Jets in the Living Room? Blame Inversions

Science Forum reader Rod Boyce wants to know why his cabin sometimes turns into highway-front property, or at least sounds like it, when it's cold outside. He lives about a mile from a major road, but he notices that the sound of passing cars and trucks seems to be amplified during the winter. I checked, and he doesn't have a faulty hearing aid.

Many people living in Fairbanks have noticed the same phenomenon. My cabin, for example, is 10 miles from Fairbanks International Airport, yet during the winter it often sounds as if I live on the runway.

According to Geophysical institute Professor Emeritus Juan Roederer, unusually loud jets, trucks, and trains in cold weather are signs of a strong temperature inversion over the city.

Temperature inversions--which occur when warm air sits above cold air--form frequently in Fairbanks because of three environmental quirks: a lack of sunlight that fails to warm the earth; clear skies that allow heat to radiate from the ground into space; and winds so calm they don't mix cold with warmer air.

A temperature inversion acts as a lid of relatively warm air that traps cold air close to the ground. Inversions are troublemakers in Fairbanks because the cold air trapped at lung level sometimes contains more than nine molecules of carbon monoxide per every million air molecules, causing the city to violate federal air quality standards.

Roederer said inversions act not only as a box that traps car exhaust, but also as sort of a reflective ceiling off which low frequency sound waves can bounce. The inversion layer traps sound energy that would otherwise escape skyward.

Under normal conditions, sound waves travel from their source in all directions. When a raven squawks, for example, waves travel directly outward from its bill as well as above, below and to the sides. These waves are like concentric ripples in a pond caused when someone throws in a stone. Sound waves, though, travel much faster than surface waves on the water; in air, sound waves zip along at about 1,128 feet per second.

Inversions act as a ceiling--sometimes as low as 10 to 15 feet--that prevent low frequency sound waves from scattering in all directions, Roederer said. When an inversion is present, the sound waves representing the rumble of a far-off jet engine bounce off the slab of warmer air, reflect downward and reach the listener's ear without being weakened by the passage through buildings, spruce trees and other obstacles. Roederer said this process is similar to the one that makes honking one's car horn in a tunnel so much fun.

High pitched noises tend to pass through an inversion layer, Roederer said. For that reason, whistles and other high frequency noises can't be heard as far away as the groan of a locomotive engine during an inversion.

These sounds can also seem louder in the winter because snow-packed streets, snow-covered trees and less activity by the noisiest of organisms---humans---combine to make a city much more quiet when it's cold.