In October 2009, an otherworldly cloud formation appeared over Moscow. The Sun (the newspaper, not the yellow ball in the sky) promptly announced the appearance of a ‘mystery UFO halo’ and, before too long, internet message boards were awash with rumours of alien motherships and Russian weather weapons.
But sadly, this strange phenomenon is not the work of some little green men: a new study published in Science this week explains how these ‘hole punch clouds’ are created by aeroplanes, not UFOs, zipping through the sky.
How to make a holey cloud
There are two key ingredients required for a holey cloud: supercooled water and a sudden temperature drop.
Supercooled water is water that seems to defy the laws of nature by remaining liquid at sub-zero temperatures. This happens in clouds at temperatures between 0 and -40°C when the surrounding air is so pure that no ice nuclei (e.g. dust particles) are present (the water droplets cannot crystallise).
However, a sudden drop in cloud temperature may help to kick-start ice formation. An aircraft can provide this cooling when it flies through a cloud, via air expansion behind the propeller tips or airflow over the wings.
The introduction of ice into a supercooled cloud triggers a domino effect of rapid ice formation, known in the meteorology world as the Bergeron-Findeisen process. When the ice becomes too heavy to remain in the cloud, it falls to the ground as snow, leaving behind a void. Et voilà…a holey cloud.
A hole new world
In their Science study, Andrew Heymsfield and colleagues from the National Centre for Atmospheric Research (NCAR in Colorado, USA) investigated satellite images of a holey cloud layer over Texas.
In total, 92 holes were spotted, produced by a wide variety of aircraft, including large passenger jets, military jets and turboprop aeroplanes. A typical hole grew for an hour after it was first detected, up to a maximum length of over 100 km in some cases (larger than the distance between London and Brighton). Several holes remained in the sky for more than four hours.
The scientists then used a computer model, configured to match the weather conditions in the Texas satellite observations, to show that there is more to holey clouds than previously thought.
They found that adding ice particles to the simulated cloud released latent heat, a form of energy associated with water changing state (e.g. from liquid to solid). This created updrafts near the hole centre that suspended the ice, allowing the hole to grow. Meanwhile, compensating downdrafts on the outer edges of the hole accelerated the evaporation of liquid water, causing the hole to widen rapidly.
Snow, glorious snow
And because cloud holes are often associated with snow, they can spell bad news for travellers, air traffic controllers, and people with an irrational fear of snowmen. Heymsfield and colleagues estimate that there is as much as a 6% probability of aircraft triggering ice formation near major airports.
If this happened during wintertime months, it could lead to rerouted flights, frantic de-icing, and grumpy passengers. Still, cloud spotters would argue that cranky holidaymakers are a small price to pay for these celestial beauties.
Based on an article written for Cosmos magazine.
Heymsfield, A., Thompson, G., Morrison, H., Bansemer, A., Rasmussen, R., Minnis, P., Wang, Z., & Zhang, D. (2011). Formation and Spread of Aircraft-Induced Holes in Clouds Science, 333 (6038), 77-81 DOI: 10.1126/science.1202851