The next time you are near an airport; look overhead not just at the planes taking off but at the clouds too. You may get to see strange wispy shapes -- UFO-like maybe, or some very interesting holey ones!

A new study suggests that airplanes may be altering the weather around airports by punching holes in clouds and even causing snow or rain. As planes land and take-off, there is a 6% chance of the planes “seeding clouds” that they punch through and causing precipitation. 

Large holes in clouds have been a source of mystery for observers for some time. One suspiciously saucer-shaped formation over Moscow even led to theories of UFO visits!

Research published in 2010 by scientists at National Center for Atmospheric Research in Boulder, Colorado, offered an explanation for the strange clouds by linking the cooling effect of airplanes to these mysterious gaps.

So what is the secret behind these holey clouds?

The Reason For Precipitation

We all know about the cycle of water. Water evaporates and becomes water vapor when the air is warm. As the warm air rises in the sky, it cools. When enough of these droplets collect together, clouds form. If the clouds are big enough and have enough water droplets, the droplets bang together and form even bigger drops.

For it to rain, temperatures inside the cloud need to be sub-zero, or below the freezing point of water. When droplets of this super-cooled water meet specks of dust, salt, or sand, they form small ice crystals. Water vapor in the cloud then freezes directly onto the surface of these crystals. When these crystals become too heavy to stay afloat, they fall as rain because of gravity. This is known as precipitation which we experience as rain. 

How The Airplanes Do It?

Under normal conditions, clouds at temperatures between 0 and minus 40 degrees Celsius (32 and minus 40 degrees Fahrenheit) contain suspended, super-cooled droplets of liquid water. An airplane, powered by jet engines or propellers, "seeds" clouds like these by expanding and cooling the air that flows under its wings or through its propellers. Cooling creates ice, which attracts the super-cooled water droplets. Together, they grow heavier and create snow or rain, which may plummet to the ground or evaporate away.

In hole-punch clouds, this effect sometimes appears as wisps of ice crystals or snow within or below the cloud. As you may have noticed, the precipitation caused by the airplanes is accidental when the jet has passed through a particular area of a cloud.

Clouds can also be intentionally seeded to alter the weather. To cause rains, clouds are sprayed with silver iodide which induces freezing of water droplets - an expensive process that is done in parts of China. The practice has its critics who do not believe we should tamper with nature and are concerned about the effects of silver iodide.