What's the temperature? Listen to the crickets

The chirping of crickets is one of the soundtracks of summer nights, but their constant chirp is not just background noise. Believe it or not, that chirp is a natural way of telling the temperature. In the late 19th century, physicist Amos Dolbear discovered that the frequency of cricket chirping varies according to the ambient temperature.

These insects are ectotherms, i.e. their body temperature depends on their environment. The warmer it is, the faster their metabolism and therefore the more frequently they chirp. When the mercury in the thermometer drops, the rate slows down. Dolbear formulated a simple equation to estimate the temperature in Celsius from the chirps per minute:

(Cpm / 5) - 9 = T

Cpm is the chirps per minute and T is the temperature. However, this method only works between 15C and 35C, as outside this range crickets chirp irregularly or not at all. Moreover, not all crickets follow this pattern: species, age, environment or context (such as mating) also play a role.

Although Dolbear did not specify which species he used for his study, later analyses point to the snowy tree cricket (Oecanthus fultoni) as the most likely. Still, his formula is useful for getting a rough estimate and enjoying a little natural experiment on a summer's night in the process. Next time you're outdoors, listen carefully. Crickets aren't just chirping: they're telling you the temperature.