T = 50 + (N-40)/4
where N is the number of field cricket chirps per minute, and T is the temperature in Fahrenheit. An easier to remember equivalent is
T = N + 40
where, in this case, N is the number of chirps in 15 seconds.
I gave it a try tonight. I counted 91.7 +/- 2.1 chirps per minute, yielding a calculated temperature of 62.9 +/- 0.5 degrees. The actual reading on the thermometer outside? 61 degrees. Pretty good accuracy, and no instrumentation required.
Bonus science trivia: All nearby crickets of the same species will synchronize their chirping — despite the fact that the males are actually competing with each other for mates.
Extra bonus science trivia: The chirping rate of crickets (and many activities of cold blooded creatures) actually follows the Arrhenius equation for the temperature dependence of chemical reaction rates. The chirping rate is a function of temperature because of the underlying biochemical reactions that give the cricket the energy it needs to chirp.