Dolbear’s law
Crickets are cold-blooded: their muscles work faster when it is warm, so they chirp faster. In 1897 the American physicist Amos Dolbear published “The Cricket as a Thermometer”, giving a simple formula for the snowy tree cricket:
T (°F) = 50 + (N − 40) ÷ 4
where N is the number of chirps per minute. A handy shortcut gives the same result: count the chirps in 14 seconds and add 40 to get the temperature in °F.
In Celsius, the equivalent is T (°C) = 10 + (N − 40) ÷ 7, or roughly: count the chirps in 25 seconds, divide by 3 and add 4.
Example
You count 35 chirps in 14 seconds. That is 150 chirps per minute, so T = 50 + (150 − 40) ÷ 4 = 77.5 °F, about 25 °C. The shortcut gives 35 + 40 = 75 °F, within a few degrees.
How to count
- Pick a single cricket: a chorus is too hard to count.
- Use a watch or phone timer and count for at least 14 seconds; a longer count of 30 or 60 seconds is more accurate.
- Count each chirp, not each pulse within it.
- The result is the temperature where the cricket sits, which can differ from the temperature at the weather station.
Which crickets work best?
The formula was written for the snowy tree cricket (Oecanthus fultoni), whose chirps are regular and often synchronized across a whole group, earning it the nickname “thermometer cricket”. Field crickets and house crickets also chirp faster when warm, but less regularly, so the estimate is rougher. Katydids follow a similar pattern with their own formulas.
Limits
- Below about 55 °F (13 °C), most crickets stop chirping.
- Above about 100 °F (38 °C), the rate stops rising.
- Age, mating behavior and the chirps of rivals affect the rhythm.
Why it works
Chirping is produced by stridulation: the male rubs a scraper on one wing against a toothed file on the other. The speed of this movement depends on muscle chemistry, and the rate of most chemical reactions rises with temperature, often doubling for each 10 °C rise. Crickets make that relationship audible.