Demonstration of Cloud Seeding

The best in-the-laboratory demonstration of cloud seeding I have seen was done by Dr. Ira Geer while he was teaching at SUNY Brockport. The demonstration centered around a small freezer chest, about 2 feet wide by 3 ft long by 2 ft deep of the type which holds ice cream in supermarkets. He painted the inside flat black. For those who can't afford a freezer either in terms of funds or space in the classroom, I've found a simple foam picnic basket with the inside painted flat black and cooled with dry ice works almost as well but see the caution at the end of this note. The demonstration is better if you use a foam chest with a cardboard box painted flat black which fits inside the foam picnic chest to separate the dry ice in the bottom from the air in the box.

Either way, you should order a small amount of Silver Iodide, AgI, and some Acetone for the next school year. And, you will need to order dry ice (there is probably a local source) the appropriate time before the demonstration. If your audio-visual department needs some lead time, order a 35 mm slide projector for the demonstration.

If you have an electrical freezer chest, turn it on the day before the demonstration so the inside gets very cold. This is also a good time to dissolve half a gram of AgI in a few milliliters of Acetone and store in a dark bottle and in a dark place. If you are using a plastic chest, you will need to cool it with chunks of dry ice half an hour before the demonstration.

Arrange the slide projector and the chest or freezer as shown in the figure to the right. Then add some warm water to the chest. Fog/cloud should form.

Scrape a little dry ice into the cloud and observe the trails and the volume of cloud material swept out.

Then add a few drops of the AgI solution to a paper towel, bring it near the top of the cloud and start the solution burning. If you are concerned about the smoke, a can or flask with hoses in both sides can be use to capture the fumes and inject it into the cloud. Tiny crystals of AgI will condense in the smoke and trails of ice should be observable in the chest and the void of cloud material should be easily seen in the ice crystals' wake.

Caution: If you're using a picnic basket, avoid prolonged smelling of the cloudy material as the water droplets absorb carbon dioxide and become dilute carbonic acid. While this isn't particularly dangerous, some people may react to it.