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effective area of insolation
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.
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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
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