Dark blue line represents the cold front moving down the Florida peninsula this afternoon.
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Freeze line across North Florida January 8, 2017 (in red)
While the cool-down is greatly appreciated, it could put an end to the sunflowers which have been in full bloom all winter if frost forms tonight or tomorrow night. Near-freezing temperatures are forecast with colder wind chills but these plants (Tithonia diversifolia) are sensitive to temperatures below 40° (4½° C). And killer frost could arrive if the winds die down, as Miguel de Unamuno said so eloquently in his book The Tragic Sense of Life (1912). . .
Warmth, warmth, more warmth! for we
are dying of cold and not darkness. It is
not the night that kills, but the frost.
Why Does Frost Form
When Temperatures Are
Above Freezing?
To explain why frost forms when temperatures are above freezing one first must understand the nature of frost.
What is Frost?
Frost is ice that sublimates directly on the surfaces on which it is found. Sublimation occurs when water vapor transforms from a vaporous state to a solid state.
To understand frost deposition one must think in terms of energy. Water vapor is in a higher energy state than liquid water and liquid is in turn in a higher energy state that solid water (ice). For water to be maintained in a vaporous state there must be a certain amount of energy available. As emergy is removed from the atmosphere (as the air is cooled) the gasses lose heat. Nitrogen, Oxygen, Carbon Dioxide, Argon, Water Vapor and all the other components of the atmosphere lose energy as they are cooled. Eventually when enough energy is removed the vapor changes state and it becomes either liquid or solid. Each liquid water molecule has less energy than each vapor molecule and each solid water molecule in turn has less energy than liquid or vaporous water.
And What is Dew Point?
The temperature where a change of state occurs as the air is cooled is called the dew point temperature when the change of state occurs at a temperature above freezing.
The temperature of condensation or sublimation is determined by the amount of moisture in the air. The greater the amount of water vapor, the higher the condensation temperature. During summer it is not uncommon to have a dew point of 75° (24° C) or higher in Central Florida. But in cooler seasons the dew point can be much closer to freezing.
Tonight, the dew point temperature in Orlando is 36° (2° C). Currently the air temperature is 40° (4° C) so it won't take much more cooling to start producing a lot of condensation across Central Florida as the humidity currently hovers around 86%.
DEW or FROST TONIGHT?
So will there be dew or frost tonight?
The forecast calls for patchy frost
in an arc all around Orlando (below)
Whether dew or frost will form is determined by two factors:
1. If the amount of moisture in the air causes the condensation temperature to be above or below freezing and
2. If the temperature will cool to the condensation temperature.
One can assume that across a large part of Central Florida the temperature will drop to at least the condensation temperature (36°) tonight with patchy frost forming in areas that are protected from the light breezes and especially in areas far from large lakes and rivers.
But the low temperature is only
forecast to be 36°
How will frost form?
What is important is not what the temperature was at the thermometer but what the temperature was where the frost formed, and if frost formed the local temperature was below freezing.
A thermometer indicates the temperature where the thermometer is located, usually a few feet above the ground. Because cold air sinks relative to warmer air and because the ground can cool very quickly, the temperature at ground level can be cooler than a few feet higher where the thermometer is located. Thus, even though the thermometer never reaches freezing, it can easily be below freezing a few feet lower or in pockets of lower elevation, on rooftops, car tops, etcetera.
Why is frost on my car
but not in the garden?
Certain materials like glass and car metal radiate heat quickly and therefore cool quickly. Also, areas like rooftops or hills, because of their exposure, lose heat through re-radiation very quickly and tend to receive frost before sheltered areas.
On marginal nights, like tonight, frost may only be seen on rooftops and cars and spotty grassy areas. Because of material makeup some areas could cool below freezing while nearby areas remain just above freezing and receive no frost.
Some Areas Have Dew,
Others Have Frost, Why?
The amount of water vapor can vary from place to place considerably. An exposed parking lot may have less vapor than a nearby garden because the plant canopy of the garden traps water vapor and plants transpire (emit) water vapor.
In the garden there may be enough vapor so the condensation temperature is a dew point temperature while in the nearby parking lot the condensation temperature is at or below freezing producing frost.
So what does any of this have to do with my sunflowers?
Most of these sunflowers are sunflower trees (Tithonia diversifolia), growing to 20-feet or more (7 m). They are a tropical species that is sensitive to frost and cold temperatures. If the temperature drops low enough for frost to form on the flowers, buds, or leaves of this species they will die back considerably and we won't expect blooms again until late spring at the earliest.
The few real sunflowers in the garden (Helianthus annus) are resistant to cold and frost and will be fine, but they are short this time of year and insignificant. Because of the short daylight hours they grow only about 3 feet (1 m) before blooming in Florida's winter.
I won't mind so much if the sunflower trees are damaged as they have grown super-tall and spindly this year with some reaching heights approaching 30 feet (10 m), due to the extremely warm temperatures and calm weather (no hurricanes, no thunderstorms, no weather at all to speak of in summer-fall 2015).
Above, a Bella Moth (or Ornate Moth, Rattlebox Moth; Utetheisa ornatrix) resting on a Sunflower Tree flower.
Above, a battered and worn-looking female Eastern Tiger Swallowtail (Papilio glaucus), on Sunflower Tree. Notice that she has lost most of the tails on both wings either to predators, age, whatever.
Above, an upside-down flying male Monarch Butterfly (Danaus plexippus) with Sunflower Trees.
REDNECK
Doesn't Know its Winter
The turtles and frogs have been active through this winter. The two nights of cool weather will send them into hiding. Above and blow, Redneck, the red-eared slider turtle doesn't seem to know its supposed to be winter. Something odd is going on in the image below. There is an extra foot protruding from underneath Redneck's shell.
Click on this or any other image for a larger view.
Below, Redneck's red-stripe is more visible when he is in the water. His colors fade on land. He spends very little time on land, ever, but especially in winter. Normally he should be in hibernation until at least the first of April.
Below, the bullfrogs have also been active this winter, mating as if it were spring.
Below, I like the images of Redneck's nostrils. Two perfect little holes. His expression changes little. He is unimpressed with me, that is obvious.
This bullfrog won't be seen for a while after tonight's cool.
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