aeromancy

aeromancy \AIR-uh-man-see\, noun:

The prediction of future events from observation of weather conditions.

The wreckage of vast sundowns predicts in sulphurous aeromancy havoc to be.
-- Robert Malise and Bowyer Nichols, Fantastica: being The smile of the Sphinx, and other tales of imagination

Scrutinize the clouds for sun-dogs, gaze into the eyes of a baby or a feline, and practice the ancient divination arts of augury and aeromancy by recognizing intuitive messages received when watching birds or airplanes fly across the sky.
-- Margie Lapanja and Andrew F. Smith, Romancing the Stove: Celebrated Recipes and Delicious Fun for Every Kitchen

Aeromancy is but one sort of divination practiced in medieval times. Some other examples: augury, based on the flight of birds; cheiromancy, palm-reading; and oneiromancy, the interpretation of dreams.