[NatureNS] Fw: Reconsideration; ground frost

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From: "David & Alison Webster" <dwebster@glinx.com>
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Date: Sun, 28 Feb 2016 08:59:52 -0400
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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.=20
    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=20
outgoing radiation.
    Sometimes it take a while to notice the obvious. =20
Yt, Dave Webster, Kentville
----- Original Message -----=20
From: David & Alison Webster=20
To: NatureNS@chebucto.ns.ca=20
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 =

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.=20

    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.=20

    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 =

    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.=20

    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 -----=20
  From: rita.paul@ns.sympatico.ca=20
  To: naturens@chebucto.ns.ca=20
  Sent: Sunday, October 11, 2015 9:02 AM
  Subject: RE: Long: Re: [NatureNS] Frost in the morning

  Very interesting Dave and Andrew.=20
  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 =

  hard work!=20

  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=20
  of the day we deduced it was an early Russian satellite - an memorable =
  Enjoy the day=20
  Paul =20

  > On October 10, 2015 at 8:21 PM "Hebda, Andrew J" =
<Andrew.Hebda@novascotia.ca> wrote:=20
  > Thanks David=20
  > That makes sense.=20
  > Now with a low tide (here at about 06:40 - Noel), that pretty well =
guarantees the cold dense air will win tonight.=20
  > A=20

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<DIV>Dear All, &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;=20
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; =
&nbsp;&nbsp;&nbsp; Feb 28, 2016</DIV>
<DIV>&nbsp;&nbsp;&nbsp; I overlooked another aspect to the question of =
shortly after sunrise; effective interception of solar =
would apply to some extent year round in all terrain and locations given =
clear weather. On a global scale, the area&nbsp;of sunlight intercepted =
by the=20
earth is a disk equal to the cross-sectional area of the globe. =
of a recent private e-mail on this aspect is pasted below. </DIV>
<DIV>&nbsp;&nbsp;&nbsp; START OF PASTE\\\\\\\\\\\\</DIV>
<DIV>&nbsp;&nbsp; As one approaches the edge of this disk (sunrise) the=20
effective area of insolation