WeatherWhys for August 10,2016 from accuweather.com

In ancient times, people believed that a very bright star called Sirius added heat to the atmosphere during midsummer. Sirius is the brightest star in the constellation Canus Major. It is also referred to as the dog star. The period in which this star rises with the sun became known as the dog days of summer. This just happens to be the warmest part of the year.