Dolbear's Law (Cricket Thermometer)

This is an empirical rule, not a physical law. Here $N_{60}$ is the number of chirps a tree cricket makes in one minute and $T_F$ is the air temperature, in degrees Fahrenheit, where it is singing. The line passes through 40 chirps a minute at 50 °F and adds four chirps for every extra degree. Dolbear published it in 1897 as a straight-line fit to one kind of singer, and nothing in it is derived from first principles: it simply describes how those crickets were found to behave.

A cricket can act as a thermometer at all because it is cold-blooded: its body sits at the temperature of the air, and biological rates speed up as things warm. The Q10 temperature coefficient describes that general trend, with biological rates roughly doubling over 10 K. A rate that doubles over 10 K traces a curve, not a straight line, so read this relation as a local fit and do not stretch it far past the temperatures where crickets sing.

The shortcut in the description is the same formula with the arithmetic done: count the chirps in 15 seconds and add 40 to read the temperature in °F. In the worked example, 28 chirps in 15 seconds is 112 chirps a minute, and the page returns 68 °F, which is 20 °C. Adding 40 to 28 gives the same 68. The anchor of the line works too: 40 chirps a minute reads 50 °F, or 10 °C.

Because the relation is linear it also runs the other way, $N_{60} = 4\,(T_F - 50) + 40$, to say how fast a cricket should be singing at a known temperature. Keep its limits in view. It is fitted to one kind of singer, so it should not be assumed to hold for other insects, and it is a field curiosity and a nice test of a thermometer, not an instrument. The Celsius reading of the same temperature is on the temperature conversion page.