It's Cold and Loud Up There

My country's weather network occasionally shows vignettes about weather trivia, and one of them furrowed the brow of my sweetheart when she heard it, and truth be told it threw me off a bit as well.

The coldest temperature ever recorded in Canada was on February 2, 1947 in Snag, Yukon. The air temperature was -63°C. The air was so cold that meteorologists in the area could hear voices from 6 kilometers away.

This lead to questions of whether sound waves can travel further in cold or warm air masses, all else being equal. So I, with my limited physics and more thorough atmospheric science knowledge take a stab at the inquiry.

Google Earth snapshot of Snag area (62° 23.954'N 140° 22.302'W)
Nearest significant hills are 15+km north

It might sound counter-intuitive, but if sound travels faster it does not also travel further. Within warmer air masses, molecules are moving (vibrating) more rapidly, and thus the propagation of sound waves, which move via molecular collision, is quicker. However, as the molecules of air in a warmer air mass are further apart due to expansion, attenuation of sound waves will occur, and thus the intensity (decibel) of the sound will diminish compared to a colder air mass. Essentially warmer air is "stiffer", not as “elastic” as colder air due to the molecules moving around with more energy.

Density is the key for the propagation of sound, and as we all know a cold air mass is denser, with more tightly packed molecules. Under those conditions, attenuation is not as pronounced, and sound waves collide molecule-molecule with ease. But these are simply the basic conditions for sound in the atmosphere, and a Chicago meteorologist explains the key factor best:

The most important factor, though, is the great difference in the thermal structure of the lower atmosphere when it is cold versus when it is hot. When air temperatures change along the path that sound waves are traveling, the waves always bend toward the colder air. In bitterly cold arctic air masses, the coldest temperatures are at the ground with higher temperatures above; sound waves do not disburse upward readily. On hot days, it's just the opposite: It's hottest at the ground and cooler above; sound waves bend up and away.

Certainly to get a -63°C temperature requires a bitterly cold arctic air mass, and early February is deep within winter, where the snow cover is widespread and the positive feedback of constant reflective albedo has had plenty of time to cool down the region. The nearest major town with historical records from Environment Canada is Burwash Landing, and Burwash shows a February daily minimum average of -25°C, with an extreme of -55°C recorded in 1968.  This highlights an extreme diurnal temperature range due to continentality. The Gulf of Alaska is ~300km away, but High pressure Arctic anticyclones (cA) sweeping down from the Beaufort Sea (which is equivalent to a white landmass) ~800km away, prevail over any weak westerlies from the northeast Pacific. The February polar jet stream is far below Snag.

I hope this clears up the same question anyone else might have about air temperature and how sound carries. Inevitably other factors, such as wind direction/speed, topography, and humidity come into play and add complexity. Back on that mighty cold day, the meteorologists also made mention that they could hear their breath freezing just after exhaling. Be careful not to shout in -63°C, or you might break your foot.

Additional Info:

• Chicago Weather Center - Sounds carry better in cold air
• Environment Canada Weather Office - Burwash Landing, Yukon