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