Last month (July 2012), my birth town of Chicago, Illinois experienced a three-day string of high temperatures that exceeded 100°F (37.8°C). The last time that officially occurred in the Windy City was 65 years ago in August of 1947: August 4th to 6th to be exact. That event was a few weeks prior to a very important date for me. For on 29 August, I was born. As I look back, I think I may have entered the world crying for more heat — though I am sure my mother was glad I was out. As I recall, August 1947 remains as the hottest August in the weather history of Chicago at 80.2°F (26.8°C). Eighteen days topped 90F and four went over 100°F (37.8°C), the 24th being the other 100°F (37.8°C) day.
According to WGN Meteorologist Tom Skilling, days prior to my birth (17-24 August) gave an eight-day string of heat with 100°F (37.8°C) reached on the 24th. Not only were the days scorchingly hot, but the nights were hot and muggy as well, with only one low temperature below 75°F (23.9°C) during the period.
Morning Weather Map for 24 August 1947
Five days after this heat wave ended, I emerged. During the night, a front had come through bringing heavy rains, that my grandmother would later remember, caused water to enter the basement of our home.
Morning Weather Map for 29 August 1947
Such stretches of hot, steamy weather were, and likely still are, know around Northern Illinois, as the dog days of summer.
The Dog Days of Summer
Why the dog days? What is the origin of this piece of weather lore?
Some claim the the dog days of summer cause dogs to go mad from the heat. Others contend its hot sultry days are "not fit for a dog." Other associations have been droughts, plagues and madness according to the ancients.
According to the old AMS Glossary of Meteorology (1959), the dog days occur from mid-July to early September in the United States and from the third of July to the eleventh of August in Western Europe. They are typically associated with the hot sultry days of summer. The Ancient Romans considered 24 July to 24 August to be the dog days. The Old Farmer’s Almanac sets the days as 3 July to 11 August for these days.
But, in reality, the Dog Days is another of those weather-related terms that really has its origins as much in astronomy as in weather and climate. Like the equinoxes and solstices, it relates to a stellar position, but in this case, the Sun has a partner, the bright star Sirius, known as "the dog star." The ancient civilizations (Egyptians, Greeks, Romans) around the Mediterranean believed that the rising of the star Sirius, just prior to our Sun’s rising at this time of year, added additional heat to the day, thus causing the peak heat of the summer.
The current official name of this star comes from the Latin Sīrius, which is derived from the Ancient Greek word Sothis meaning ("glowing" or "scorcher"). The Greeks observed that the re-appearance of Sirius in the sky after a brief absence heralded the onset of the hot and dry Greek summer. The ancient Egyptians named it after their goddess Sopdet or Sothis, who, it was said, gave birth to the mid-summer heat. Alternate names for this star include The Sparkling One and The Scorching One and The Star of Isis.
Known also as the “dog star,” Sirius, or Alpha Canis Majoris can be found in the constellation Canis Major (large dog) and one of Orion's two hunting dogs). It is the brightest star (a visual magnitude of -1.46, twice as bright as any other star in our sky (excluding Ol’ Sol), and at a distance of 8.7 light years, the fifth-nearest known star. Sirius is actually a binary star; the larger and brighter Sirius A accompanied by the smaller white dwarf, Sirius B.
Left: The image of Sirius A and Sirius B taken by Hubble Space Telescope (Credit: NASA). Right: Location of Sirius in Canis Major
The ancient Egyptians started their calendar year with the rising of Sirius, a time that was associated with their god Osiris, god of the afterlife, the underworld and the dead and his wife Isis. Osiris had the power to grant life, including the sprouting of vegetation and the fertile annual flooding of the Nile River. For a period of 70 days as it crosses behind the Sun, the Egyptian sky was without the light of Sirius. This period was considered a time of death and rebirth. When Sirius reappeared in the east at dawning, it signified a rebirth has begun.
The ancient Egyptians observed that shortly after Sirius returned to view, the annual flooding of the Nile Delta would typically occur. Thus, the star's arrival thus heralded the rise of the Nile, a warning to those living near the shore. But more importantly it announced the return of fertility to the land, a time to open the irrigation channels and plant the crops.
For this reason, those priests charged with calendar-keeping functions kept an eye out for Sirius's return. During this era, the new year began with the return of Sirius after its solar conjunction. To announce the event, a jewel was placed in the forehead of a statue to the goddess Isis in the temple of Isis-Hathor at Denderah. When the light of the star returned to againshine upon the gem, the priest would announce the New Year. In the temple an inscription reads: "Her majesty Isis shines into the temple on New Years Day, and she mingles her light with that of her father Ra [the sun god] on the horizon."
In ancient times, Sirius rose in the sky just prior to or simultaneous with the rising of the Sun shortly after the summer solstice. With the progression of the equinox over the millennia, however, this is no longer true and in 2012, this will be around 2 August in the Middle East and a week later in Greece.
What makes Sirius interesting from a meteorological/climatological perspective is that it rises in conjunction or near conjunction with the Sun for over a month (currently from about 3 July to 11 August) and that corresponds to the hottest period of the year in northern Africa and southern Europe. Since the conjunction of Sirius with the Sun coincides with the hottest period of the year, it was naturally assumed their combined beams caused the heat.
The Romans began using the term The Dog Days to describe the peak summer's heat and the period encompassed the 20 days before to 20 days after the conjunction, but all were not convinced even then. Around 70 BCE, the astronomer Geminus remarked: "It is generally believed that Sirius produces the heat of the Dog Days, but this is an error, for the star merely marks a season of the year when the Sun's heat is the greatest."
Many cultures have considered a special relationship between Sirius and dogs. Canis Major is considered in mythology to be Orion the Hunter’s dog as the constellation of Canis Major lies behind and below the constellation of Orion — at Orion’s feet, so to speak — and in pursuit of Lepus, the Hare. Homer wrote in the Illiad,
"Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion's Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity."
The Greeks believed emanations from Sirius adversely affected dogs, making them behave abnormally. Romans termed these days as dies caniculares, with Sirius called Canicula, the “little dog.” They believed excessive panting of dogs in hot weather placed them at risk of dehydration and disease. Because the panting of heat-stressed dogs often caused lathering around the mouth, it was thought to cause rabies.
Today, many still believe that the origin of the term dog days refers to the languid behavior of dogs during the hot and humid weather of high summer. Rather than risk overheating during the heat, domestic canines will sprawl out and pant to lose heat, avoiding exertion in the hot conditions.
The Dog Days have descended on us here in the Central Canadian Rockies, and it is too warm to spend time sitting at my computer. I would rather laze in the shade with a cool drink and a hot book as I ponder the trail of my six and a half decades across the skies.
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Written by Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
August 1, 2012 Osiris image by Jeff Dahl, based on New Kingdom tomb paintings, source, Wikipedia Sirius photo from Hubble Telescope, courtesy NASA.