In the late 1980s, in response to a request from the Canadian military, a Department of National Defence scientist named Dr. Rita Crow set out to unravel a long-standing Canadian mystery. "What factors," she asked, "make cold-wet conditions so chilling?"
The idea that damp cold is more shiver-inducing than dry cold is often taken for granted – at least, it is in my family. My parents grew up on the Prairies and my older brother lives in Saskatoon, while I grew up and still live in Toronto. Minus 20 on the steppes is better than a slushy -2 C in Toronto, they tell me.
But is there really a difference? As Crow soon discovered, the answer is surprisingly elusive – even today.
"Ah, yes, the old 'damp cold' debate," said Dr. Ollie Jay, a thermal physiologist at the University of Sydney and former head of the University of Ottawa's Thermal Ergonomics Laboratory, when I put the question to him. "It is very odd that there seems to be very little data on this idea."
When he posed this question in undergraduate lectures in Ottawa, Jay recalls, students from the Prairies would swear that their dry cold wasn't as bad as similar temperatures in the capital.
The idea is intuitively plausible, since humidity has a clear effect in hot weather. In humid air, sweat evaporates from your skin more slowly, which makes you feel hotter.
In cold air, of course, sweating isn't usually an issue.
Instead, the usual explanation is that damp air transfers heat more quickly than dry air, allowing more heat to escape from your body and making you feel colder.
"The rate of heat transfer should be affected by humidity in the air," explains Dr. Ira Jacobs, the dean of the University of Toronto's faculty of kinesiology and physical education and a former Defence Research and Development Canada researcher. That's a straightforward consequence of the fundamental laws of thermodynamics.
But even though the theory is sound, the difference is unlikely to be noticeable, according to Jay. That's because the maximum amount of moisture in cold air is much smaller than in warm air, meaning that differences in relative humidity (and thus also in heat conduction) are trivial.
Consider a hot summer day at 30 C. If the relative humidity is 80 per cent, a kilogram of air holds 22 grams of water; if the humidity drops to 20 per cent, the water content drops to just 5 grams – a massive difference.
At 0 C, on the other hand, the same air holds only 4 grams of water even at 100-per-cent humidity, since slower-moving water molecules are more likely to condense or freeze. By the time you get down to -25 C, that drops to less than a gram of water. Below freezing, it's fair to say that all cold is dry cold.
This objection was noted by Crow, whose exhaustive review, published in 1988, summarized the results of a series of military experiments in Canada, Britain, and the United States in the late 1950s. Hapless subjects were exposed to near-freezing temperatures at low and high humidity, with and without clothes, while having their skin and rectal temperatures measured and their subjective responses noted. Humidity didn't make any measurable difference.
Still, this apparent debunking conflicts with the experience and convictions of millions of Canadians, so Crow considered other theories.
One possibility is that droplets of water vapour – fog, essentially – get into your winter clothing and reduce its insulation. There's no doubt that wet clothing loses heat more quickly, as quantified in peer-reviewed studies such as "Impact of wet underwear on thermoregulatory responses and thermal comfort in the cold," by Norwegian and Danish researchers.
This explanation may be plausible for "British winter" conditions with fog and temperatures just above freezing. But at colder temperatures, it runs into the same problem: The amount of moisture in the air is negligible no matter what the humidity is.
In her search for answers, Crow turned to Environment Canada to get weather records for 12 Canadian cities and an air base in Germany. The cold-wet sensation, she determined, wasn't associated with wind or atmospheric pressure, but did seem linked to cloud cover.
This line of reasoning makes sense. By some estimates, Crow reported, direct sunlight can make the air feel 5 degrees warmer than it actually is. And Environment Canada's map of annual sunlight (compiled for solar power development) could easily be mistaken for a guide to where people boast about their "dry cold," with a broad hot spot across the Prairies.
The top sunlight cities in Canada: Regina, Saskatoon, Calgary, Winnipeg, Edmonton.
In the end, then, it may be that cold, dry air – as measured in a laboratory climate chamber, at least – feels no different than cold, damp air. But cold, dry air under a clear blue sky feels much nicer than the cloudy grey skies that afflict cities such as Toronto and Vancouver, whatever the humidity.
It's a nice theory, but its confirmation will have to wait for further study. "You know those raw days when you feel chilled to the bone the moment you step outside?" Crow wrote in a 2009 memoir, more than 20 years after her literature review was published. "I still don't know why this makes one feel so cold."