A heavily overlooked epithet of Zeus is that of Rain Bringer; Ombrios (Ομβριος), or Hyetios (Ὑετιος). More than likely these two were the same Theos, in a different epithet; 'Ombrios' comes from the Hellenic 'ombros', meaning rain. 'Hyetios' most likely comes from the mountain range of Hymettos, where at the very top, an altar to Zeus Hymêttios (Ὑμηττιος) stood. Oftentimes, the worship of Zeus took place on mountains, but because almost all epithets of Zeus would be worshipped on mountains, it was deemed somewhat inappropriate to favor one aspect of Zeus over another, and give Him a specific title; instead, the sanctuary was dedicated to an epithet of Zeus that gained the name of the mountain itself. This way, Zeus could be worshipped on any mountain by any epithet; so while there is no evidence of other mountain sanctuaries to Zeus Ombrios than the one at Hymettos, that does not mean other mountain sanctuaries would not have attracted pilgrims looking for rain. There might have been a small difference between the two, however, as Zeus Ombrios was seen as the bringer of lasting rain, while Zeus Hyetios brought heavy rain (or storms).

The sanctuary of Zeus in the mountain range of Hymettos (Υμηττός) is located in the Athens area, in East Central Greece. Numerous offerings have been found on the site of the sanctuary that stood near (or on) the very top of the mountain, including a pit full of shards and a large stele with cuttings for a small bronze statue, possibly of Zeus Hymêttios. These offerings have been dated to the 8th-7th centuries BC. Pausanias makes mention of this sanctuary (as well as one on Parnes) in his History of Greece:

"The Athenians have also statues of gods on their mountains. On Pentelicus is a statue of Athena, on Hymettus one of Zeus Hymettius. There are altars both of Zeus Rain-god and of Apollo Foreseer. On Parnes is a bronze Zeus Parnethius, and an altar to Zeus Semaleus (Sign-giving. There is on Parnes another altar, and on it they make sacrifice, calling Zeus sometimes Rain-god, sometimes Averter of Ills. Anchesmus is a mountain of no great size, with an image of Zeus Anchesmius." [1.32.2]

The mountain would, in ancient times, have served as a natural weather station; in fact, it is still regarded in that capacity today. A thick concentration of clouds near the summit would undoubtedly have spelled rain. Zeus in his epithets of Rain Bringer is more than likely an agricultural deity; small shrines to Zeus Ombrios would have dotted the land, often located on or near fields for easy access and as markers: those who sacrificed to him would expect he rain to fall on or near the shrine. That said; sometimes the rain simply did not come. A fictional account noted down by famed writer Alciphron describes a sacrifice to Zeus Ombrios that was in vain:

"A drought is upon us. Not a cloud is to be seen in the sky, and we want a regular downpour. You have only to look at the ploughed land to see how dreadfully parched the soi is. I am afraid all our sacrifixes to Jupiter Pluvius [Zeus Ombrios] hve gone for nothing, and yet all we villagers outdid each other to make a good sacrificial show. Each man brought what he could according to his means and ability. One brought a ram, another a goat, another some fruit, the poor man brought a cake, and the positive pauper some lumps of decidedly mouldy incense. No one could run to a bull, for our Attic soil is thin and cattle are scarce. But we might have saved our expense. Zeus it would seem is 'on a journey' and cannot attend to us."

Sacrifices of the animal kind were not at all scarce in relation to Zeus Ombrios, but there is also mention of human sacrifice. When the Eleans consult the oracle during a prolonged drought, they are instructed to sacrifice a noble boy to Zeus. A youth named Molpis volunteers, rain falls, and the Eleans build a sanctuary of Zeus Ombrios, setting up a statue of Molpis there. Of course, these accounts are old and not altogether trustworthy.

Personally, I rarely pray to Zeus Ombrios for rain; I mostly pray for him to keep the rain away. When I still worked outdoors a lot, I would pray to him every day I went out, to just keep back the rain until I got home--especially during late autumn and winter. In winter, I also prayed to Khionê to keep the snow at bay. Quite recently, I have been praying for rain for a farmer friend of mine every day; he's in Arkansas and dealing with hot and dry weather that are threatening his crops. so far, he has gotten two days of rain, and it looks like he might salvage his harvest. I have a close emotional connection with Zeus Ombrios, and regard Him very fondly. Perhaps you might look into His worship and subsequent aid now too.