听力原文：M: Have you ever looked really closely at the snowflake? W: Sure, but they usually melt too fast for me to get a close look. Why do you ask? M: I'm just curious. I was reading an article about the formation of snowflakes, and I real- ized that I had never paid much attention to them before. W: WeI1, there is a big variety, isn't there? M: Yes, but they all have one of the three basic forms: hexagonal columns, thin hexagonal plates and the blanching starshaped form. W: I wonder why are forms different, maybe because ice starts to form. on dust particles with different shape. M: Well, I thought it might have something, to do with the water saturation of the air. But we're both wrong. The author of this article did extensive research and concluded that. the shapes of snow crystals largely controlled by the temperature of the air. For example, the featherly star-shaped snowflake that everyone thinks is typical occurs only at a specific temperature. W: Doesn't the relative humidity has anything to do with the shapes? M: Apparently not. The effect of super saturation is simply to alter the growth rate. The greater the saturation, the faster the snowflakes form. W: Why, next time when it snows. I'll make a point of taking a close look. What are the people discussing?