单项选择题 Snow has obliterated surface features and the sky is covered with uniform，altostratus clouds. There are no shadows and the horizon has disappeared. What is this condition called? （）

单项选择题 Relative humidity is the percentage of water vapor that is in the air as compared to the maximum amount it can hold at（）.

单项选择题 Recurvature of a hurricane’s track usually results in the forward speed（）.