The data were collected from Iowa during a recent 10-year period.  The
variables are:
    precip:  annual total precipitation (in inches)
    temp:    average annual temperature (in degrees Fahrenheit)
    corn:    amounts of corn produced (in millions of bushels)
    acres:   acres harvested (in thousands of acres)


The dataset is taken from Triola, M.F. Elementary Statistics, 8th ed. 
Addison-Wesley, 2001.

Presumably the "precip" and "temp" variables are statewide averages.