Predicting frost

[konarikcy]

Hi all. We have had one of the warmest winters on record but the cold is here this week in Cyprus with temperatures around 12C in the day an around 6C at night. However according to all the weather forecasts, tomorrow it will fall to 2 C only for a couple of nights then its going up again. I have tried repeatedly from forecasts to work out if the combination of information can predict frost as this info is not available but to no avail.Dew point is listed as -2C. Is that of any use. Can someone explain the info required and how to predict. Meanwhile, my small palms, will share my old quilts and blankets for those nights.

 

[JLM]

26 minutes ago, konarikcy said:

Hi all. We have had one of the warmest winters on record but the cold is here this week in Cyprus with temperatures around 12C in the day an around 6C at night. However according to all the weather forecasts, tomorrow it will fall to 2 C only for a couple of nights then its going up again. I have tried repeatedly from forecasts to work out if the combination of information can predict frost as this info is not available but to no avail.Dew point is listed as -2C. Is that of any use. Can someone explain the info required and how to predict. Meanwhile, my small palms, will share my old quilts and blankets for those nights.

 

That's a good question. 
When temps fall below 39-40F, with clear skies and no wind, frost will form. Dewpoint plays a large role, if the dewpoint is near the air temp, and these values are below 37-39F then frost is possible or likely. 
If someone has a much more detailed explanation, it would not only help you better understand but would help me as well.

[Swolte]

One of the reasons, from my understanding, its hard to predict whether frost will occur is that the thermometers are about 5feet above the surface and its easily possible for the ground temperature (where our dear plants are) to be much lower. A lower dew point, clear skies, no wind (or a hard cold wind), etc... (like JLM mentioned) can all play a role in whether frost develops or not. 

In your situation, with dew points in the negative, there may be a chance for frost to occur. If your predicted dew points are above 32F (0 C), you will likely not see frost.

Not an expert though and I find many of the explanations on the dew point not very clear. We do have a meteorologist amongst us, I believe...

 

[mnorell]

Humidity and dewpoint are very much entwined, and in high-humidity environments, dewpoints will generally be closer to the air temperature at dawn. When the air temperature reaches the dewpoint, and the dewpoint is 32F or below, frost can form. Underneath the canopy of an evergreen tree or the eave of a building, the lack of radiation upward to the open sky will stop any frost from forming, even at very low temperatures. But even in that case, below about 27F, the tissues of the plant will freeze and the cells will possibly burst. But frost can indeed occur at higher temperatures due to the process of super-cooling, especially on materials like metal (cars, rooftops). Also, plants that have leaves that lay at a horizontal, as opposed to vertical, will be the first to suffer from frost and you will see damage on those leaves first. If you are in a marginal climate (USDA 9b/10a), you can have a very successful garden if you very carefully plant things under the overhangs of buildings or under evergreen canopy trees (like high-branching, evergreen oaks). This is why the first thing experienced landscapers and tropical plant growers will tell you upon planning a landscape is to plant some fast-growing, tall-arching, evergreen canopy, because you can plant many, many things in that environment. In the American Deep South, where there will be several freezes every winter with high humidity/dewpoint that almost guarantee plant damage at 32F or so, many savvy gardeners and landscapers utilize the Southern Live Oak (which uniquely allows a very nice level of filtered light beneath it) as a way to grow gingers, heliconias, sensitive palms and other tender tropicals in that climate.

On the other hand, in a low-humidity desert (such as the Sonoran Desert in NW Mexico and SW USA) the humidity is so low, and dewpoints are often near 0F, and even the most sensitive plants under open sky (Plumeria, Adenium, many palms, et al.) will show absolutely no damage after 28-30F for several hours because no frost can form at that low level of humidity.

[konarikcy]

 

Thanks but I find this all very difficult despite being a scientist. I know prediction is not an exact science but from your comments I am still having difficulty. So I attach above the predicted forecast for tonight,  for someone to give me a simple guideline (I am sure many other non meteriologists will value such a guideline) .