It was observed in ancient times that a number of things happened almost simultaneously, and with surprising regularity, in mid-July in the eastern reaches of the Mediterranean sea.
First, as we noted yesterday in "Weather Eye", for a 40-day period around this time of year Sirius, the brightest of the stars, rises and sets at nearly the same time as the sun. The Greeks and Romans concluded that it was the extra radiant power of this star, the "Dog Star", added to that of the sun, that provided the palpable increase in heat that characterised what they called the "Dog Days"; a period of several weeks of almost unbearably hot, dry and very sultry weather of the kind that last week affected south-east Europe.
But also at this time of year it was noticed that across the sea in Egypt the annual flooding of the Nile began. And third, over the Greek islands, a pleasant and surprisingly persistent, northerly, cool breeze set in.
This last was such a regular annual visitor that the Greeks called it the etesian wind, from the Greek word etos, meaning year. It was a very welcome guest, as it will be more than ever this particular year, blowing southwards over the Aegean Sea. It tempered the sultry heat of the "Dog Days", and was a benign, dependable friend of ancient mariners. Horace, for example, includes it in his valedictory ode:
And he who rules 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 different happenings down to the influence of Sirius. They thought, as we have seen, that the extra heat which flowed from it caused the "Dog Days", and also that its energy caused the etesian wind. And the etesian wind, in turn, they wrongly believed, caused the flooding of the Nile by blowing against the river and preventing it from flowing to the sea.
But the etesian wind is no longer explained by reference to the stars. We now know it to be part of a large circulation of air caused by low pressure to the east over northern India - which also brings the Indian summer monsoon - and the Azores high in the Atlantic to the west.
The combination gives 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 Greece and the Aegean islands. It lends a pleasant coolness to those idyllic surroundings and will be particularly welcome in the current year.