next message in archive
no next message in thread
previous message in archive
previous message in thread
Index of Subjects
> of artifi
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head>
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type"/>
</head><body style="">
<div>
<span style="font-size: 12pt;">Very interesting Dave and Andrew.</span>
</div>
<div>
<span style="font-size: 12pt;">Up and out before the sun and not coming in til after dark<br/></span>
</div>
<div>
<span style="font-size: 12pt;">was very interesting at this time of year. even if harvesting potatoes was<br/></span>
</div>
<div>
<span style="font-size: 12pt;">hard work! <br/></span>
</div>
<div>
<span style="font-size: 12pt;">One evening after the sun went down and we were finishing up one<br/></span>
</div>
<div>
<span style="font-size: 12pt;">member of the crew pointed out a bright light going across the sky.<br/></span>
</div>
<div>
<span style="font-size: 12pt;">More like the sun reflecting of metal rather than a light. It wasn't very high<br/></span>
</div>
<div>
<span style="font-size: 12pt;">higher than an airplane but not as high as satellites nowadays. From the newspapers</span>
</div>
<div>
<span style="font-size: 12pt;">of the day we deduced it was an early Russian satellite - an memorable sighting.</span>
</div>
<div>
<span style="font-size: 12pt;">Enjoy the day</span>
</div>
<div>
<span style="font-size: 12pt;">Paul  <br/></span>
</div>
<div>
 
</div>
<div>
 
</div>
<div>
<br/>> On October 10, 2015 at 8:21 PM "Hebda, Andrew J" <Andrew.Hebda@novascotia.ca> wrote:
<br/>>
<br/>>
<br/>>
<br/>> Thanks David
<br/>>
<br/>> That makes sense.
<br/>>
<br/>> Now with a low tide (here at about 06:40 - Noel), that pretty well guarantees the cold dense air will win tonight.
<br/>>
<br/>> A
<br/>> ________________________________________
<br/>> From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on behalf of David & Alison Webster [dwebster@glinx.com]
<br/>> Sent: October-10-15 8:09 PM
<br/>> To: naturens@chebucto.ns.ca
<br/>> Subject: Re: Long: Re: [NatureNS] Frost in the morning
<br/>>
<br/>> Hi Andrew & All,
<br/>> I didn't express my thoughts very clearly. As in "This rise of a warmer
<br/>> air mass above high ground will lead to a compensating upward flow of cooler
<br/>> air from lower levels". And I left out a lot, such as expansion of the
<br/>> rising warmed air body and concurrent cooling which as I recall occupies
<br/>> about four chapters in an introductory Met. book and lets not go there.
<br/>> So I will take another run at it. The initial state of the model is
<br/>> sunlight hitting some area of high ground, which is located at the summit of
<br/>> a long east-facing slope, and this insolation leads to warming of the
<br/>> overlying air. This will result in an updraft accompanied by a downdraft
<br/>> around the perimeter of the warmed area and inflow of unheated air in under
<br/>> the rising air body which will then be warmed; a Stirling engine without
<br/>> pistons, cylinders or drive shaft. As the sun rises further the insolated
<br/>> area of the slope increases, increasingly lower elevations of the slope will
<br/>> be warmed and increasingly lower elevations of cooler air will become drawn
<br/>> into this circulation.
<br/>> But such updrafts and air pockets have no bearing on the pooling of
<br/>> coldest air into frost hollows. This is best visualized by considering the
<br/>> flow of air in a long straight east-facing ravine which heads in a large
<br/>> barren at high elevation. Shortly after sundown has reached this ravine the
<br/>> air will start to cool, become more dense and flow down the ravine to
<br/>> create, over a period of several hours, a river of cold air. If there are
<br/>> low areas of ground in undulating terrain beyond the mouth of this model
<br/>> ravine then one may expect the coolest air to pool in these 'frost pockets'.
<br/>> The same applies, in a less dramatic way, to any saucer-like depression at
<br/>> the lower reaches of a slope, however gradual the slope is. Coldest air,
<br/>> being most dense, will find its way to the frost hollow, displace any warmer
<br/>> air from it and then stagnate.
<br/>> These updrafts and air pockets can be quite intense even at mid-day when
<br/>> flying in light aircraft between terrain that is readily warmed (e.g. dry
<br/>> cultivated soil) and areas which are less readily warmed (e.g. lakes,
<br/>> woodland and moist meadows). You rise rapidly over the warmer area and can
<br/>> drop so rapidly when entering the cool that, with seatbelt not fastened, the
<br/>> plane roof drops with a bang onto your head.
<br/>> Meanwhile we may have our first frost tonight in the yard and I trust it
<br/>> knows how to do it.
<br/>> And I hope the second and third paragraphs make more sense that the
<br/>> brief version did.
<br/>> Yt, DW
<br/>>
<br/>>
<br/>>
<br/>> ----- Original Message -----
<br/>> From: "Hebda, Andrew J" <Andrew.Hebda@novascotia.ca>
<br/>> To: <naturens@chebucto.ns.ca>
<br/>> Sent: Saturday, October 10, 2015 3:05 PM
<br/>> Subject: RE: Long: Re: [NatureNS] Frost in the morning
<br/>>
<br/>>
<br/>> >
<br/>> > Thanks David
<br/>> >
<br/>> > So what would the mechanism for the wider-scale "frost-hollow" phenomenon
<br/>> > be? If denser, colder air is then drawn upward to replace the
<br/>> > warmer-less-dense air, would that be creating a density gradient with
<br/>> > colder/most dense air remaining at low levels?
<br/>> >
<br/>> > Andrew.
<br/>> > ________________________________________
<br/>> > From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on
<br/>> > behalf of David & Alison Webster [dwebster@glinx.com]
<br/>> > Sent: October-10-15 2:28 PM
<br/>> > To: naturens@chebucto.ns.ca
<br/>> > Subject: Long: Re: [NatureNS] Frost in the morning
<br/>> >
<br/>> > Hi Paul, Andrew & All, Oct 10, 2015
<br/>> > That is an interesting question Paul. In common with most natural
<br/>> > phenomena there are no doubt several forces at work and I will describe
<br/>> > what
<br/>> > I suspect to be the major possible mechanism from first principles.
<br/>> > I don't buy the explanation you suggested Andrew. Sun on high ground
<br/>> > does not draw heat upward causing cold air to be drawn down.
<br/>> > Sun on high ground will warm the local air causing it to expand,
<br/>> > decrease the density of this air mass and it will rise by the same forces
<br/>> > that cause a hot air balloon to rise. This rise of a warmer air mass above
<br/>> > high ground will lead to a compensating upward flow of cooler air from
<br/>> > lower
<br/>> > levels.
<br/>> > It will help, I think, to consider some of the effective ways of
<br/>> > warding
<br/>> > off a marginal frost because this sheds some insight on mechanisms.
<br/>> > (A) Nothing is gained by working all night when frost is unlikely so
<br/>> > the
<br/>> > first step is deciding when to prepare for action. In the days before
<br/>> > weather forecasts frost omens were the combination of calm air and absence
<br/>> > of cloud cover when evening temperatures were marginal. Thus, in the
<br/>> > absence of lateral air movement, local cooling can be rapid if out
<br/>> > radiation
<br/>> > is not compensated by in radiation (from clouds).
<br/>> > (B) And one time-tested strategy for frost avoidance is putting frost
<br/>> > sensitive crops on a slope. Even in otherwise calm air the very slow
<br/>> > downward flow of air under cooling conditions offers some protection. That
<br/>> > this works also with tree fruits where the sensitive tissue is well above
<br/>> > ground level suggests that transpiration from plant organs (leaves or
<br/>> > flowers) is also a cause of cooling; thus airflow displaces this sheath of
<br/>> > cooled air adjacent to the transpiring plant organs [Stomata will be open
<br/>> > at
<br/>> > night and the latent heat of vaporization is about 590 calories per gram
<br/>> > of
<br/>> > water at 10o C; enough heat to melt 7 grams of ice at 0o C.] This effect
<br/>> > of
<br/>> > air flow also applies to D.
<br/>> > (C) One ancient way to avoid frost is smudge pots; fires in portable
<br/>> > containers smothered with anything which will generate smoke and moved as
<br/>> > necessary so smoke will drift over the crop. The heat generated by these
<br/>> > fires is insignificant so protection, if any, is provided by back
<br/>> > radiation
<br/>> > of heat from the smoke which will be able to absorb some out radiation. A
<br/>> > modern variant of this uses aerosols generated by spraying liquid onto a
<br/>> > vibrating surface(I think).
<br/>> > (D) Because frost develops when objects with the potential to cool
<br/>> > rapidly are not warmed somewhat by a compensating inflow of warmer air or
<br/>> > incoming radiation, one very effective way to avoid frost is the
<br/>> > generation
<br/>> > of artificial wind; either by large fans on tall towers or by flying low
<br/>> > over the crop with a small plane throughout the critical dawn hours.
<br/>> > E) Probably the most efficient and convenient means of frost protection
<br/>> > is sprinkler irrigation when frost seems imminent. This will lead to
<br/>> > significant ice formation on leaves and blossoms but the crop is protected
<br/>> > because temperature can not drop below 0o C.
<br/>> > F) For small plants in gardens (e.g. newly planted tomato plants) a
<br/>> > gallon glass jug with the bottom cut off (cotton twine wet with gasoline
<br/>> > then lit and jug placed in a tray of water just after flame dies) is
<br/>> > effective if placed over the plant due to a greenhouse effect; heat from
<br/>> > the
<br/>> > soil and plant is trapped within the jug, except for a slow loss due to
<br/>> > conduction, and this demonstrates that out radiation is a significant
<br/>> > cause
<br/>> > of cooling.
<br/>> > G) Again for small gardens, e.g. Tomato plants tied to poles or
<br/>> > sprawling, an old sheet draped over them and left overnight will help to
<br/>> > keep in soil/plant heat which would otherwise be lost to radiation.
<br/>> >
<br/>> > Getting back to the original question (why is frost hazard maximal near
<br/>> > dawn ?) probably one major factor is a large reduction in back radiation;
<br/>> > an
<br/>> > object cools when out radiation exceeds in radiation. The haze which
<br/>> > develops when air becomes saturated with water vapor and condenses onto
<br/>> > dust
<br/>> > particles probably provides significant in radiation (my guess). i.e.
<br/>> > haze,
<br/>> > water droplets so small they are evident only in a beam of light, can be
<br/>> > expected to intercept radiation from plants or soil and radiate a portion
<br/>> > of
<br/>> > this back towards the ground and in effect act as low-lying clouds. How
<br/>> > high
<br/>> > this haze blanket extends I don't know but presumaby as high as the zone
<br/>> > of
<br/>> > saturation and this haze blanket will quickly thin from the top down as
<br/>> > these tiny accumulations of water on dust particles are warmed by the
<br/>> > first
<br/>> > rays of the sun. So this haze blanket may be thought of as an
<br/>> > earth-fitting
<br/>> > wedge shaped layer which retreats westward as the sun rises, the upper
<br/>> > surface being a tangent to the horizon and aimed at the sun.
<br/>> > NOTE: these dust particles, really spores, pollen grains, salt crystals,
<br/>> > smoke particles, very fine sand, silt or clay, etc. will also lead to some
<br/>> > back radiation but not as effectively as water droplets due to the heat
<br/>> > capacity of water and the capacity of haze sized droplets. In the absence
<br/>> > of
<br/>> > particles to serve as a nucleus water can not readily condense from
<br/>> > saturated air due to a relatively high heat of fusion (79.7 cal/gram at
<br/>> > 15o
<br/>> > C). If two water molecules got together to start a water droplet then the
<br/>> > heat released would be roughly equivalent to the heat required to raise
<br/>> > their temperature by ~80o C; a difficult way to stay cool. Ionizing
<br/>> > radiation also aids condensation (Wilson chamber) but this is a side
<br/>> > issue.
<br/>> > It is possible perhaps that the high concentrations of CO2, especially
<br/>> > within plant intercellular spaces and on the underside of leaves, helps to
<br/>> > hold tissue heat in (micro-greenhouse effect) and at the first rays of
<br/>> > light
<br/>> > this buildup of CO2 will start to decrease as photosynthesis kicks in.
<br/>> > The outer surfaces of leaves and fruits carry an electrostatic charge
<br/>> > because in the application of pesticides as tiny droplets at low volume
<br/>> > the
<br/>> > particles are given an opposite charge so they will be attracted to leaf
<br/>> > and
<br/>> > fruit surfaces (to minimize drift). Drawing on memory, these spray
<br/>> > particles
<br/>> > were given a negative charge which implies a positive charge on plant
<br/>> > surfaces. I don't know how this charge develops but wonder if it is
<br/>> > dependent on or reversed by photosynthesis. And if a wind of charges from
<br/>> > the sharp tips of plant hairs could cause cooling. Electrostatics is full
<br/>> > of
<br/>> > surprises.
<br/>> >
<br/>> > Yt, Dave Webster, Kentville
<br/>> >
<br/>> >
<br/>> > ----- Original Message -----
<br/>> > From: "Hebda, Andrew J" <Andrew.Hebda@novascotia.ca>
<br/>> > To: <naturens@chebucto.ns.ca>
<br/>> > Sent: Friday, October 09, 2015 9:55 AM
<br/>> > Subject: RE: [NatureNS] Frost in the morning
<br/>> >
<br/>> >
<br/>> >>
<br/>> >> Hi Paul
<br/>> >>
<br/>> >> I remember reading somewhere that the reason was that the sun would hit
<br/>> >> the top of the hills first, drawing heat upwards (convection?), that
<br/>> >> would
<br/>> >> then draw the colder air down the slopes to the hollows.. can't remember
<br/>> >> where I saw that, but it seems to make sense (from the point of physics
<br/>> >> of
<br/>> >> air mass movement)
<br/>> >>
<br/>> >> Andrew
<br/>> >>
<br/>> >> S________________________________________
<br/>> >> From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on
<br/>> >> behalf of rita.paul@ns.sympatico.ca [rita.paul@ns.sympatico.ca]
<br/>> >> Sent: October-09-15 9:31 AM
<br/>> >> To: naturens@chebucto.ns.ca
<br/>> >> Subject: [NatureNS] Frost in the morning
<br/>> >>
<br/>> >> Hi All
<br/>> >> In the old days on the farm we would be out before the
<br/>> >> Sun in early October. Not so much anymore! Then we would notice
<br/>> >> that the white frost would form just as the sun broke the horizon
<br/>> >> on a morning like this.
<br/>> >> This morning that happened also. When I went out before sun up there
<br/>> >> was no white frost anywhere but half an hour latter everything was white.
<br/>> >> I assume it was enough to kill tender plants but we need wait and see.
<br/>> >> There used to be an explanation for that but I forget - I was never sure
<br/>> >> if
<br/>> >> it was correct or not! Perhaps someone will remember.
<br/>> >> Enjoy the beautiful day
<br/>> >> Paul
<br/>> >>
<br/>> >>
<br/>> >> -----
<br/>> >> No virus found in this message.
<br/>> >> Checked by AVG - www.avg.com
<br/>> >> Version: 2015.0.6172 / Virus Database: 4435/10783 - Release Date:
<br/>> >> 10/08/15
<br/>> >>
<br/>> >
<br/>> >
<br/>> > -----
<br/>> > No virus found in this message.
<br/>> > Checked by AVG - www.avg.com
<br/>> > Version: 2015.0.6172 / Virus Database: 4435/10791 - Release Date: 10/10/15
<br/>> >
<br/>>
</div>
</body></html>
next message in archive
no next message in thread
previous message in archive
previous message in thread
Index of Subjects