A man and a woman are caught in a small, awkward moment that goes nowhere. The speaker coolly "figures" that some loss of warmth, some evaporation, is only to be expected, and he buttons his collar against the twilight. It reads as a quiet joke about desire cooling off, and about his habit of treating feelings like a forecast.
The woman, meanwhile, takes a bolder step: she slips off her underwear and twirls it on her finger. He answers with nothing, and she fills the silence by recalling a weather map at the bus station, with low pressure expected in the Northwest. She seems to be mocking his detached, calculating tone, using his own language back at him. When he mumbles that he figured something like that, the spell breaks. She turns away and puts her clothes back on, sadly.
The mood is dry, comic and a little sad. The poem suggests a chance missed through too much caution.