Artificial rain

In the troposphere, there is decrease in temperature of the atmosphere with altitude so the clouds in the troposphere have lower temperature as they rise in altitude and vice versa. The clouds thus formed are basically of two types:

i) Hot/Warm clouds, &

ii) Cold clouds 

When the temperature of a cloud is above 0°C, the cloud is called a warm cloud; when it is below 0 °C it is called a cold cloud. 

In a warm cloud, small water drops Collides and combines to form larger droplets and  breaks the upward force i.e  the buoyancy of the cloud and falls as rain to the ground. Likewise, inside a cold cloud, ice crystals can also grow in size where they can break the buoyancy and fall as rain. As during fall the temperature gets more than 0°C, the ice crystals melt and gets converted into rain. But if the water droplets are unable to grow in size or there is deficiency of ice crystals, rainfall will not be there even if the clouds are present.

So there is an option of artificially creating ice crystals or helping water molecules to grow in size, and this phenomenon is called as artificial rain.There are many ways to make rain in a warm cloud, e.g. spraying water drops, hygroscopic powder or liquid (sodium chloride), etc, and letting the water drops grow though collision to become rain. There are also many ways to make rain in a cold cloud, but the most common way is to use dry ice or silver iodide. This is mainly because the temperature of dry ice is -78 °C. Spreading dry ice in a cold cloud lacking ice crystals will rapidly reduce its temperature. Therefore we directly transform supercooled water into ice crystals, which will eventually become rain without the help of ice-crystal.

Now a days cloud seeding technique is pretty common which is simply a weather modification leading to formation of artificial rain by sprinkling dry ice or silver iodide. The use of hygroscopic  substance such as table salt for cloud seeding is increasing day by day.  The technique can be used in drought hit areas or the areas which have water scarcity. Although the success and effectiveness of this technique is yet to be seen but it has its successful trials in many countries.