当水汽凝成雨水，会造成局域性的大气压力下降。
As the vapor condenses into rain, the local atmospheric pressure drops.
一个可能的解释是存在一个缓慢的大气二氧化碳浓度的下降，始自3百万年前。
One possible explanation is that there was a slow decline in the concentration of carbon dioxide in the atmosphere, starting around 3 million years ago.
此外在没有降雨的时候，颗粒物通常会停留在大气上层，而不会下降到街道路面的高度。
And, when the weather is not rainy, particles usually stay lofted in the air and do not reach street level.
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