Extra - July 11, 2014
Dog days
Sirius blamed for extra heat

By Philip Eden

The 'dog days' of 2014 began a week ago last Thursday, and only in the Midlands, East Anglia and the Southeast was it warm. This period of high summer which lasts from July 3 to August 11 is traditionally supposed bring the hottest weather of the entire year, and the expression 'dog days' conjures up an image of lethargy and laziness.

The Romans called the period of greatest summer heat [IT]caniculares dies[RO] or dog days because they wrongly believed that Sirius, the brightest star in the heavens, reinforced the power of the sun's rays and so caused the weather to become even warmer although the sun had passed its highest point in the sky on June 21. In those days, Sirius, often called the dog-star because it is the brightest in the constellation [IT]Canis Major[RO] or the Great Dog, rose and set at approximately the same time as the sun between early July and mid August. We now know, of course, that the heat energy supplied by all the stars in the sky is negligible; it is, in fact, less than one ten-millionth of the energy we get from the sun.

It is true that our highest temperatures, on average, come well after the summer solstice - in mid to late July in inland parts of the UK, and in early to mid August in coastal districts. This lag of three to six weeks is because it takes time to heat up the Earth's surface. The heat capacity of the oceans is much larger than the continents, thus central Asia reaches its warmest barely two weeks after the solstice while mid-Atlantic water temperatures do not peak until the beginning of September.

During the twentieth century there were 21 days when the temperature reached or exceeded 95F (35C) somewhere in the British Isles. Only ten of these occurred during the dog days; the earliest such occasion occurred on June 26, whereas the latest happened on September 2. That extraordinarily late burst of heat happened 73 days after the solstice. The highest of all occurred in the 21st century - 2003 to be precise - when 100.6F (38.1C) was logged at Kew Gardens on August 9.

For those who believe in ancient country weather lore there is little comfort this year in the old saying, rather reminiscent of the St Swithin's Day lore:

[IT]"If it rains on first Dog Day, it will for forty days thereafter."[RO]

By Philip Eden

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