The [url=http://www.noaa.gov/]NOAA[/url] keeps close track of various kinds of weather data at a number of locations around the U.S. The NOAA station closest to me is in [url=http://en.wikipedia.org/wiki/Chillicothe,_Missouri]Chillicothe, Missouri[/url]. A graph of the hourly surface soil temperature in Chillicothe, starting on July 30, 2009, is shown below. (Source data [url=ftp://ftp.ncdc.noaa.gov/pub/data/uscrn/products/soilsip01/CRNSIP01_MO_Chillicothe_22_ENE.txt]here[/url]. Warning: this is a 5mb text file!)
Find a sine or cosine equation that matches this set of data as closely as possible, and check it by graphing it as you did before. As usual, you can clear your graph(s) by clicking the bluish reset icon in the upper right.