Sentence:  "The air under each cloud has spent time near the surface, has been slowed and backed by friction—it is a <WSD>lull</WSD>."<!-- in case anyone's wondering, yes, this surfing usage is a special case of the general sense of lull as a pause, but in surfing a lull means specifically a pause between sets of waves --> 
Meaning of the word marked with <WSD> tags:  A period without waves or wind.