How Things Work: Man-Made Weather

Man-made snow

Snow gun, snow cannon, snow maker: Call it what you want, just don’t call it artificial. Snow produced by these machines is still snow — it’s just man-made.

Natural snow that falls from the sky forms crystals that consist of six arm-like structures, called dendrites. This differs from the composition of man-made snow, which is denser and theoretically more durable than natural snow.

There are two basic kinds of snow makers. One requires a compressed air supply, while the other uses a fan.

A snow maker produces snow by compressing air and water in the machine’s body at a temperature below 30°F. The precise mix of water and air depends on the day’s humidity and temperature.

The air then breaks up the water stream into miniscule droplets and simultaneously blows them out into the surrounding air. Once they leave the machine, the droplets turn into snow, which falls to the ground. Mountain resorts with a lot of land to cover often transport piles of snow to other areas of the mountain, where machines groom the snow.

Snow makers also produce two types of snow, wet and dry. Wet snow sticks to everything, whereas dry snow is suitable for winter sports.
The airless snow gun, also called a snow fan, works like an amusement park spray fan. Water goes into the machine, atomizes, freezes, and is then blown out of the machine by an enormous fan.

Snowmaking equipment isn’t just for ski slopes anymore. At the price of $2400, the Blizzard Sport snow maker will turn even the blandest of backyards into a winter wonderland.

Backyard Blizzard, the company that sells this snow maker, claims that with the help of an ordinary garden hose, Blizzard Sport produces 50 cubic feet of snow per hour. In comparison, a commercial snow maker produces 20,000 cubic feet of snow in under 20 minutes.

Man-made precipitation

Sometimes, the best way to change the weather is to go straight to the source. In a method called cloud seeding, substances are added to clouds to induce rain or snow.

Most of the time, scientists induce precipitation in clouds with the compound silver iodide, which is similar in composition to ice crystals naturally formed in clouds.

Silver iodide can either be fired into clouds from rockets on the ground or deposited directly using planes that release silver iodide flares into clouds.

Once there, the silver iodide causes the supercooled liquid inside of the cloud to freeze through a process called heterogenous nucleation. This is the process by which a system begins to transition from on phrase of matter to another.

Other materials, including dry ice and propane gas, can cause the temperature of an entire cloud — not just the liquid inside it — to decrease to lower than ­–40° C. This cooling process causes homogenous nucleation, which is similar to heterogeneous nucleation except that it occurs in a uniform system.

Both of these processes cause ice particles to form within clouds. The vapor pressure is lower over water than it is over ice, and consequently, when the ice particles become sufficiently large, the cloud is no longer able to support them. The particles exit in the form of snow, though they may melt into rain during their descent.

When rain or snow falls from a cloud that would not have otherwise yielded precipitation, the process is called static seeding. Static seeding typically occurs in mid-latitude clouds. Warm-season clouds, on the other hand, rely on a process called dynamic seeding to produce precipitation. Dynamic seeding increases the size of a cloud using heat that is released as a byproduct of freezing.

There is no definitive proof that cloud seeding increases rainfall since it is impossible to know how much rain would have fallen from the clouds had they not been seeded. The general consensus among experts, however, is that the seeding is worth the cost and effort.

Pouring tens of millions of dollars into cloud seeding, China hosts the world’s most comprehensive seeding system. Though the per capita water resources are 40 percent over the world average, northern China receives only 65 percent of the world’s average yearly rainfall.

Thanks to the use of silver iodide in forming precipitation, every cloud really does have a silver lining.