According to folk legend, when you hear the first song of the dog-day cicadas, it means there's just six weeks until frost.
While this may not be a precise predictor, there is some merit to the claim. Dog-day cicadas, as their name implies, appear during the long, hot summer days of July and August.
Unlike their cousins, the periodical cicadas, dog-day cicadas have relatively short life cycles of just 2-5 years. Their emergence is staggered so that some individuals emerge each year; dog-day cicadas also go by the name annual cicadas for this reason.
On hot days late in the summer, you can hear the male dog-day cicadas calling for females from high in the trees. Their song, which is made by vibrating membranes on the abdomen, sounds more like a loud hum or whine than an actual song.
Count forward six weeks from the first time you hear this common summer sound, and you'll have a very rough estimate of when cool, fall weather will arrive.