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
http://www.ces.ncsu.edu/depts/hort/hil/wx-index.html
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Now, let's delve into the concepts mentioned in the article you provided by Mike Moss:
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Frost Formation Factors: Mike Moss explains that forecasting frost involves considering the minimum air temperature and the likely dew point at that time. If the temperature drops close to the dew point, and the dew point is at or below freezing, frost is likely to form.
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Complexity of Frost Formation: The article highlights the complexity of frost formation, noting that frost can occur on surfaces exposed to clear skies even when air temperatures, measured a few feet above the ground, are above freezing. This is due to radiational cooling and light winds allowing surfaces to drop to freezing or below.
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Black Frost: The term "black frost" is introduced, describing a situation where the temperature at ground level is well below freezing, but no frost is present. This occurs when the air is very dry, and the dew point at grass level remains a few degrees below the temperature at that same level.
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Temperature vs. Frost Presence: Mike emphasizes that the actual temperature at ground level is more critical than the presence of frost. He contrasts examples, stating that a temperature of 25 degrees may not result in frost if the air is very dry and the dew point at grass level remains below freezing.
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Benefits of Frost: The article mentions potential benefits of frost, explaining that when frost forms, the deposition of ice onto the grass releases latent heat, preventing the grass from getting even colder. However, there might be concerns about mechanical damage if someone walks on the grass with external ice crystals present.
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Turf Health and Frost Prohibition: Mike acknowledges that he is not certain about the reason for prohibiting walking on the grass when frost is present, speculating that there could be a mechanical damage issue with external ice crystals being ground into the blades. He suggests the possibility of benefits from the heat released during frost formation.
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NC State Extension Service Resources: The article provides a reference to the NC State Extension Service, which offers valuable resources on frost and other weather impacts on horticulture. The link provided directs readers to online references for further information.
In summary, the article discusses the various factors influencing frost formation, the complexity of the process, the concept of black frost, the importance of ground-level temperature, potential benefits of frost, and the reference to additional resources for in-depth information.
