MIKE MOSS SAYS: Chris, Frost can be forecast with reasonable success if one gets a good idea of the minimum air temperature that will occur along with the likely dew point at that time. If the temperature drops to near the dew point, and the dew point is at or below freezing, then frost has a good chance of forming. The situation can be more complex than that, since it is not unusual for frost to form on the ground or on other upward facing surfaces exposed to clear skies, when air temperatures (measured about 4-5 feet above the ground) are above freezing, but strong radiational cooling and light winds allow the surfaces themselves to drop to freezing or below. For this reason you might see frost on the field on a morning when the air temperature is 35 or 37 degrees, for example.
On the other hand, if the temperature is 25, as in your examples you mentioned, there may be no frost if the air is very dry and the dew point at grass level (also called frost point in sub-freezing conditions) remains at least a few degrees below the temperature at that same level. This situation, in which the temperature at ground level is well below freezing but no frost is present, is called a "black frost" and I'm not sure there is any reason to let anyone walk on the grass under these conditions if they are not allowed when frost is present. In other words, the actual temperature at ground level is more critical than whether frost has formed. In fact, when frost does form, it can be somewhat beneficial because the deposition of ice onto the grass releases latent heat into the blades and the air just above them that may prevent them from getting even colder. I suppose there could be a mechanical damage issue with the external ice crystals being ground into the blades when someone walks on them, but I don't know enough about turf health to know if that's the reason you prohibit walking on the grass when frost is there.
The NC State Extension Service has some excellent references on frost, among other weather impacts to horticulture, available online at