Our intuition about temperature tells us that when we take an object out of the refrigerator and place it on the kitchen counter at room temperature, say 70 F, then after a while, this object will also reach 70 F. After all, the air, being at 70 F, is in contact with the object, and transfers the heat to the object.

On a clear, dry and cloudless night, you can pretty much throw this intuition out the door. Why? Dry air is a very poor heat conductor, and thus, only slowly conducts a lot of heat away from the plants. The primary means by which objects loose or gain heat is via electromagnetic radiation, e.g. infrared heat waves.

How does this work? Consider the fact that at around 39 F, frost will form on surfaces directly exposed to a clear night sky such as a well insulated house roof, or car windows and car roofs. Basically , even though the air is well above freezing, the surface cools off much more rapidly than the air, to the point that the surface can be well below freezing even if the air is well above freezing.

You can verify this with a thermometer: on a very clear dry night, place a thermometer in such a way that it faces the sky. Make sure it is well insulated from the surface beneath it. You will end up with much colder temperatures than what the air temp. actually is.

Does this mean that a plant leaf exposed to the same night sky will also drop to those temperatures? Not necessarily, because the thermometer has a different radiation loss factor than a plant leaf, and also may not store the same amount of heat as the plant leaf. A mercury thermometer will give a different reading than an alcohol thermometer.

Of course, even though dry air doesn't conduct heat very well, the air does play a small factor by forming a mini-inversion layer above a horizontal surface. Warm air rises and cold air sinks. As a result, a very cold layer of air can form if there is nowhere for the cold air to drain away.

Note that the above is true for radiative freezes. The definition of a radiative freeze is when the overnight air temperature drops below freezing as a result of radiative losses. In contrast, a convective freeze is a result of air that is already sub-freezing which moves into a particular area. Of course, once the air settles in and winds drop off, then on a clear and dry night, radiative effects will work to lower the air temperature even further.

So how do we measure the air temperature? Well, there really isn't a real consistent way to do this. Thus, the National Weather has defined a standard way of measurement, which requires a thermometer to be placed inside a well-ventilated box at about 5 feet above the ground. The thermometer needs to be placed inside the box in such a fashion that it is insulated from the box itself (the box can be several degrees cooler than the air.)

The subtropical gardener can immediately draw some great conclusions from all this:

1) MAKE SURE TO COVER SENSITIVE PLANTS ON A CLEAR NIGHT. It doesn't matter to create an air-tight cover, just make sure to use a cover which will act as a radiation shield. The latest advance in modern agriculture is the use of "frost row covers" which can give as much a 10 F protection during a radiative freeze. These covers are made of white, insulating fiberglass/polyester which reflect radiation and trap rising warm air. But if you don't have any of this stuff, then use a thick white blanket. Don't use plastic, it's useless as radiation escapes right through it.

2) Plant tissue is often hardier than hardiness ratings show. Take for example King Palms. The palm fronds themselves will not suffer any damage if the cells remain above 24 F. (provided no ice crystals form on the plant tissue surface, e.g. frozen condensation) Yet king palms out in the open are known to show damage when "official" readings reach 27 F. This is again because the palm fronds exposed to the clear cold night sky will drop into the low 20's, or even the upper teens. From this one can conclude that a king palm growing under a canopy or with some kind of overhead protection will survive air temperatures of 24 F without any trace of damage. This is actually the case.

Alas, much of the evidence about hardiness is anecdotal and comes from amateur temperature readings. A thermometer next to the house will give warmer readings than ambient air temperatures, but if the thermometer was in the open, then the readings might be much lower.

For Central Coast gardeners and other folks in zones 9 and 10 who have to contend with mainly radiative freezes, factors such as overhangs, tree canopies, and manually covering a plant are all climate modifiers which can guarantee a full half-zone improvement. We usually don't have to contend with frozen soil, so the covers really make a huge impact. I realize some of the readers here grow subtropicals in zones far colder than anything I am used to. Some subtropicals conceivably could widthstand prolonged freezes, and a cover to prevent further cooling as a result of radiative losses could conceivably make a difference.

The Cloudforest Gardener

Link to many online weather readings