其实帮助里面有现成的答案，看不懂别问我，哈哈(因为我头晕)

Atmosphere Shadows group

These controls let atmospheric effects cast shadows.

On: When on, atmospheric effects cast shadows as the light passes through them. Default=off.

Note: This control is independent of the On toggle for normal Object Shadows. A light can cast atmospheric shadows but not normal shadows, or vice versa. It can cast both kinds of shadows, or neither.

Opacity: Adjusts the opacity of the shadows. This value is a percentage. Default=100.0.

Color Amount: Adjusts the amount that the atmosphere's color is blended with the shadow color. This value is a percentage. Default=100.0.

其实应该养成看帮助的习惯。
关于这几个参数：比方说你的场景里的灯光打开了投射阴影开关，场景里的物体当然有阴影了。假设又增加了一个质量雾大气效果，那些雾是没有阴影的。再打开Atmosphere Shadows的开关，灯光就可以投射雾的阴影。下面的两个值就好理解了，一个是大气阴影的不透明度，另一个是阴影与大气效果颜色的混合度。