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Wednesday, August 20, 2014

Dolbear's Law and the Crickets of August

Dani and Gina just stopped me on the way to the lake for their swim.  They wanted to know about crickets...  Their grandpa was out with us last night for the Wolf Howl, and other Nature "Stuff" that we talk about, and they wanted to know if it was true that if you count the number of times a cricket chirps in 13 seconds and add 40 that it would be the temperature.

They had tried it, and had come up with 57 degrees. Could that possibly be correct?  We consulted the Fahrenheit thermometer, and it was sitting there at just below 60 degrees.  It is most likely that they were counting a Field Cricket, who are notoriously sloppy compared to Snowy Tree Crickets, in their thermal calculations.  Perhaps it was a little cooler down in the grass where the cricket was chirping. Or perhaps they missed a critical chirp... or were out a half second in the time. Whatever, it was pretty dang close.

So how does that work?  Thank Professor Dolbear.

field cricket
Here's the scoop:

   You have no doubt heard crickets chirping away in the grass and bushes. The male cricket chirps at regular intervals in order to attract female crickets, ward off predators, or establish territory. He creates the cricket chirping sound by rubbing one of his wings on a hard rigid structure located on its other wing.
   You can use these cricket chirps as a way to determine the temperature. But first, a little lesson on why this works:

snowy tree cricket

Cricket Metabolism
   Like all insects, crickets are cold blooded. This means that unlike you and me (warm blooded creatures) a cricket produces little or no body heat of its own. Instead, a cricket’s body temperature tends to match the temperature of its surroundings
   Because the metabolism of an insect is proportional to its body temperature, if it is too cold the cricket cannot even move. As the temperature of its environment warms, the cricket's body also becomes warmer. Its metabolism increases and the insect can move faster.
   You have probably witnessed this increase in insect metabolism as the temperature increases. For example, ants will run must faster when on a hot sunny sidewalk as compared to when it is cool.


Calculating Cricket Temperature
    What does all this cold-blooded-temperature-of-the-environment stuff have to do with finding the temperature using crickets? Plenty. You see, the warmer the ambient temperature of the crickets surroundings, the warmer the cricket. And the warmer crickets increased metabolism allows it to chirp faster.

   In 1898 Amos Dolbear noticed that warmer crickets seemed to chirp faster. Dolbear made a detailed study of cricket chirp rates based on the temperature of the crickets environment and came up with the cricket chirping temperature formula known as Dolbears Law:

T = 50 + (N - 40) / 4

T = temperature in degrees Fahrenheit.
N = number of chirps per minute.

For those of you who cannot remember your elementary school math, here is the wordy version of the same cricket temperature formula:

Count the number of chirps the cricket makes in 1 minute (60 seconds)
Subtract 40 from this number.
Divide the result by 4
Add 50.

The result of this calculation is close to the temperature of the environment the cricket is in. For example, if the cricket chirps 120 times in one minute then the temperature is about 50 + (120 - 40)/4 = 70 degrees.

If you are working in degrees Celsius here is the Dolbears Law formula for degrees C:
T = 10 + (N - 40) / 7

A simplification of Dolbears Law can sometimes be found listed as something like:

Fahrenheit Temperature = Number of chirps in 13 seconds + 40


   Which seems to closely agree with the more complicated formula Amos Dolbear came up with.
When Determining Temperature Using Crickets
    It is important to note that the cricket chirp temperature formula is based on the temperature of the cricket, which is not necessarily the temperature of where you are. Be aware that the temperature of the grass or bushes close to the ground where the cricket is may be quite different than the temperature several feet off the ground.

     Another factor that must be considered is that Amos Dolbear came up with his revolutionary cricket temperature formula while experimenting with Snowy Tree Crickets. Other crickets may give varying results based on the cricket species and age.

   Estimating the temperature using cricket chirps is a good approximation of the temperature. Give it a try and amaze your friends!

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