Camping in Antarctica last December, I noticed that even though it was extremely cold, -10°C to -20°C, our exhalations didn’t make visible clouds. When the helicopter landed to pick us up, though, our breath appeared as dense white clouds. Why was this?
Adam Lewis, a glacial geologist from North Dakota State University, initially made this observation while we were walking in the Friis Hills, a 1300m-high plateau about 50km up the Taylor Valley in the Dry Valleys region of Antarctica. Part of a group of geologists led by Tim Naish from Victoria University’s Antarctic Research Centre and Richard Levy from GNS Science, we speculated as to what extent the lack of visible breath had to do with the dryness of the air and the lack of cloud condensation nuclei.
The clouds we expect to see when someone exhales on a cold day are made from tiny droplets of liquid water. These out-breaths create a visible cloud for the same reason clouds form in the sky. Air – which is primarily composed of nitrogen and oxygen – also contains a small amount of gaseous water, and the warmer the air, the more water gas it’s capable of holding. When air cools, it can hold less water gas, and some of the gas will condense out of the air and start to form droplets of liquid water. In sufficient quantities, these droplets can form a visible cloud. Droplets of liquid water tend to form around condensation nuclei, tiny particles of sea salt, dust, clay or soot.
Back in Wellington, I asked MetService’s Erick Brenstrum why our Antarctic breath-clouds were different from those we experience in New Zealand.
“When you breathe out, you’re breathing warm air with high water content,” he explained. In the Friis Hills, the warm, moist breath coming from our lungs would have mixed with the surrounding air, which had a temperature of about -10°C and perhaps only 10% humidity (compared with the 60-90% humidity typical of New Zealand). As warm air from our lungs mixes with the cold air outside, “it cools, and wants to make a cloud. But if the air it’s mixing with is incredibly dry, the water gas is never going to reach a concentration where it forms a liquid and makes a cloud. So if you’re in an incredibly dry environment, even when you breathe out high-humidity air, you don’t get a cloud.”
So what difference did the proximity of the helicopter make? “When you burn fuel in an engine, one of the by-products is water gas. So it could be that locally, under and around the helicopter, you’re suddenly getting a large quantity of water gas. So when you breathe out moist air, you’re able to reach 100% humidity and you get a little cloud.”
Condensation nuclei play a part too. “It’s highly likely that pollutants from the burning of the fuel would act as condensation nuclei. It’s also likely there would be some on the ground, thrown up by rotor action.”
That said, a definitive answer to this question would demand controlled testing and repeatable results. Perhaps a good reason to return to Antarctica?
This story was originally published in The Listener http://www.listener.co.nz/current-affairs/science/clouded-judgment/