|MadSci Network: Earth Sciences|
Good question! I'm sure many others have wondered the same thing.
For frost to form, the temperature of the object upon which frost forms must be 32 degrees F of lower.
You will often see frost form on automobiles before it forms on the ground. This is because metal radiates heat quicker than soil does.
The minimum air temperature that your local meteorologist gives, or even the air temperature that you measure in your own yard, can be misleading. Under ideal radiational cooling nights...clear skies and calm winds...the temperature at the earth's surface can be several degrees cooler than the temperature at the height of your thermometer.
Therefore, if your thermometer measures a temperature of 37 degrees F, it is quite possible that, under the radiational cooling conditions mentioned above, the temperature on the ground is below freezing.
Try the links in the MadSci Library for more information on Earth Sciences.