The Old Farmer's Almanac lists the traditional timing of the Dog Days: the 40 days beginning July 3 and ending August 11, coinciding with the heliacal (at sunrise) rising of the Dog Star, Sirius.
The rising of Sirius does not actually affect the weather (some of our hottest and most humid days occur after August 11), but for the ancient Egyptians, Sirius appeared just before the season of the Nile’s flooding, so they used the star as a “watchdog” for that event.
Since its rising also coincided with a time of extreme heat, the connection with sultry weather was made for all time. And, just how hot is it where you live?
Dog Days bright and clear
Indicate a happy year.
But when accompanied by rain,
for better times our hopes are vain.