In the Mediterranean countries, it was observed in ancient times that a number of events occurred almost simultaneously, and with surprising regularity, with the approach of the middle of July. Firstly the Dog Star, Sirius, became visible above the horizon in the night sky, and ushered in the "Dog Days", a period of hot, dry and very sultry weather over Italy and further west that you may have read about in Weather Eye on Saturday. Meanwhile across the sea in Egypt the annual flooding of the Nile began. And in the eastern areas of the Mediterranean, a surprisingly persistent, cool and welcome, northerly breeze set in.
Now the ancient Greeks were very organised about their winds, and liked to give them names. Apeliotes, for example, was the showery east wind, Notos was the moist and sultry southerly, and Zephros was the gentle breeze wafting across the Mediterranean from the west. The northerly breeze that coincided with the Dog Days they called the etesian wind - presumably because of its regular annual occurrence, since the name comes from the Greek word etos, meaning "year". It was a welcome guest; it tempered the sultry heat, and was a benign, dependable friend of ancient mariners, as Horace notes in one of his valedictory odes:
. . . the raging wind
To thee, O Sacred Ship, be
kind;
And gentle breezes fill thy
sails,
Supplying soft etesian gales.
The Greeks put all these happenings down to the influence of Sirius. They believed that the extra heat which flowed from it was the reason for the Dog Days; this extra heat, they reckoned, also supplied the energy for the etesian winds, and these in turn caused the flooding along the banks of the Nile by blowing against the flow, and obstructing the progress of the mighty river towards the sea.
Summers in the eastern Mediterranean are still dominated by the gentle northerly breeze of the etesian winds, which lend a pleasant coolness to the idyllic landscape all around. But, as we now know, they have no connection whatever with Sirius or any other star. They are part of a large circulation of air which comes about because of low pressure to the east over India - which also brings the Indian summer monsoon - and the Azores High in the Atlantic to the west. The combination leads to a northerly flow of air between the two, and the topography of the region is such as to cause a concentration of this northerly drift over mainland Greece and the islands of the blue Aegean.