shallow surface cold layer grew a few hundred feet deep. Meanwhile, cool air from a snow-covered Quebec rushed southward along the eastern seaboard behind a cold front. This cool layer was about 4,000 feet deep. In short – very cold air near the surface, warm air aloft.”
Since temperature increased with height, sounds bent toward the ground, Capella said. The artillery fired sound waves that emanated in all directions, including upwards, but bent back toward the ground, he said. This process allows listeners to hear a bit louder, a bit further away.
In certain cases, Capella said, these temperature profiles can induce a process known as ducting. Sounds can bend towards the ground, reflect off the ground back up into the cold air, refract back towards the ground, reflect off the surface and repeat. This “skipping” can allow someone to perceive sounds from much greater distances and much louder than normal.
“The temperature profile was perfect for sound ducting in southeast Pennsylvania on the morning March 15,” Capella said. “The cold air near the surface allowed sound waves that started at Fort Indiantown Gap to bend back toward the ground, reflect off the surface, and keep skipping along toward nearby towns.”