Crickets stridulate. The word comes from the Latin verb stridere, meaning "to creak" and is more familiar to us in the adjectival guise of "strident".
Only the males, it seems, are stridulators, the ends of their wings being equipped with a special apparatus to make it possible for them to make their well-known chirping sound. In the case of Gryllus domesticus, the hero of The Cricket on the Hearth, the process is well described by Charles Reade: "Good Heavens how it chirped!" he says of the little insect. "Its shrill, sharp, piercing voice resounded through the house, and seemed to twinkle in the outer darkness like a star. There was an indescribable little trill and tremble in it, at its loudest, which suggested its being carried off its legs and made to leap again by its own intense enthusiasm." But of more interest to meteorologists is this cricket's country cousin, the field cricket or Gryllus campestris. According to one authority, the male of this species sits stridulating at the mouth of its burrow on warm evenings until a female approaches, at which point "the louder notes are succeeded by a more subdued tone whilst the successful musician caresses with his antennae the mate he has just won".
Foremost among meteorological researchers on these matters in the last century was Prof Andrew Dolbear. He lived in a part of the US where crickets are common, and had the impression that crickets chirped faster on very warm nights. Since observations are the key to all meteorological investigation, he monitored the proceedings closely over a lengthy period.
Two sets of measurements had to be compared. Every night he listened to the crickets, and counted carefully the number of stridulations in a minute. At the same time, he noted the temperature as "6ft above cricket level". He found that 13 degrees Celsius was a critical temperature: below this value the insect does not stridulate at all, but at 13C he chirps at around 60 chirps per minute. Above 13, the rate of chirping increases steadily with temperature. In 1897 Dolbear published an important paper called "The Cricket as Thermometer", in which he unveiled what has come to be known as "Dolbear's Law". This tells us that the temperature of the air on a warm night can be found from the formula T = 10 + (S-40)/8 , where T is the temperature in degrees Celsius, and S the stridulating rate per minute.
Those of you with a mathematical bent will be able to work out, therefore, that if your local cricket chirps at 140 chirps per minute, the air temperature must be a balmy 22.5 degrees Celsius.