‘Ottawa’ Why Your Voice Carries Farther on a Cold Morning
This winter, Postmedia’s Tom Spears looks at what makes our coldest season tick. It’s a series we call The Science of Winter, and today we ask why sound itself behaves differently on a very cold day.
“Never tell a secret on a cold day,” says David Phillips, who is Environment Canada’s senior climatologist and the country’s resident expert in all things weather.
Cold air slows down the speed of sound compared to warm air. But it also makes sound travel farther.
Phillips likes to tell the tale of Canada’s coldest recorded day, Feb. 3, 1947, when it reached -62.8 C in Snag, Yukon. Canada used Fahrenheit at the time, so the clippings from a 1947 newspaper show it down in the minus-80s.
People could hear their breath hissing as the moisture turned to ice crystals and fell to the ground. They heard banging and booming like gunshots as the expanding ice cracked on the White River.
But weirdly, they could also hear voices from far away. People at the airport, more than six kilometres from town, could hear dogs barking in town. They could hear distant people’s conversations as well.
Snag’s frigid day was an extreme version of what we also get on very cold winter days in Ottawa and elsewhere.
The cold air forms a dense, heavy mass that sits down at ground level. Air a little above that is actually warmer — a condition called a temperature inversion. And Tuesday morning here was no exception: The early morning temperature at the Gatineau Airport was -23.4 C while it was -20.6 C higher up at Ottawa’s International Airport.
Sound waves tend to refract, or bend, from material where they travel faster toward one where they travel more slowly, so in this case toward the colder, denser air. This means sound waves spreading out from someone at ground level are refocused back down toward the ground. The sound follows a curving path, and it travels farther under those conditions. Those with a thirst for mathematical explanations with sines and angles can look up Snell’s Law, named Snell’s because it was first described by a guy named Ibn Sahl, and later rediscovered and published by one named Descartes. Willebrord Snellius was in between those two. (Oh, just look it up.)
Snag had its own weather station on the record cold day in 1947, and weather officer Gordon Toole confirmed that Snag residents could eavesdrop on a conversation several kilometres away when the temperature dropped.
Very cold days often have still air because that dense layer at ground level isn’t moving anywhere. That means there’s no competing noise from wind, and other sounds are easier to hear.
But this oddity in the movement of sound is a feature of summer as well.
If you have a summer cottage, you may have had the experience of seeing visitors learn this the hard way. They head out on the lake, maybe in a boat, maybe on a raft or a sandbar, and say something indiscreet, not knowing that sound also travels a surprisingly long way across the water.
The same effect is in play as during the winter. Even in July, the air temperature down at water level is cooler than the air above it, and secrets told on the water can be refracted back down to listening ears on land.
But that, of course, is the science of summer.