There was a time when a farmer would sit out in his field, watching a lonesome cloud float away, taking with it the last hope of a much-needed burst of rain. As humans take control over more and more natural processes – not necessarily for the betterment of civilisation – rain too seems to have finally been leashed in.
Just like a farmer throws seeds on ploughed land to harvest plants, clouds can also be seeded with chemicals to induce rain! But to understand how cloud seeding works we must first learn some basic facts about the weather.
Over 99 per cent of a cloud is air and air contains moisture. This moisture is the result of evaporation of water by the heat of the sun. When this warm air or water vapour rises from the earth’s surface and begins to cool, tiny droplets are formed and thus, clouds are made.
For it to rain, temperatures inside the cloud need to be sub-zero, or below freezing point of water. When droplets of this super-cooled water meet specks of dust, salt or sand, they form small ice crystals (see image at the right). Water vapour 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.
Formation of such ice crystals takes a long time. Due to changes in climate, certain areas do not get adequate rain. Lack of rain dries up the land, causing drought. To avoid drought and to increase water resources, scientists have over the years studied cloud formations. They conclude that clouds lack ice nuclei. Therefore, they argue, additional ice nuclei should result in more rain-producing clouds.
As an experiment, scientists sprayed a chemical called silver iodide or dry ice, onto a cloud formation. Sure enough, the chemical induced freezing of the water droplets, and what followed was rain. Silver iodide has a crystalline structure similar to that of ice and this increases ice crystal concentration.
So the final recipe for rain is: one large cloud, though a bunch of smaller ones would do nicely too; a not too large serving of silver iodide (remember, too much and it’s called pollution) and the services of a plane. Shower the silver iodide onto the cloud. Blend well and serve cold. We now have rain!
It’s important to plan things well. Since clouds don’t produce rain until 20 or 30 minutes after they have been seeded, one needs to make sure they deliver at the right place.
The flow of air into the clouds and the liquid water content also suggests if a cloud is likely to produce rain.
Cloud seeding is most commonly done out of airplanes. Planes fly into selected cloud formations and release packets of microscopic silver iodide particles using flares. When the particles meet cool moisture in the clouds, they trigger the formation of ice crystals and raindrops. The amount of silver iodide that is used is small enough to make sure that it doesn’t pose a pollution risk.
Another method is to use smoke machines from the ground. The machines send silver iodide particles billowing from the ground into the atmosphere.
So, the next time you feel like a rain dance, plan ahead. Make rain a part of the menu.