NARRATOR：

Now listen to part of a lecture on the topic you just read about.

FEMALE PROFESSOR：

It’s not clear that cloud seeding is all that effective and there are reasons to question each of the arguments you just read.

First, it may be true that under laboratory conditions, silver iodide creates snow instead of hail.

However, in real life, silver iodide can actually prevent any precipitation at all from forming in the clouds—snow, rain, or hail.

This is a bad thing, because if you seed all the clouds in areas where it doesn’t rain very often, you run the risk of causing a drought.

In this case the crops simply get damaged for a different reason—lack of water.

Second, it’s not clear that the positive results with cloud seeding in Asia can be repeated in the United States.

The reason is that cloud seeding in Asia was tried in urban areas—in cities.

And cities tend to have a high level of air pollution—from car traffic, industry, etc.

Surprisingly, pollution particles can create favorable conditions for cloud seeding, because they interact with clouds and the seeding chemicals.

Such favorable conditions for cloud seeding may not occur in an unpolluted area.

This means that the cloud seeding method that works in polluted cities may not work in unpolluted farming regions in the United States.

Third, the local study mentioned in the passage isn’t very convincing either.

That’s because the study found that hail damage decreased not just in the area where the cloud seeding actually took place, but also in many of the neighboring areas to the east, south, and north of that area.

So, the fact that the whole region was experiencing a reduced number of hailstorms that particular year makes it more likely that this was a result of natural variation in local weather and had nothing to do with cloud seeding.