According to paragraph 2, what is the condition for crystallisation?

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 39 to 43.

During droughts water is scarce but is it possible to make it rain to provide water. Experiments in cloud seeding suggest that it may be possible to artificially create rainfall.

Rainfall occurs when supercooled droplets of water – those that are still liquid but are at a temperature below the usual freezing point of zero centigrade – form ice crystals. Now too heavy to remain suspended in the air, these then fall, often melting on their way down to form rain.

Even in dry areas the air usually contains some water. This can be made to come together and form ice crystals by seeding the atmosphere with chemicals such as silver iodide or dry ice. They work to promote rainfall by inducing nucleation – what little water is in the air condenses around the newly introduced particles and crystallises to form ice. The ‘seeds’ can be delivered by plane or simply by spraying from the ground.

But does it work? It’s hard to tell for sure. As is often the case with weather and climate, it’s impossible to carry out a controlled experiment – so, in areas of increased precipitation, we can’t know whether it would still have rained even if the clouds hadn’t been seeded.

Success has been claimed for trials in Australia, France, Spain and the US. In the United Arab Emirates, the technique is ascribed to the creation of 52 storms in the Abu Dhabi desert, while China boasts of having used the technology in reverse to keep the Beijing Olympic Games of 2008 dry. Recent research, however, suggests that it’s not as effective as was previously believed.

(Source: http://www.physics.org/)