When I woke up this morning it was -37 F with a windchill of -64F. I'm glad that I didn't forget to plug my truck in. This is the coldest temp we have had in Kotzebue so far this winter. Because we live on the coast we don't usually get those brutal temps of -50F and -60F like they do in the interior part of the state. Every now and then though, we have a bit of a cold snap like we are having right now.
This morning I was outside when the morning Alaska Airlines flight was getting ready to take off. Because Kotzebue is only 1 mile long and a half mile wide and the airport is located on the south end of town you can always hear the jets and other planes landing and taking off. You hear it so much that you sometimes you don't even notice it. But this morning, the sound of the jet taking off was deafening. It was like being right underneath it. This is something we experience in the winter. The colder it is, the louder the sound and the father it travels. I have also noticed this out on the ice when I can hear someone else's voice from over 1/2 to one mile away.
Ned Rozell, a science writer at the Geophysical Institute, University of Alaska Fairbanks wrote an excellent article that discusses this phenomenon. "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." Mr. Rozell explains. "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 create favorable conditions for a temperature inversion." When conditions are right "A temperature inversion acts as a lid of relatively warm air that traps cold air close to the ground."
So we now know what causes the inversion, but why does it make everything so dang loud? Ned goes on to explain...
"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."
The Alaska Science Forum is an excellent website to read all sorts of interesting articles about the how's and why's of Alaska. The website is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Check it out.