Hagno
An Arcadian nymph, who is said to have brought up Zeus. On Mount Lycaeus in Arcadia there was a well sacred to and named after her. When the country was suffering from drought, the priest of Zeus Lycaeus, after having offered up prayers and sacrifices, touched the surface of the well with the branch of an oak tree, whereupon clouds were formed immediately which refreshed the country with rain. The nymph Hagno was represented at Megalopolis carrying in one hand a pitcher and in the other a patera. (Description of Greece by Pausanias viii)
From Smith's Dictionary of Greek and Roman Biography and MythologyFrom Greek Popular Religion by Martin P. Nilsson
It was seldom necessary to pray for rain in Greece, for the course of the seasons is much more regular there than in northern Europe. Late autumn and winter bring rain; summer brings drought and heat. On the other hand, the weather is not so regular that certain days of the year could be fixed upon for weather magic. This is the reason why as weather god Zeus had few festivals. Sometimes heat and drought were excessive. Myths have much to tell about these disasters, and it is related that they were sometimes so great that the most extreme of all sacrifices, a human sacrifice, was offered. Two such sacrifices are recorded from historical times, one to Zeus Lykaios and one to Zeus Laphystios. Zeus Lykaios received his name from the high mountain in southwestern Arcadia, Lykaion, on the top of which he had a famous sanctuary. Zeus Laphystios was named after the mountain Laphystion in Boeotia, although his cult belonged to Halos in Thessaly. On Mount Lykaion there was a well called Hagno. When there was need of rain the priest of Zeus went to this well, performed ceremonies and prayers, and dipped an oak twig into the water. Thereupon a haze arose from the well and condensed into clouds, and soon there was rain all over Arcadia.
From The Mycenaean Origin of Greek Mythology By Martin P. Nilsson
From this point of view it is possible to explain the outstanding rôle of the fleece in the myth. On the top of Mount Pelion there was a sanctuary of Zeus Acraeus to which a procession went in the severest heat of the summer, about the time of the rising of Sirius; the men were girt with sheep fleeces. This Zeus is the cloud-gatherer and rain-giver, and the procession is in accord with quite a number of similar customs the aim of which was rain-magic. That the fleece served as a means for weather-magic is proved by various known facts. Hence we are able to understand the curious fact that the Cloud, Nephele, is said to be the wife of Athamas. She is the longed for rain-cloud which in another instance of weather-magic, people saw rising from the well of Hagno on Mount Lykaion.
