Article — Cricket Chirp Thermometer
Cricket chirp thermometer: how to read temperature from cricket song
The cricket chirp thermometer estimates outdoor air temperature from the chirp rate of a snowy tree cricket. Count chirps in 15 seconds and add 37 to get the temperature in Fahrenheit. Accuracy is within ±1 to 2°C between 55°F and 100°F. The relationship was published by physicist Amos Dolbear in 1897 and is known as Dolbear's Law.
Cricket chirp rate is one of the cleanest examples of a biological process tracking ambient temperature. The chirp depends on muscle contraction speed, which depends on enzyme reaction rates, which obey Arrhenius kinetics — each 10°C rise approximately doubles the rate. The result is a near-linear relationship between chirp count and temperature across the cricket's active range.
What is Dolbear's Law?
Amos Dolbear published the formula in 1897 in The American Naturalist as a short article titled "The Cricket as a Thermometer." His original form: T (°F) = 50 + (N − 40)/4, where N is chirps per minute. The 14-second shortcut (N + 40) and the 15-second shortcut (N + 37) are arithmetic simplifications that give the same answer at typical chirp rates.
Dolbear noticed that crickets in the same area chirp at the same rate regardless of distance, and that the rate changes with temperature. He spent a summer counting chirps and recording thermometer readings, then derived the linear formula that bears his name. The accuracy of the cricket chirp method — within a few degrees Fahrenheit — surprised contemporaries who expected animal behavior to be too variable.
Dolbear was a physicist at Tufts College who patented a wireless telephone in 1882 — 14 years before Marconi. He spent his later career on bio-acoustic research, and the cricket thermometer paper was a side project published the same year he turned 60. The formula has been verified in dozens of studies and remains the most cited result in cricket bioacoustics.
How the cricket chirp thermometer works
Four equivalent forms of Dolbear's Law are in common use, differing only in the counting interval. The 15-second shortcut is the most popular because the math is trivial in the field. The 8-second shortcut converts directly to Celsius. The 60-second form is Dolbear's original.
T°F = chirps/15s + 37 15-sec shortcutT°F = chirps/14s + 40 Older 14-sec formT°C = chirps/8s + 5 Celsius shortcutT°F = 50 + (chirps/min − 40)/4 Dolbear originalAll four are linearizations of the same exponential relationship between chemical reaction rate and temperature. The exponential becomes effectively linear across the relatively narrow temperature band where crickets are active (10 to 38°C / 50 to 100°F), which is why the simple addition formulas work so well.
Counting cricket chirps accurately
Reliable cricket chirp counts require a quiet environment, a single calling male, and good rhythm sense. Multiple crickets calling simultaneously make individual chirps hard to separate — pick out one cricket by sound and direction, then focus on it. Each distinct chirp counts as one, even if it consists of several pulses (a chirp pattern called a "trill").
For best accuracy, count twice and average. Start the 15-second timer at the leading edge of a chirp and stop at the leading edge 15 seconds later. Smartphone timer apps work well — many have a built-in stopwatch with a lap function that captures the count directly. Tap a knee or finger with each chirp if hand-counting is hard.
The cricket chirp thermometer is most reliable after sunset on a still, dry evening. Wind disrupts the count by breaking the sound pattern. Rain silences crickets entirely. Daytime calls are sparse and irregular for tree crickets. Late summer through early autumn (July to October in temperate climates) is peak cricket season.
Which cricket species to count
Dolbear derived his formula from snowy tree crickets (Oecanthus fultoni) and the formula works best for that species. Snowy tree crickets call from low shrubs and woody plants, produce evenly-spaced single chirps about half a second apart, and respond cleanly to temperature. The species is widespread across the eastern and central United States.
Other tree crickets in the Oecanthus genus (broad-winged, four-spotted, narrow-winged) follow similar relationships with small species-specific offsets. House crickets (Acheta domesticus, the species sold as pet food) have erratic chirp rates that depend on age, hunger, and individual male fitness. Field crickets (Gryllus species) call in unpredictable trills and bursts.
If more than one cricket calls in the same area, the perceived chirp rate adds across individuals. Two crickets at 35 chirps each per 15 seconds will sound like 70 chirps, which translates to 107°F — far above any realistic ambient. Pick a single cricket by ear and ignore the chorus.
Cricket chirp accuracy and limits
Accuracy of the cricket chirp thermometer is ±1 to 2°C (±2 to 4°F) between 55°F and 100°F for the snowy tree cricket. Below 55°F crickets stop chirping reliably and the formula breaks down. Above 100°F crickets enter heat stress and chirp rates plateau or drop. The linear approximation also weakens at temperature extremes because the underlying exponential reaction rate curve flattens.
Sources of error in real-world counts: multiple calling crickets, wind noise, traffic or other background sound, age and fitness of the individual male, and observer rhythm. Five separate counts averaged together produce accuracy similar to a $20 thermometer, which is more than enough for the "is it warm enough to go outside" question that the cricket thermometer is usually answering.
The cricket thermometer in the field
Practical uses of the cricket chirp thermometer are educational, recreational, and historical. STEM educators use it to introduce the Arrhenius equation, ectothermy, and biological rate processes to elementary and middle school students. Naturalists and citizen scientists use it to track temperature in remote locations without instruments. Old-time campers and hikers use it as a charming alternative to a thermometer.
The Old Farmer's Almanac has published Dolbear's Law continuously since the 1950s. The National Weather Museum and NOAA both feature the cricket thermometer as an example of biological climate indicators. The Globe Program — a NASA-NSF citizen science initiative — has students count cricket chirps as part of climate monitoring projects.
The biology of cricket song
Cricket chirps come from a process called stridulation — rubbing one forewing against the other. The scraper edge of one wing catches the file teeth of the other, producing one sound pulse per stroke. Each chirp contains 50 to 5000 sound pulses depending on species. Snowy tree crickets produce simple single-pulse chirps about 60 to 200 per minute; field crickets produce complex trill patterns.
Only males chirp. Females are silent and evaluate males partly by call rate. Faster, more regular chirping signals a fitter male in better condition — a reliable cue because chirping is metabolically expensive. The temperature dependence is incidental from the cricket's perspective: a male calling on a warm night is simply running his physiology at the speed the temperature allows.
- 15-second shortcut = chirps + 37 = °F
- 8-second shortcut = chirps + 5 = °C
- Best species = snowy tree cricket (Oecanthus fultoni)
- Accuracy = ±1–2°C in 55–100°F range
- Below 55°F = crickets stop chirping reliably
- Above 100°F = heat stress disrupts the rate
- Best time = after sunset, still and dry weather
- Year published = 1897, The American Naturalist