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> Hi Paul, Andrew & All,
Thanks David... What starts out as a simple question requires as deep consideration as a complex one..
Appreciate being reminded of that.
Andrew
________________________________________
From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on behalf of David & Alison Webster [dwebster@glinx.com]
Sent: February-29-16 6:50 PM
To: naturens@chebucto.ns.ca
Subject: Re: [NatureNS] Fw: Reconsideration; ground frost
No Andrew I don't think you have missed anything and there is nothing
fundamental involved. As Steve pointed out I used the term "incidence"
incorrectly to mean the angle between a tangent on the surface and the
incoming light.
I was approaching the grazing angle from the light side; starting with
the sun directly overhead and moving to the edge of the insolated surface as
a way of explaining that the energy falling on unit horizontal area just
after sunrise is relatively small. The element we are interested in is that
narrow ring of illuminated surface which just recently became exposed to
sunlight.
Using correct terminology, with the incident angle being the deviation
from normal, the energy intercepted per unit area would be proportional to
the Cos of the incident angle. Which I now see is known as Lambert's law.
I hope this unmuddifies the picture.
Dave
----- Original Message -----
From: "Hebda, Andrew J" <Andrew.Hebda@novascotia.ca>
To: <naturens@chebucto.ns.ca>
Sent: Sunday, February 28, 2016 10:54 PM
Subject: RE: [NatureNS] Fw: Reconsideration; ground frost
>
> David
>
> I think I am missing something here.
>
> I can see the relationship with the sine of the angle of incidence.. but
> until light strikes the surface, the effective angle of incidence is zero
> (as it is during the dark period), so am no sure how it can approach zero
> if it is there already.... or have I missed something fundamental here?
>
> Andrew
>
> ________________________________________
> From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on
> behalf of David & Alison Webster [dwebster@glinx.com]
> Sent: February-28-16 8:59 AM
> To: NatureNS@chebucto.ns.ca
> Subject: [NatureNS] Fw: Reconsideration; ground frost
>
> Dear All, Feb 28, 2016
> I overlooked another aspect to the question of frost shortly after
> sunrise; effective interception of solar radiation. This would apply to
> some extent year round in all terrain and locations given calm clear
> weather. On a global scale, the area of sunlight intercepted by the earth
> is a disk equal to the cross-sectional area of the globe. A portion of a
> recent private e-mail on this aspect is pasted below.
> START OF PASTE\\\\\\\\\\\\
> As one approaches the edge of this disk (sunrise) the effective area of
> insolation realtive to the area of earth insolated (the interception of
> radiation per unit area) approaches zero. Maximum insolation will be when
> the sun is directly overhead. Elsewhere the incoming radiation per unit
> area will be proportional to the sin of the angle of incidence.
> An object will continue to cool until incoming radiation exceeds
> outgoing radiation.
> Sometimes it take a while to notice the obvious.
> END OF PASTE
> Yt, Dave Webster, Kentville
> ----- Original Message -----
> From: David & Alison Webster<mailto:dwebster@glinx.com>
> To: NatureNS@chebucto.ns.ca<mailto:NatureNS@chebucto.ns.ca>
> Sent: Tuesday, October 13, 2015 7:15 AM
> Subject: Reconsideration; ground frost
>
> Hi Paul, Andrew & All Oct 13, 2015
> This didn't get posted the first time; perhaps because it had become
> too large so I have pasted the original with several earlier exchanges
> clipped.
>
> START OF PASTE\\\\\\\\\\
> Hi Paul, Andrew & All, Oct 11, 2015
> Having thought this over again, I suspect I overlooked the key factor
> which bears on both the value of air movement and the timing of greatest
> risk.
>
> All bodies will radiate heat at rates (as I recall) which depend only
> on their temperature and at temperatures under consideration there will
> always be some heat lost by out radiation. Consequently thin or small
> organs (with consequently low heat content), such as leaves and flowers
> will continue to cool relatively rapidly by out radiation, and cool the
> adjacent air by conduction unless this heat loss is offset by heat gain.
> One possible way to replenish this heat loss, on a cloudless night, is in
> radiation from haze or nearby warm objects. But I suspect that air flow,
> provided the air is warmer than the sheath of cold air in the vicinity of
> the leaf or flower, is far more effective in general. Thus the value of
> air flow down a slope, wind machines and low aircraft.
>
> Probably for much of NS the concepts of land breeze and sea breeze
> apply: land breeze at night, when land is cooling off faster than the sea,
> and sea breeze in the daytime, when land is warming rapidly relative to
> the sea.
>
> If the overall flow or air, when winds do not confuse matters, is
> downhill at night and uphill in the daytime then there must be a period of
> slack flow when downhill flow slows to zero and uphill flow is still also
> zero. This turnaround period should start shortly after there has been
> appreciable warming of high ground but not enough warming to initiate a
> reverse of flow from low ground to high. Which roughly would be shortly
> before sunrise at low elevation to an hour or so after sunrise (a guess).
> As I recall this fits experience with frost quite well.
>
> When in doubt consult the manual; so I dug out my old Met. book last
> evening and to my surprise found nothing on frost but a fair amount on
> condensation and sublimation. And found that I had misremembered the role
> of various particles and humidity in air.
>
> Condensation nuclei are usually salt crystals or droplets of sulfuric
> acid or smoke particles and sublimation nuclei are usually particles of
> soil (according to this 1954 text).
>
> Condensation nuclei are hygroscopic and start to collect water at about
> 80% RH but grow much faster at 96% RH or above. Sublimation starts only
> after RH exceeds saturation by several percent.
>
> DW comment: Sublimation is the process which would be involved in frost
> formation.
>
> Yours truly, DW
> My final answer. I promise; sort of.
>
> ----- Original Message -----
> From: rita.paul@ns.sympatico.ca
> To: naturens@chebucto.ns.ca
> Sent: Sunday, October 11, 2015 9:02 AM
> Subject: RE: Long: Re: [NatureNS] Frost in the morning
>
> Very interesting Dave and Andrew.
> Up and out before the sun and not coming in til after dark
> was very interesting at this time of year. even if harvesting potatoes was
> hard work!
> One evening after the sun went down and we were finishing up one
> member of the crew pointed out a bright light going across the sky.
> More like the sun reflecting of metal rather than a light. It wasn't very
> high
> higher than an airplane but not as high as satellites nowadays. From the
> newspapers
> of the day we deduced it was an early Russian satellite - an memorable
> sighting.
> Enjoy the day
> Paul
>
>> On October 10, 2015 at 8:21 PM "Hebda, Andrew J"
>> <Andrew.Hebda@novascotia.ca> wrote:
>>
>>
>>
>> Thanks David
>>
>> That makes sense.
>>
>> Now with a low tide (here at about 06:40 - Noel), that pretty well
>> guarantees the cold dense air will win tonight.
>>
>> A
> END OF PASTE\\\\\\\\\\
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2016.0.7442 / Virus Database: 4537/11713 - Release Date: 02/28/16
>
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