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Hi Paul, Andrew & All, Oct 10, 2015
That is an interesting question Paul. In common with most natural
phenomena there are no doubt several forces at work and I will describe what
I suspect to be the major possible mechanism from first principles.
I don't buy the explanation you suggested Andrew. Sun on high ground
does not draw heat upward causing cold air to be drawn down.
Sun on high ground will warm the local air causing it to expand,
decrease the density of this air mass and it will rise by the same forces
that cause a hot air balloon to rise. This rise of a warmer air mass above
high ground will lead to a compensating upward flow of cooler air from lower
levels.
It will help, I think, to consider some of the effective ways of warding
off a marginal frost because this sheds some insight on mechanisms.
(A) Nothing is gained by working all night when frost is unlikely so the
first step is deciding when to prepare for action. In the days before
weather forecasts frost omens were the combination of calm air and absence
of cloud cover when evening temperatures were marginal. Thus, in the
absence of lateral air movement, local cooling can be rapid if out radiation
is not compensated by in radiation (from clouds).
(B) And one time-tested strategy for frost avoidance is putting frost
sensitive crops on a slope. Even in otherwise calm air the very slow
downward flow of air under cooling conditions offers some protection. That
this works also with tree fruits where the sensitive tissue is well above
ground level suggests that transpiration from plant organs (leaves or
flowers) is also a cause of cooling; thus airflow displaces this sheath of
cooled air adjacent to the transpiring plant organs [Stomata will be open at
night and the latent heat of vaporization is about 590 calories per gram of
water at 10o C; enough heat to melt 7 grams of ice at 0o C.] This effect of
air flow also applies to D.
(C) One ancient way to avoid frost is smudge pots; fires in portable
containers smothered with anything which will generate smoke and moved as
necessary so smoke will drift over the crop. The heat generated by these
fires is insignificant so protection, if any, is provided by back radiation
of heat from the smoke which will be able to absorb some out radiation. A
modern variant of this uses aerosols generated by spraying liquid onto a
vibrating surface(I think).
(D) Because frost develops when objects with the potential to cool
rapidly are not warmed somewhat by a compensating inflow of warmer air or
incoming radiation, one very effective way to avoid frost is the generation
of artificial wind; either by large fans on tall towers or by flying low
over the crop with a small plane throughout the critical dawn hours.
E) Probably the most efficient and convenient means of frost protection
is sprinkler irrigation when frost seems imminent. This will lead to
significant ice formation on leaves and blossoms but the crop is protected
because temperature can not drop below 0o C.
F) For small plants in gardens (e.g. newly planted tomato plants) a
gallon glass jug with the bottom cut off (cotton twine wet with gasoline
then lit and jug placed in a tray of water just after flame dies) is
effective if placed over the plant due to a greenhouse effect; heat from the
soil and plant is trapped within the jug, except for a slow loss due to
conduction, and this demonstrates that out radiation is a significant cause
of cooling.
G) Again for small gardens, e.g. Tomato plants tied to poles or
sprawling, an old sheet draped over them and left overnight will help to
keep in soil/plant heat which would otherwise be lost to radiation.
Getting back to the original question (why is frost hazard maximal near
dawn ?) probably one major factor is a large reduction in back radiation; an
object cools when out radiation exceeds in radiation. The haze which
develops when air becomes saturated with water vapor and condenses onto dust
particles probably provides significant in radiation (my guess). i.e. haze,
water droplets so small they are evident only in a beam of light, can be
expected to intercept radiation from plants or soil and radiate a portion of
this back towards the ground and in effect act as low-lying clouds. How high
this haze blanket extends I don't know but presumaby as high as the zone of
saturation and this haze blanket will quickly thin from the top down as
these tiny accumulations of water on dust particles are warmed by the first
rays of the sun. So this haze blanket may be thought of as an earth-fitting
wedge shaped layer which retreats westward as the sun rises, the upper
surface being a tangent to the horizon and aimed at the sun.
NOTE: these dust particles, really spores, pollen grains, salt crystals,
smoke particles, very fine sand, silt or clay, etc. will also lead to some
back radiation but not as effectively as water droplets due to the heat
capacity of water and the capacity of haze sized droplets. In the absence of
particles to serve as a nucleus water can not readily condense from
saturated air due to a relatively high heat of fusion (79.7 cal/gram at 15o
C). If two water molecules got together to start a water droplet then the
heat released would be roughly equivalent to the heat required to raise
their temperature by ~80o C; a difficult way to stay cool. Ionizing
radiation also aids condensation (Wilson chamber) but this is a side issue.
It is possible perhaps that the high concentrations of CO2, especially
within plant intercellular spaces and on the underside of leaves, helps to
hold tissue heat in (micro-greenhouse effect) and at the first rays of light
this buildup of CO2 will start to decrease as photosynthesis kicks in.
The outer surfaces of leaves and fruits carry an electrostatic charge
because in the application of pesticides as tiny droplets at low volume the
particles are given an opposite charge so they will be attracted to leaf and
fruit surfaces (to minimize drift). Drawing on memory, these spray particles
were given a negative charge which implies a positive charge on plant
surfaces. I don't know how this charge develops but wonder if it is
dependent on or reversed by photosynthesis. And if a wind of charges from
the sharp tips of plant hairs could cause cooling. Electrostatics is full of
surprises.
Yt, Dave Webster, Kentville
----- Original Message -----
From: "Hebda, Andrew J" <Andrew.Hebda@novascotia.ca>
To: <naturens@chebucto.ns.ca>
Sent: Friday, October 09, 2015 9:55 AM
Subject: RE: [Nature