October 1, 2022

The science says there’s no difference between a dry or a wet cold. Sorry

CBC Alberta and Saskatchewan have teamed up for a new pilot series on weather and climate change on the prairies. Meteorologist Christy Climenhaga will bring her expert voice to the conversation to help explain weather phenomena and climate change and how it impacts everyday life.

The wet cold versus dry cold debate happens every year. It can be a point of pride depending on where you are in Canada.

Even among weather experts, there can be disagreements on whether you can say a wet cold is worse than a dry cold.

So let’s settle it — what is the science behind the wet cold/dry cold debate?

The answer? At very cold temperatures, humidity actually doesn’t make a noticeable difference. 

The surprising science of a wet cold versus dry cold

CBC meteorologists Christy Climenhaga and Jay Scotland explain why wet and dry cold aren’t necessarily what people think they are. 2:35

While that may come as a shock (and before the outrage flows in the comment section below), let’s dive into the science.

As we know, air is made up of a number of different gases including nitrogen, oxygen and carbon dioxide. It also contains amounts of water vapour that are constantly changing because of our water cycle. (Remember the water cycle we all learned about in elementary school?)

Absolute humidity is the mass of water vapour in a specific volume of air without taking temperature into account. 

Relative humidity is what you would see on your daily weather forecasts. It looks at the concentration of water vapour in the air as a percentage of what it would be if saturated. 

When we’re talking about relative humidity in the summer heat, of course a 30 C day at 20 per cent humidity will feel different from one at 90 per cent humidity . 

A humid heat can be excruciating, even dangerous. 

The higher amount of water vapour in warm air at high humidities can make it difficult to shed excess body heat through sweat. Without that evaporation, you can get overheated very quickly.

In the heat, humidity will play a big factor in how hot it feels outside. (Ben Nelms/CBC)

Temperature makes a big difference   

When the air temperature drops, the relationship between water vapour and relative humidity changes. 

When the air cools down enough and the relative humidity is 100 per cent, condensation occurs. 

Condensation is that process where water vapour changes to liquid (again, back to that water cycle). Those water droplets make up the clouds we see and when the droplets are big and heavy enough, they fall as rain or snow.

So, when the air cools, the relative humidity rises.

That means that a 90 per cent humidity at 25 C and 90 per cent humidity at –10 C would have different amounts of water associated with them.

At cold temperatures, even at high relative humidities, you would have far fewer grams of water per kilogram of air. 

So in a Canadian winter, the amount of water you are dealing with in a wet cold is pretty similar to that of a dry cold. 

David Phillips is a senior climatologist with Environment and Climate Change Canada. He says the wet cold versus dry cold debate is one of the topics he gets the most questions about. 

Phillips explains that in a kilogram of –20 C air that is almost saturated, you’ll only see around an extra half gram of water. 

“So there’s very little difference. So clearly, if the dry air is more comfortable than the moist air, it can’t be the humidity, it can’t be the water,” he says.

“It’s just not enough water molecules in that air to conduct the heat away from your body, you see, it’s negligible.” 

This principle has been put to the test and was reviewed in a 1988 research paper with Defence Research Establishment Ottawa.

The paper studied a number of experiments performed in the 1950s, where subjects were exposed in a lab environment to cold temperatures at different relative humidity levels.

According to the report, “researchers [did] not find any significant difference in the physiological responses at low and high humidities.”

Why does it feel different?

So if this colloquialism is actually a myth, why does a cold day on the East Coast feel so bone chilling, whereas a –30 C day on the Prairies may be more tolerable? 

Well, put simply, it’s all about the other elements at play and not the moisture in the air. It also comes down to people’s tolerance to the cold and their ability to stay dry themselves.

That’s why some people need to wear a tuque at –2 C in the Maritimes to protect their ears but that same person in Regina won’t need to wear a tuque until it’s –10 or –15. 

Though humidity won’t make a difference in how the temperature feels, wind most definitely will (we will discuss wind chill in the future.) 

A –15 C day in Kingston, Ont., with winds whipping at 50 kilometres per hour will feel every bit as cold as a calm –30 C day in Edmonton or Regina. 

There is also an element of sunlight that comes into play. 

With higher humidities, you are often dealing with a little more cloud cover than those cloudless winter deep freezes on  the Prairies. 

That little bump of solar radiation can make us feel a little warmer in a dry cold situation.

Another factor is clothing. Winter fashion on the Prairies tends to look a little different from that in Toronto. Bundling up in our parkas will obviously keep us warmer than something much lighter. 

And when looking at your clothing, Phillips says staying dry is key. If you become damp, of course that will cool you down in a hurry.

So this winter, it doesn’t really make a difference if you are seeing a wet cold or dry cold day because, well, cold is just cold.

Our planet is changing. So is our journalism. This story is part of a CBC News initiative entitled Our Changing Planet to show and explain the effects of climate change and what is being done about it.