Moscow Mayor Wants To Control Snowfall: Is It Possible?

 (Above) Heavy snow over Moscow. (Courtesy of wintercenter.com)

Dr. Greg Forbes, the severe weather expert for The Weather Channel, recently discussed the plan of Moscow’s mayor to control snowfall in the Russian Capital. Here is his article from weather.com:

The mayor of Moscow, Russia  has made international headlines in recent days by promising to prevent major snowstorms this winter in Moscow through weather modification efforts. Hmmm. Good luck with that!

Moscow is a good-sized city, about 25 to 32 miles in diameter, so that’s an area of about 340 square miles (similar to Minneapolis MN) that needs to be kept relatively snow-free. Climate data from Moscow that I could find weren’t very specific about how much snow falls there, but indicated that snow falls on 79-118 days — mainly from November through March. During those months Moscow gets a liquid-equivalent of 7.30-8.98 inches of rain. If that all fell as snow, they would get more than 73-90 inches! One data source indicates that Moscow averages a snow depth of 17 inches on the ground throughout the winter, so they get quite a bit.

Snowflakes fall at about 3 feet per second, and are usually generated about 6000 feet or a little higher above ground. That means that the snowflake takes a bit more than a half hour to reach ground. Allowing for some time for the snowflakes to grow, you would have to modify clouds when they were more than a half hour away from Moscow in locations where the snow would form and grow.

But can it be done, and how?
From just a logistical standpoint, it seems like it would take a huge effort. Big snowstorms develop in association with large low-pressure systems that have a slow but steady rising motion within them that continually generates clouds and regenerates snow as fast as it can fall out. Snowstorms can last for 24 hours or more, slowly drifting over a city. Seems to me like it would take quite an air force of aircraft (or some kind of surface-based launchers) to constantly modify a swath of clouds at least 25-32 miles wide heading for Moscow.

In the past, the Russian Air Force was hired to prevent rain on parades on Victory Day in May and City Day in September. The idea then — which appears to be the same concept they have in mind now — was to seed the clouds with silver iodide, dry ice, or cement powder to make the clouds drop their rain before they reached the city. At this type of year, relatively small shower clouds can theoretically be induced to rain by adding these materials as sites upon which raindrops can grow. I don’t think this approach is right for winter snow clouds.

But could it be done for snow, theoretically?The winter snow-producing clouds are widespread layers of rather persistent cloud, as described above. From a logistical standpoint, it may not be possible. From a theoretical standpoint, it might be possible, but in a different manner. The possibility would be to seed the clouds with so many tiny particles called “small aerosols”, that none of them can grow big enough to fall as snowflakes. The aerosols are one type of “condensation nuclei” upon which clouds and precipitation form.

There are several researchers who feel that winter precipitation suppression is already inadvertently being done in parts of California and other parts of the West. Givati and Rosenfeld in 2004 and in 2006 and Rosenfeld, Woodley and others in 2008 have found that there is a reduction of winter precipitation downstream of urban areas by 15-25%, thought to be due to air pollution. The culprits might be sulfate particles and other aerosols, possibly a result of emissions by diesel engines.

So maybe the Russian Air Force could emit enough small aerosols to inhibit clouds from forming snow, theoretically.
The ideal situation for precipitation suppression, however, seems to be relatively small, shallow clouds in orographic areas — not the kind of widespread clouds that would affect Moscow. There is also evidence that larger aerosols could increase rather than decrease precipitation. Do it wrong and face court-martial? (weather.com)