Posted 30 June 2011, by Dave Mosher, National Geographic, news.nationalgeographic.com
Mysterious holes in clouds made by aircraft may owe their huge sizes to a little bit of heat, a new study suggests.
For decades people have seen gargantuan holes form in high, thin clouds made of supercooled water—liquid droplets that are chilled below the freezing point but that don’t have any particles around which ice crystals can form.
In the absense of dust, these cloud droplets can turn to ice if the water gets cooled beyond -40 degrees Fahrenheit (-40 degrees Celsius). At such chilly temperatures the water molecules slow down enough to freeze spontaneously.
Researchers previously knew that plane wings, propellers, and turbines could chill supercooled water via rapid expansion of air in their wakes—making things cold enough to force the liquid to become ice. This mechanism is thought to be what creates hole-punch clouds.
Watch an animation of satellite images showing circular and x-shape holes forming in supercooled clouds over Arkansas, Louisiana, Texas, and Oklahoma in January 2007. Video courtesy NCAR/NOAA
As the water freezes, though, the change of state releases energy in the form of what’s called latent heat, and the role of this heat was suspect.
“I didn’t think the latent heat would be so important, but it drives the whole feedback cycle, in some cases for hours after a plane flies through,” said study co-author Gregory Thompson, an atmospheric scientist at the National Center for Atmospheric Research (NCAR).
“That’s why the holes can grow to the size of cities under the right conditions.”
Research Flights Affecting Cloud Data?
The researchers theorized that, as latent heat rises, it carries freshly frozen ice—material that would normally float down—back up into the cloud.
There, supercooled water droplets migrate to the ice crystals, feeding a chain reaction of ice formation. Eventually the ice patch becomes too dense and falls out as a flurry of snow.
To see if latent heat does lead to hole-punch clouds, the researchers ran cloud-model simulations with and without the effect.
The first simulation, which incorporated latent heat, showed that the heat suspended ice in the cloud, powered nearby evaporation, pulled surrounding vapor into the zone of crystallization, and created snow. The model ultimately formed holes in clouds that closely matched real images of the phenomenon.
The simulation without the latent-heat effect didn’t replicate what’s been documented in nature.
Thompson emphasized that this finding almost certainly doesn’t change our understanding of the role of aircraft in global climate. Nor do hole-punch clouds cause significant snowfalls around airports, he said: “It’s likely too minor for that.”
However, researchers “spend an awful lot of time flying through clouds to collect data, which we use to build models that mimic natural clouds. We may be altering that data as we measure it,” he said.
“It’s not a big effect, but it’s something to be mindful of in future atmospheric modeling.”