According to a saying of the time, the arrivals of treasure fleets were like light summer rains that wet the roof tiles for a moment and then evaporate.
He wrote that in the morning, after “the layer of dew evaporated . . . , upon the surface of the wilderness there was a fine flaky thing, fine like hoarfrost upon the earth.
The glands secrete the oil layer, so thin that it doesn’t distort your vision, yet keeps the tear film from evaporating and causing uncomfortable dry spots on the eye between blinks.
Whether this cloud layer heralds rain in continental Europe the following day or simply evaporates in the sun’s heat depends on a temperature difference of only a few tenths of a degree.