I live in a climate that is not frost free, but we rarely get freezes. OK. What does that mean? What is the difference bewteen a frost and a freeze... or a hard frost and a freeze? What is the difference between frost, Rime or simple ice? And how can there be frost on my lawn when my outdoor thermometer says it only got down to 38F? And why is this plant damaged when that one right next to it is not? And why are some leaves of this plant damaged while others are not? I didn't really know before I wrote this article. Though it's not all clear now, I at least have a better idea. Maybe if I'm lucky, you will, too.
First of all, what is frost? In one of Gwen Bruno's articles, she covered the basics on frost. Frost can be both an event as well as a form of frozen water. A frost event (also known as Radiation Frost) is something that occurs on calm, clear nights when the air temperature near the ground gets to or below freezing, and ice crystals are able to form. Almost always the air temperature higher up is above freezing at some point and this is called in inversion. If there is no inversion and the air temperature is below freezing at all levels, that is a true freeze (aka hard freeze), not a frost event (though frost, the actual stuff, can exist).
One needs to have a concept of dew point to understand at what conditions frost will form. Dew point is a measure of how much water is in the air (basically is a measure of humidity,but NOT relative humidity). When temperature falls to the same temperature of the dew point, dew will form, as that is temperature at which the air can no longer hold all its moisture. So if the dew point is 50F and it cools to 50F in the air, dew will form on surfaces. But when the dew temperature is below freezing, as can happen in very dry climates with very low humidities, dew will never form. But instead, as the air temperature falls to this below freezing dew point, frost will form instead of dew. These subfreezing dew points are called Frost Points. So when the dew point temperature is below freezing, and the air cools to that dew point temperature, no more moisture can be contained by the air, and ice crystals form. Those crystals are called Hoar frost and visible crystals will form on surfaces that are below freezing.
These photos from Wikipedia show Hoar frost on vegetation and grass after a frost event
Close up of Hoar frost from Wikipedia
If the dew point is extremely low (in very low humidity situations) and the temperature still drops below freezing, then you might experience what is sometimes known as "Black Frost" and no or very few visible ice crystals will form. On the other hand, if the the dew point is high (lots of water in the air) but the temperatures still fall below freezing, there will already have been dew formation, so instead frost one will see frozen dew (not the same thing). On roads, this frozen dew is called Black Ice.
I tossed this graph in mostly because it had me confused for a long time. This just shows that the relative humidity is dependent on both dew point and air temperature, and it is not the same as absolute humidity (which dew point is a more accurate reflection of). High dew points mean lots of water in the air and you will perceive these high dew point temperatures as really muggy and uncomfortable. But the relative humidity could indeed be fairly even if the point is high, so you might feel uncomfortable mugginess in the face of a low relative humidity. Not really sure what the point of knowing what the relative humidity is, but we hear it a lot on the weather channel, so here it is sort of explained in a simple graph. This graph does not include Frost Points as the graph ends at freezing, but you can just continue the lines and see that it would be impossible to have a relative humidity of 100% below freezing. It doesn't really tell you anything else.
"Black frosts: tend to be even more damaging to plants. This is because Hoar frost is associated with ice formation and newly formed ice tends to stay at a certain temperature (30F-31F), even if the surrounding temperatures drop a bit lower. Though frost will damage sensitive leaves, it can also be somewhat protective to those plants that are a bit tougher by coating and protecting them from the colder surround air temperatures. On the other hand, if there is very low humidity and no ice can form on the leaves, the leaves will be more subject to ever lowering ambient temperatures, and the temperature may get low enough that ice will form IN the leaves (hence killing them). And, if the humidity is much higher and temperatures drop below zero, then ice will have formed on the foliage and though also potentially somewhat protective, its far heavier weight will often damage more than just leaves, but entire limbs. Exactly what the difference is between a black frost and a hard freeze is beyond me (they may indeed be the same thing).
Frosts can still form at much lower temperatures than at or just below freezing, however, as long as there is a very low frost point temperature. I used to live in northern New Mexico and temperatures would drop down in the teens. But the humidity was so low, we might have a frost point of 20F. And if the air temperature dropped below 20F, frost will form on the lawn on surrounding vegetation. However, the lower the temperature that this happened, the lower the frost point and therefore the lower the humidity... and there would be less and less visible frost to even form (more black frost sitatutions than hoar frost conditions). The grass would be crunchy and one could barely see the frost, but it would not be too impressive a sight.
Rime frost is usually associated with much lower temperatures than is Hoar frost and forms when damp, freezing winds blow over relatively fine objects, coating them in an icing-like material. Rime frost occurs during advective freezes, not radiational frosts. It is also incorrectly sometimes called wind frost or advective frost. And if taken very literally, rime frost is actually not a true frost. It does not form from moisture solidifying at the freezing dew point. It is actually frozen precipitate- from a freezing fog to a fine drizzle and is usually associated with a significant wind speed. If a very delicate layer of frost-like ice forms on objects, and looks similar to Hoar frost, it is called soft Rime. If a more substantial, solid structure accumulates with a thicker, white, spiculated, comb-like appearance that can coat and even weigh down branches and other bendable structures, it is called hard Rime. Hard Rime is sort of half way between a frost like substance and solid, clear ice. Clear ice is what forms during a freezing rain. The last can be a devastating event pulling down large tree limbs, power lines etc. But we digress, as these are NOT forms of true frost.
Photos from Wikipedia showing Soft Rime (left) and Hard Rime (right) - both the result of a freezing, moist cold front
On a cool, clear night, if there is an inversion and temperatures are right to form frost, the wind is virtually nil or at least less than 5 miles per hour. If the air speed goes above 5 miles per hour, generally that amount of wind mixes the warm air found at slightly higher elevations with the cold air near the ground surface and the overall effect is no frost formation. This is why windy nights in winter around my area usually result in less plant damage (unless associated with a rare severe cold front) than calm, clear nights. In my local climate near the Pacific coast, winds will actually increase air temperature. Our proximity to the Pacific Ocean usually prevents severe arctic storms from reaching us without being modified a bit and warmed up. But cold fronts inland and throught most of the country, and even along the east coast, tend to be more brutal as those fronts travel over land instead of ocean to get there, retaining most of their freezing cold from the arctic. Cold fronts almost always bring freezing cold winds in those areas of the country. Freezing cold wind can be a plant's enemy all by itself causing foliar trauma. But it also causes severe desiccation in low humidity situations and virtually sucks the life out of plant tissues.
If winds are associated with an incoming arctic cold air mass an advective freeze (sometimes also incorrectly referred to as an advective frost) occurs. During advective freezes, clouds are often present, and may be carrying moisture as well, resulting in freezing rain or even snow. But if situations are right and the humidity is high enough but it does not rain, advective frost may form (also called wind frost). This sort of frost tends to form on the windy side of structure where the humidity is higher, leaving the relatively drier leeward side frostless. Wind frost is basically similar to Rime frost although it is a true frost. Note that advective frost, the substance, is a true frost, but an ‘advective frost', the event, is actually a freeze.
One question I have often asked, is if radiation frost forms at 31F, why does if form on roofs, car windows and lawns when my outdoor thermometer says it is only 38F-40F outside? How is that possible? Frost occurs in very localized areas where the temperature is freezing, and the temperature on the lawn, roofs and car windows may be a lot colder than where the gage for the outdoor weather station is located. Lawns, car windows and roofs tend not to hold much heat over night and that radiative cooling activity will bring local temperatures down more easily to freezing than in a nearby air mass, or on a wall, sidewalk, street or possibly even a plant, where my outdoor weather station probe may be sitting. If you put these probes on the roof or lawn, the temperature reading will probably be significantly lower (below freezing if frost actually forms). So you can quite easily experience frost on those cooler surfaces despite air temperatures being up to nearly 10 degrees higher. Virtually on any still, cloudless night when it gets down below 40F, I often have frost on the lawn in the mornings.
frost on my lawn despie measured air temperatures around 39F
Though frosts here only seem to occur in marginally freezing situations, the absolute low as well as the time the frost conditions remain can affect how much damage is done to plants. Most frosts we experience where I live are brief occurrences, almost always beginning sometime in the middle of the night or early morning and ending as soon as the sun comes up. All cold sensitive plants have their individual narrow range of cold tolerance. If temperatures drop below that cold tolerance on the surface of the plant, damage will occur. The degree of damage depends a lot on the position of the foliage relative to the earth, as well as the time the frost situation exists, the wind speed (even subtle winds below 5 mph will decrease frost damage), the dew point etc.
A few years ago, we had a particularly hard frost in which the temperatures dropped into the mid-20s and remained there most of the following day. The damage was extensive relative to what I had experienced during any other frost occurrence in my yard. I saw Hoar frost that early morning, but it vanished with the sunlight. However, the temperatures did not rise for many more hours (so the dew point must have dropped, probably due to a decreasing humidity). This might better have been labeled a true freeze, then. That borderline between a true freeze and a ‘hard frost' is not always easy to distinguish (at least by me) and this is where I still have a hard time with my definitions. Probably the event began as a radiational frost and continued on into a true freeze. Either way, I hope it never happens again!
frost damage in my yard after frost continued on for over 7 hours
Foliage position is, as just mentioned, another factor affecting damage from frost. For example, on a palm tree, new leaves are pointing up towards the sky, older ones are more parallel to the earth's surface and the oldest ones tend to droop downward towards the ground. Cold air is relatively heavy, and will always fall, relative to warmer air (this is why gardens on hillsides tend to have much less frost damage than those at the bottom of the hill, as the cold air will ‘slide on past', but pool at the bottom of the hill). Frost more easily forms where the cold air falls and stays. Surfaces that are flat (parallel to the ground) will collect cold air and frost will occur there. This is why a palm or banana on a frosty night tends to show more damage on those leaves parallel to the ground rather than the ones pointing up or down. Why is this good to know? If one knows ahead of time that a frost might occur overnight, bundling up the branches of such plants so they are all facing up or down can save a lot of foliage from getting frost bite.
Some of the more sensitive plants we grow here in Southern California showing showing leaf position... the lateral flat leaves parallel to the ground are the ones prone to frost damage
The low temperature the plant experiences also depends on a lot of other factors, most which are covered in the term microclimate. The more variation there is in a landscape, there more potential for a variety of microclimates to exist. Flat fields covered with crops generally have few microclimatic variations and damage to crops will often be relatively consistent. But if there is any breeze at all one might experience pockets of frost here and there, even on such a flat plain. And even without any measurable wind, other factors such as soil moisture, the presence or absence of mulch and the proximity of heat retaining structures (buildings, walls, ponds, hills etc.), and anything overhead will affect frost formation and are also included under the umbrella of microclimate. If you then include slopes, valleys, roads, large bodies of water, overall elevation, nearby canopy etc. the variables of microclimate become even more complex.
This lathe house in Balboa Park, San Diego, encloses a wonderful miniature arboretum with a lot of very tropical plants within it. The only protection these plants have over the outdoor plants is the slatted structure- there is no added warmth or protection, but that microclimate under the lathe is enough to keep a lot of very tropical things happy.
In my garden, which is far from the flat-field crop situation, I have a LOT of marginal plants growing in various microclimates about the yard. Those plants growing near the house are a lot less likely to be adversely affected by frosts since the house gives off (radiates) heat at night. Also, as the garden ages, more plants are growing tall and the farther foliage is from the ground where all the cold air collects, the less damage is done to its foliage. And these taller plants also act as a canopy to smaller plants, thereby protecting the smaller plants from frost, too. Even a thin banana leaf leaning a few feet over another plant may be enough of a canopy to keep that smaller plant from getting frost on its leaves. I have a somewhat hilly front yard and those plants at the top of the hill, despite being a bit more exposed, are less likely to show frost damage as cool air sinks and pools in troughs. So it is the plants at the bottom of the mounds that are more likely to have cold air wash over them and create the right situation for frost formation. This situation would be markedly exaggerated for those who have large yards with lots of hills, or who live on a side of hill, top of a mountain or in a valley, particularly with various exposures to breezes. Mulch, though usually thought of as a beneficial thing, can be a problem in frost situations as mulched ground tends to store much less heat and dry out faster, protecting plants growing out if it far less than moist or even soggy ground might. This is why farmers do not plow or cultivate on days when there is a frost warning the following night as cultivated ground gives far less frost protection than more solid, damp ground does. Many will also water the previous afternoon to insure the ground will absorb more heat and tend to give off more overnight. Some will even keep the water on all night. The formation of ice on the leaves themselves can be protective to some degree, particularly if a severe frost or freeze is anticipated. Like hoar frost, ice can protect leaves at a constant, barely subzero temperature while the surrounding air might get much colder and potentially do far more damage. Ice formation obviously has its down sides, though, as it can break limbs and tear of foliage and fruit from the sheer weight of it.
This Sabal yapa palm had frost on its leaves and one can see the darker discolored areas mostly on the leaves paralell to the ground (these leaves all ended up dying- left photo); right is an Aloe diolii that had frost on it for a while and it turned the leaves to mush (discolored leaves)
What else can you do to prevent frost damage, if you know it is going to be a chilly night? One of the reason orange groves often have large fans is to protect against radiation frost - to stir up the air and mix the higher, warmer air of the inversion with the colder air near the ground. And also moving air is less likely to allow frost to form on leaves and fruit. As mentioned above, watering the ground before the cold night can also help. Heaters in the form of smudge pots (now mostly not allowed anymore), oil heaters with blowers and large pools of warmed water can also help keep frost at bay. Overhead protection, either in the form of shade cloth, plastic etc. can be a life saver for marginal plants on frosty nights. Some have even tried creating artificial fog, though this is rather not too cost effective. If one has a few prized plants, one can wrap them, or just gently bind the leaves so they are either pointing up or down, rather than parallel to the ground, or drape some Christmas lights on them. Often all that is needed is a mere one to two degrees in local temperature elevation to save a plant from severe frost damage.
Initial frost damage may not appear bad (left), but after a few days one can really assess the damage on those plants more exposed (right). The Alocasia macrorhiza had no overhead protection, while another near it had a few palm leaves above it... that latter one did fine.
What can be done in the face of an advective freeze? Not much. Freeze protection is a lot more difficult and one is more at the mercy of the climate in situations and locations where advective freezes are common (in the path of arctic storms, for example). Freezes kill by literally freezing the plant, causing ice to form within its cells, disrupting them and basically killing all the tissues. But freezes can also kill with their winds, which, if dry, can literally suck the life out of a plant. Cold, dry wind is a very desiccating force, far more even than hot dry wind (which we experience here often). You can always wrap up your larger plants and take the potted ones indoors. Or move to a better climate.